Paleogeography, Diagenesis, and Paleohydrology of a Trinity Cretaceous Carbonate Beach Sequence, Central Texas

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1972 Paleogeography, Diagenesis, and Paleohydrology of a Trinity Cretaceous Carbonate Beach Sequence, Central Texas. Richard Francis Inden Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Inden, Richard Francis, "Paleogeography, Diagenesis, and Paleohydrology of a Trinity Cretaceous Carbonate Beach Sequence, Central Texas." (1972). LSU Historical Dissertations and Theses. 2220. https://digitalcommons.lsu.edu/gradschool_disstheses/2220

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INDEN, Richard Francis, 1943- PALEOGEOGRAPHY, DIAGENESIS, AND PALEOHYDROLOGY OF A TRINITY CRETACEOUS CARBONATE BEACH SEQUENCE, CENTRAL TEXAS.

The Louisiana State University and Agricultural and Mechanical College, Ph.D., 1972 Geology

University Microfilms, A XEROXCompany, Ann Arbor, Michigan

THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED PALEOGEOGRAPHY, DIAGENESIS, AND PALEOHYDROLOGY OF A TRINITY CRETACEOUS CARBONATE BEACH SEQUENCE, CENTRAL TEXAS

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College 1n partial fulfillment of the requirements for the degree of Doctor of Philosophy

1 n

The Department of Geology

by Richard Francis Inden B.S., University of Wisconsin, 1965 M.S., University of Illinois, 1968 May, 1972 PLEASE NOTE:

Some pages may have

i nd i st i net print.

Filmed as received.

University Microfilms, A Xerox Education Company ACKNOWLEDGMENTS

Dr. C. H. Moore suggested this prob'em and served as my committee chairman throughout Its endurance. I am in debted to him and to Dr. Jeffrey Hanor, Dr. D. R. Lowe,

Dr. G. F. Hart, Dr. Bob F. Perkins, Dr. Ray E. Ferrell, and Paul L. Stelneck for critically reviewing the manu script and making many useful comments. The author gra ciously acknowledges the guidance of Dr. P. E. Schilling, without whom the statistical analyses could not have been done; Mr. Mac Jervey and Alice Blenvenu assisted 1n the coding and keypunching of data for the computer analysis.

Thanks are due to Frank Lozo and Fred Stricklin of

Shell Oil Company for their help during the early stages of field work. The Texas Bureau of Economic Geology allowed the author to sample two donated Shell Oil cores. Sue

Mouledoux and Tom LeFebvre aided 1n the preparation of samples.

Gratitudes also go out to John Robinson, who did much of the photographic work; C liff Duplechln, for draft ing of the final plates; and Bunny Thlbodaux for typing.

Special thanks are due James Kennedy for preparing the final plates.

Financial support was provided by the Geology De partment, Louisiana State University, throughout the author's residency, and by a Sigma X1 Grants-1n-A1d of

Research Award. 11 TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...... 1i

LIST OF TABLES...... v

LIST OF FIGURES...... vi

ABSTRACT...... vH I

INTRODUCTION ...... 1

Purpose ...... 1 Study A r e a ...... 3 Procedures ...... 3 Regional Geological Setting ...... 5

MIDDLE TRINITY LITHOFACIES ...... 7

Stratigraphy ...... 7 Llthofades Analysis...... 12 Sycamore Conglomerate ...... 12 Middle and Upper Hammett Shale ...... 18 Cow Creek Limestone ...... 24 Hensel Sandstone ...... 44 Summary and PaleogeographlcReconstruction .... 51

CARBONATE DIAGENESIS ...... 57

G eneral ...... 57 Statistical Analysis ...... 59 Hammett Shale D1agenes1s ...... 61 Clay Mineralogy, Diagenesis, Dolomite Injection . 61 Cow Creek Limestone D1agenes1s ...... 71 Grain D1agenes1s ...... 71 Intergranular D1agenes1s ...... 96 "Marine" Fung-Algal Caliche ...... 124 Summary ...... 134 Hammett Shale ...... 134 Grain D1agenes1s ...... 135 Intergranular Diagenesis ...... 137 Comparison with Other Beaches ...... 139

111 Page

GENERAL DIAGENETIC-PALEOHYDROLOGIC CONCEPTUAL MODEL FOR MIDDLE TRINITY T I M E ...... 142

General Paleohydrology and D1agenet1c Environments 142 Subsea Diagenesis ...... 143 Subaerial D1agenes1s ...... 144 Cow Creek Strandllne Phase ...... 147 Post-Cow Creek Strandllne Phase ...... 152

CONCLUSIONS AND IMPLICATIONS ...... 160

REFERENCES...... 168

APPENDIX A. Procedures ...... 183

APPENDIX B. Discussion of Significant Variations 1n the Outcrop and Outcrop by facies Interaction Analyses . . . ! ! . . . 190

APPENDIX C. Clay Mineralogy of the Hammett Shales and Carbonates . ! ! 1 ^ ^ 7~. . 202

APPENDIX D, Other Dlagenetic Aspects of the Cow Creek Limestone .. I ! I ! I ! . . 207

D1. Early L1th1fIcation of the Nodules and Nodular B e d s ...... 207 D2. Grain Dolom1t1zat1on (RjE) ...... 213 D3. The Irregular, Medial Crystal Juncture L i n e ...... 217 D4. Recrystallized Cements ...... 220 D5. Equilibrium Calculations ...... 223 D6. Dldgsnetic Code from Folk (1965) . . . 225

APPENDIX E. Measured Sections ...... 227

VITA ...... 263

1 v LIST OF TABLES

Table Pag <2

1. Clay mineral analysis of "marine11 caliche and Hensel alluvial plain sediments ...... 49

2. Summary chart of Middle Trinity lithofacles . . 5 2

3. Distribution of grains with micrlte or dolomlcrlte envelopes ...... 76

4. Distribution of mlcrltlzed grains ...... 82

5 . Distribution of recrystal 1 1zed-1nverted (N-j) grains and moldic cavity filled grains . . . 90

6. Matrix types and distribution ...... 98

7. Cement distributions ...... 109

v LIST OF FIGURES

Study area and location of measured sections 4

Trinity division nomenclature ...... 8

Generalized stratlgraphlc column of Middle Trinity (Lower Cretaceous) rocks: Colorado River area ...... 10

Stratlgraphlc cross-section of Middle Trinity rocks, Colorado River area ...... 11

Generalized paleogeography and cross-section showing envlronment-facies relationships during Middle Trinity time ...... 13

Photographs: Llthofacles lb and V ...... 16

Photographs: Llthofacles V-VII ...... 19

Photographs: Llthofacles VII ...... 25

Photographs: Llthofacles VIII ...... 29

Photographs: Llthofacles VIII-X ...... 31

Photographs: Llthofacles X and XI ...... 36

Photographs: Llthofacles XII and XIII . . . 42

Hammett Shale limestone and dolomite .... 62

Hammett Shale dolomite; mlcrlte envelopes 65

Insoluble residue content of Hammett Shale limestone and dolomite ...... 66

Mlcrlte envelopes and mlcrltlzed grains . . 74

Recrystal 11 zed-1nverted g ra in s ......

Moldlc cavity fills , solution features and matrix types ...... ,

v 1 Figure Page

19. Carbonate cements ...... 105

20. Carbonate cements and "marine" fung-algal c a l i c h e ...... 114

21. Diagram showing the relations among the metas table Iron hydroxides and slderite at 25°C and 1 atmosphere total pressure ...... 123

22. Plot of insoluble residue content 1n "marine" fung-algal caliches ...... 126

23. "Marine" fung-algal caliche fabrics ...... 128

24. Cow Creek strandllne dlagenetic phase .... 148

25. Post-Cow Creek strandllne dlagenetic phase . . 156

v 11 ABSTRACT

The Hammett Shale. Cow Creek Limestone, and uppermost Hensel Sand (Lower Cretaceous) of central Texas repre sent a transgresslve-regress1ve deposltlonal cycle. Ham mett shale and carbonates (Insoluble poor lime packstone and clayey, silty dolomite wackestone and mudstone) were deposited In low energy offshore marine llthotopes during a decrease 1n the Influx of clastic detritus. Landward from these low energy environments nearshore oyster beds, carbonate shoals, lagoon, and a carbonate beach developed

(Cow Creek Limestone). The oyster beds are represented by clayey, silty oyster dolomite packstone with large, un abraded, convex-up oyster valves. Shoal sediments are composed of thickly bedded, sorted and rounded 11me grainstone and bloturbated arenaceous Hme packstone. Overlying the shoal unit 1s a lagoonal sequence made up of sometimes dolomltlzed foss111ferous, very f1ne-f1ne grained quartz arenlte with a carbonate mud matrix, and arenaceous (very fine-fine grained) pelleted lime packstone; arenlte beds are thin and Irregular whereas the packstone occurs as

Irregular, branching nodules (crustacean burrows), sphe roidal concretions, or lenticular. Irregular beds. The beach Interval consists of basal medium ant coarse grained

11me gralnstone and packstone with festoon cross-beds, and

v 111 thick accretion beds of medium-very coarse lime grainstone; sorting and rounding of grains becomes progressively better upward from the festoon cross-beds (upper offshore) into accretion beds (beach foreshore).

Sporadically capping the beach sequence is a nodular

"marine" caliche which, for the most part, is of fungal and algal origin, and a thin, coarsely crystalline supratidal dolomite. Cal1ch1f1cat1on took place contemporaneously with beach progradation (caliche clasts are reworked into the beach) and thus, the previously hypothesized uncon formity, which was thought to separate the Cow Creek Lime stone from Hensel Sand, becomes untenable. Hensel Sands and red mudstones therefore represent the alluvial plain equivalent of the Cow Creek and Hammett marine sequence.

The entire section was deposited under arid climatic condi tions as indicated by smectite and caliche nodules 1n Hen sel red mudstones, and the unweathered nature of potash feldspars 1n the alluvial channel f ills .

A statistical analysis of point counts on matrix, cement types, and allochem dlagenetic modifications reveals that early dlagenetic modifications 1n the marine carbo nates have distinctive vertical distributions. Fewer beds are dolomltlzed above the Hammett Shale, 1n which only the clay-rich carbonates were replaced. Lagoonal strata are sometimes dolomltlzed but consist, mainly of recrystallized pelleted lime packstone nodules, which underwent early

1 x 11thlflcation and recrystal 11 zed mud matrix in the quartz arenlte; beach gralnstones are never dolomltlzed.

Aragon1t1c bioclasts underwent dissolution In the beach, but stabilized by Inversion 1n lower facies; micritized original calcltlc bioclasts, on the other hand, are most abundant In beach foreshore sediments. Also, mlcrlte and dolomlcrlte grain envelopes are most prevalent 1n the beach unit.

Iron-free ca ld te cements, and the largest cements, predominate 1n the beach foreshore; cements below the beach are smaller, less abundant, and are ferroan-ca1d t e .

Mlcrltlzed and mlcrlte enveloped grains are the most abundant In the foreshore sequence because of grain stabi lization and the presence of mud-free pores, thus allowing bacteria, algae, and fungi to In filtrate the sediment.

The strandllne meteorlc-vadose zone and the mixed zone separating the local and regional meteor1c-phreat1c water lenses from marine Interstitial fluids were the loci of most dlagenetic alteration. Caliche development, early cementation, and grain leaching were effected by equili brating vadose waters. Cementation was Inhibited, and in version was allowed to proceed 1n the local mlxed-phreatlc zone. Dolom111zat1 on of Hammett carbonates took place in regional m1xed-phreat1c waters, and final cementation by

Iron-free cements occurred 1n oxidized phreatlc or vadose zones.

x INTRODUCTION

Purpose

During the past decade carbonate petrologists and petrographers have awakened to the fact that carbonate rocks contain information which not only helps one deline ate carbonate deposltlonal environments, but also aids in identifying specific dlagenetic environments. Friedman

(1964) made one of the first significant contributions to carbonate petrology, and provoked much controversy in making the statement that carbonates only stabilize and become ce mented when exposed to fresh waters. Since that time, much research has been done in detailing not only the diagenetic effects imposed on carbonates In fresh waters (Bathurst,

1964; Land, 1966; Matthews, 1967, 1968; Robinson, 1968;

Folk, 1965; Gavish and Friedman, 1969), but also those cement types and grain matrix modifications which are unique to marine and hypersaline diagenetic water environments

(Bathurst, 1966; Shinn and others, 1965; Deffeyes and others, 1965; Shinn, 1969; Moore, 1970).

Few studies, however, have related dlagenetic altera tions to specific deposltlonal environments or to specific fresh, diagenetic water regimes. Calcite cements contain ing iron seemingly can only be precipitated below the fresh

1 2

water table (Evamy, 1969) and Runnells (1969) notes that the mixing of diagenetic waters plays a significant role in the diagenetic history of a rock. Purdy (1968) in a very lucid treatment of marine diagenetic modification, discusses many of the environmental controls which help or hinder their development. Roehl (1967) was one of the first researchers to relate diagenetic fabrics to specific depositional environments; more recently, Allen (1970),

Moore and Allen (1971), and Land (1970) have identified vertical variations 1n the types and frequencies of cements and grain modifications through Cretaceous and Pleistocene beaches; these variations are apparently induced by the different processes acting within depositional and post deposltlonal environments. Thus, diagenetic changes may be peculiar to certain depositional environments (therefore, they are facies specific) or hydrologic environments, which in coastal regions are intimately related to the strandllne depositional environment (I.e ., beach vs. tidal flat) and climate.

The purpose of this dissertation is first, to iden tify the depositional environments and paleogeography rep resented by the Cow Creek Limestone and Hammett Shale, and second, but more Important, to use this depositional analy sis to relate the dlagenetic modifications of the Cow Creek

Limestone and Hammett Shale to the deposltlonal environ ments, and hydrochemica1 situations to which the sediments 3

were subjected soon after deposition.

Study Area

Eighteen strat1graphic sections of the lower Hensel

Sandstone, Cow Creek Limestone, Hammett Shale, and upper most Sycamore Conglomerate (Lower Cretaceous) were measured and sampled over a 350 square mile area, which includes parts of Travis, Burnet, Hays, and Blanco Counties, Texas

(Fig. 1). All locations described are along tributaries of the Colorado or Pedernales Rivers. The Hensel Sand stone and Hammett Shale are typically slope formers, and fresh, unweathered exposures are rare. The Cow Creek Lime stone, however, 1s well exposed 1n all outcrops visited, but extensive recent leaching has created the problem of obtaining samples suitable for petrographic analysis. At two localities, numbers 17 and 18 (Cow Creek and Hamilton

Pool, respectively) samples and measured sections were ob tained from Shell 011 Company cores drilled a few hundred feet back from the outcrops; supplementary samples of the upper part of the Cow Creek Limestone were collected from the outcrop at Location 17.

Procedures

The field and laboratory methods employed 1n the analysis of the Cow Creek Limestone and Hammett Shale are in Appendix A. Detailed field descriptions of the 4

•LakbMk usr tmas (by permission of F.L. Strickl1n)

CONQHOAKH * *

VI * \H O

PEDKKNALKS ft IVIR d*

J o ho ton City

L o c h t t o n MILES dapositional atrika of Cow craak Limaatona

Figure 1. Study Area and Location of Measured Sections. 5

strati graphic sequence were made and point count, Insoluble residue, and clay mineral analyses were performed on the samples collected. The measured stratlgraphic sections are gi ven i n Appendi x E.

Regional Geological Setting

The study area 1s situated near the eastern margin of the northwest-southeast trending Llano Uplift (Fig. 1), which along with Its southeastward continuation, the San

Marcos Arch (Plummer, 1950), controlled sediment distribu tion in central Texas throughout the early Cretaceous.

In the study area, basal Cretaceous sediments were in itia lly deposited upon an irregular, eroded substratum of faulted and tilted Carboniferous carbonates and elastics

(Marble Falls Limestone and Smlthwick Formation) and Ordo vician dolomites (Ellenburger Group) (Sellards and others,

1966; Plummer, 1950; Barnes, 1951). This surface was de pressed to form a slight structural embayment on the south east flank of the Llano Uplift, immediately to the north of the San Marcos Arch {F1g. 1). Exposed Pre-cambr1an igneous and metamorphic rocks of the Llano lay to the west, while the broad marine continental shelf extended eastward.

Lower Cretaceous clastic sequences proximal to their source area are composed of terrigenous detritus derived from rocks exposed 1n the Llano. Llthic fragments of ig neous and metamorphic rocks, and virtually all resistant 6

Paleozoic formations are represented 1n the sandstone and conglomerate facies of the Sycamore Conglomerate (Barnes,

1951; Damon, 1940) and Hensel Sandstone. Both formations contain abundant pebbles and cobbles derived from Ordovi cian Ellenburger Group dolomite.

Boone (1968) reports pebbles and cobbles of Llano- type llthologles 1n the Sycamore Conglomerate 75 miles to the north of the present study area. Indicating that the

Llano area acted as a major sediment source during Early

Cretaceous time. Central portions of the Llano Uplift remained posi tive and underwent erosion until covered by sediment onlap sometime during Glen Rose time (Murray, 1961). A complete but diminutive sequence of Trinity Division sediments, deposited over the San Marcos Arch, indicates that the arch was a shoal to slightly positive area during Trinity time (Lozo and Stricklin, 1956). MIDDLE TRINITY LITHOFACIES

Strati graphy

Complete synopses of the evolution of Lower Creta ceous stratigraphic nomenclature are presented in Lozo and

Stricklin (1956) and Boone (1968). Lozo and Stricklin

(1956) partition the Aptian and Alblan Trinity Division

into three subdivisions (F1g. 2). The lower subdivision

includes the Sycamore Conglomerate and the Sligo Limestone, the middle, the Hammett Shale and Cow Creek Limestone, and the upper, the Hensel Sand and Glen Rose Limestone. Thus, each is a cyclic unit consisting of a clastic Interval at

Its top; a non-deposltional disconformity 1s inferred to mark the top of each subdivision. They treat these sub divisions as conceptual "tectonic-sedimentary lithogenetic entities" (p. 68). The interpretation offered for each cycle is that of an initial uplifting of the source area, resulting in increasing erosional rates and supply of det ritus to surrounding depositional areas. Transgression takes place, and with time the supply of terrigenous mate rial wanes; shallow marine carbonate depositional environ ments come Into existence, regression occurs, and eventually sea bottom is converted to land.

Stricklin and Smith (1968) recognize the Cow Creek

7 BASAL CRETACEOUS STRATIGRAPHY

GLEN ROSE LS.

s *H n

COW CREEK LS.

H a MM, sHal

SLIGO LS. (after Lozo % and S tric k lin , 1956) **r.

Figure 2. Trinity Division Nomenclature. 9

Limestone as a regressive beach sequence. They subdivide

it into three facies: 1) a basal lime mudstone containing

whole fossils; 2) an overlying sandy calcarenite, and

3) an upper coarse grained calcarenite-coquina. The basal

and intermediate facies were deposited in gradually shoal

ing waters, whereas the coquina unit represents a prograd

ing beach deposit. Cross-bedding azimuths indicate that

longshore currents flowed to the southwest and wave ap

proach was from the south. The beach deposits are "over-

lain disconformably by the Hensel Formation" (Stricklin

and Smith, 1968). A more detailed account of this Middle

Trinity sequence Is given in Stricklin and others (1971).

Figure 3 illustrates the generalized Middle Trinity

sequence in the study area. The Cow Creek Limestone and

Hammett Shale progressively thin north-westward onto the

Llano Uplift (F1g. 4). Because no erosional unconformities

were noted 1n the section, thinning is attributed to onlap,

offlap, and nondeposition. As the sequence 1s traced north

westward, the Hammett Shale disappears firs t, and at the

plnchout, the Cow Creek Limestone is a thin oyster bed

overlying a bored, oyster-encrusted dolomite pebble zone,

which represents the top of the Sycamore Conglomerate.

The Hensel Sand 1s approximately 80 feet thick near the

Llano Uplift and thins south eastward down depositional dip. In contrast to those of the Hammett-Cow Creek sec

tion, the component lithologic units of the Hensel are COW CREEK LIMtSTONE I HtNSEL SAN SYCAMORE Figure 3. G eneral ized ized eneral G 3. Figure CONGLOMERATE tocvout rci Col ado o d ra lo o C rocki; t) u o v c rtto C MMTIOHAL M GM RNCOS AKTNS TI FAG BS WITH BOS, FLAGGY THIN PACKSTQNES; t l i ARENACEOUS L LTY-Fl* S. YTR OOIE PACKSTONl. DOLOMITE OYSTER SO. * l F - Y T IL S DIP. LOWER L IA IT PACKSTOHi. IN BDS.. SOME LH. SHARP.CONTACT OYSTER IAREG. NOLLUSK AREH. BURROWED IK-MASSIVE FINE CSE. E-. S. RNCOS LUK RISOE; K. IK GRAINSTONES; 1 t l * i CONTACT LOWER CONTACT LONER L SHARP SHARP-GRADATIONAL ZONES; ARENACEOUS BEOS. WLLUSK BRECCIA ACCRETION CSE. HED-V. NMULAA , y ; t M T S M ARENACEOUS HACKS AND TONE OOLOmTE TONE-WHS LIME ARENACEOUS DQLOniTIC AIH CAT. HR LNR CONTACT LONER SHARP CLASTS. PEBBLES REWORKED CALICHE BASE; DOL. HITH BASE ICAR t AT SS. SS. OTZ. OYSTER WITH CLAYSTONE OOII OTIA DLNT OZ S, N VERT AND SS, OTZ. DQLONITE LOWERCONTACT SHARP-GRADATIONALTHROUGHOUT. OETRITAL DOLOMITIC , f . V - . F CSE-FINE .: K B X FESTOON CONTACTS SCOUR BDS.. GRAOATIONS; PHSSIW N NLUK AKTNS DLMT INJECTED DOLOMITE PACKSTONES; t l NACJCSTONLS i ARENACEOUS DOLOMITE OYSTER L NOLLUSK AND t i l F CONTACT NORMALLYLONER SHARP REUA CNAT; t AIAIN. NODULES LAMINATIONS. tt O S CONTACTS;IRREGULAR THIN- .: S S SUBAMCOSIC CSE. t COW. PEBBLE DOL. NO It TN; BURROWED; LOWER CONTACT STONE; t I U INTO CN MDTNS IH AIH (Lift f i L ( CALICHE MUDSTONES WITH -CRN. 1 0E HTLD- HR LOWEP SHARP HOTTLED.- Z04ES ) I atirpjc oun Mi e le d id M l o column c igraphj 1 t ra

ZOUES

River

area.

lithofacies Trinity I I -1 I I I a- lb VIII XIII I I I V I V

IV I X I I I X

10 ;'/f lum» iymb«l> 01 Fig. 3)

0 1 2 3 ftMJn

Figure 4. St ratigraphic cross-section of Middle Trinity rocks,Colorado River area. 1 2

lost not by stratigraphic plnchout, but by complex thinning

and interfingering throughout the study area (Fig. 4).

Lithofacies Analysis

The sequence from the uppermost Sycamore Conglomer ate through lowermost Glen Rose Limestone is here divided into 13 lithofacies. These units are delineated on the basis of their sedimentary structures, composition, and texture. Point count data on grain-matrlx types and fre quencies of each Cow Creek lithofacies, and clay mineral and Insoluble residue analyses of the Hammett Shale are given in Appendices B and C, respectively. The lateral facies relations and generalized paleogeographic recon struction are depicted in Figure 5. Explanations of the diagenetic sedimentary structures of Lithofacies V, V III, and XI are presented 1n following sections concerned with diagenesis.

Sycamore Conglomerate

Dolomite Pebble Conglomerate (Lithofacies la) Pebbly Dolomite (Llthofacles lb) Festoon Cross-Bedded Sandstone (Lithofacies II) Intercalated, Fossi11ferous Sandstone and Limestone TITthofaTie? ITI)------

Desc ri pt1 on

Only the uppermost part of the Sycamore Conglomerate was examined as part of the present study. The formation (same symbols as Figure 3)

Figure 5. Generalized Peleoteography and cross-section showing environment- facies relationships during Middle Trinity time. 1 4

consists mainly of red and green texturally immature feld-

spathic subgreywacke and subarkose (Folk, 1965), and well

rounded, poorly sorted polymlctlc grain-supported con

glomerate. The conglomerate units are massive (up to 10*

tk.) and homogeneous or graded, and are composed of quartz,

chert, granite, limestone, and dolomite cobbles and pebbles

set 1n a sandstone matrix (Lithofacles la). Interbedded

lenticular or massive sandstone 1s cross-bedded or inter

nally homogeneous. The conglomerate 1s typically scoured

into underlying sandstone or mudstone, and normally grades

upward Into sandstone. Interbeds of red and green mud

stone containing dolomltic limestone nodules are sparsely

present. Fossil turtle shells have been found within the

Sycamore (B. F. Perkins, personal communication, 1971).

The contact between the Sycamore Conglomerate and

Hammett Shale 1s generally placed at the base of a thin

zone of bored, oyster-encrusted dolomite pebbles. Imme

diately below this zone are three very different llthologic

types and sequences. At locations 1, 4, 5, 16, 17, and 18

(Figs. 1, 4), Sycamore conglomerate and sandstone (Litho

facles la) grades upward into thinly bedded (5-60 cm),

fossi11ferous, texturally mature sandstone and siltstone, with interbeds of 11me packstone and wackestone (Litho-

facies III) . At other outcrops (7, 8, 9, 13, and 15;

Figs. 1, 4), the contact 1s sharp, and the uppermost Syca more consists of silicate and dolomite pebbles suspended 15

in a matrix of sugary dolomite (Fig. 6 A, B; Lithofacies

lb); downward through a 5'-20* Interval, pebbles become

more abundant until the rock becomes grain supported. The

dolomite matrix contains caldte filled cracks and rare

laminated structures resembling pisolites (Fig. 6 C, D, £).

Clasts of this material are reworked into the lowermost

Hammett beds (Fig. 6 F).

The top of the Sycamore at outcrop 6 is represented

by a basal festoon cross-bedded, texturally mature-

supermature sandstone which 1s overlain by gently dipping

accretion beds composed of well-rounded, well-sorted dolo

mite pebble conglomerate (Lithofacies II), The total

thickness of this sequence 1s approximately 15 feet; 1t is

succeeded by thin, laminated beds of fossi11ferous, fine-

medium grained sandstone and slltstone (Lithofacies III) .

Sycamore Alluvial Plain ( la )-Cal1 che Paleosol-Beach Envi ronments ( Ib; 11); Basal Hammett Intertidal- Subtida1 Environment fTTTT------

Interpre tati on

Oamon (1940) hypothesized that the Sycamore Con

glomerate and sandstone represent alluvial terrace and

channel deposits. The lithologies, sedimentary structures, and llthologlc sequences of the uppermost Sycamore confirm

such an origin for some of the rocks (Lithofacies la).

However, L1thofa1ces lb, 1n which pebbles float in a dolomite matrix containing expansion desiccation cracks Figure 6. Photographs: Lithofacies lb and V.

A. Uppermost surface of Sycamore Conglomerate; pebbles and cobbles of dolomite and quartz which float 1n a dolomite matrix (Lithofacies lb)

B. Polished slab of Pebbly dolomite (Lithofacies lb)

C. Pebbly dolomite from uppermost Sycamore Conglomer ate showing brecclated dolomite matrix (gray) and angular pebbles (whlte-Hght gray).

D. Irregular fractures 1n arenaceous csely. crystal line dolomite; fractures filled with calclte spar and s ilt to sand size qtz. and dol. (vadose s ilt); Lithofacies lb.

E. Pisolites (Lithofacies lb).

F. Reworked pebbles and caliche (Lithofacies lb) clasts from Sycamore Conglomerate 1n lowermost Hammett Shale carbonate.

G. Brecclated, contorted v. thin oyster lime- packstone-wackestone beds; Lithofacies V of Hammett Shale.

H. Compactiona1 flow aligned oysters 1n Hammett Shale dolomite (Lithofacies V); white area Is thin, dis rupted lime packstone. 16

Figure 6. 1 7

and pisolite structures (Dunham, 1969) 1s Interpreted as a

paleocallche soil horizon (Amsbury, 1967). The accretion-

bedded and festoon cross-bedded sandstones of Lithofacies

II represent a beach foreshore-upper offshore package, as

Interpreted from the process-response beach model of Ber

nard and others (1962). In summary, the uppermost Sycamore

portrays deposition 1n alluvial plain to strandllne en-

vi ronments.

The texturally mature sandstones, siltstones and

limestones (Lithofacies I II ) above the bored pebble zone

at the base of the Hammett Shale represents the transition

into 1ntertidal-high subtldal sedimentation during the

regional marine transgression over the Sycamore. At the

same time, alluvial deposition and calIchif1cation of the

uppermost Sycamore proceeded to the west of the strandllne.

Wave reworked dolomite pebbles, or those freshly brought

to the deposltlonal site by streams, were attacked by bor

ing mollusks and worms 1n the nearshore zone (Perkins,

1971). As the strandllne moved westward, the bored pebbles were covered by deeper water and encrusted by oysters.

This Interpretation of the Sycamore-1owermost

Hammett establishes the pa 1eogeographic setting for the development of the superjacent Hammett, Cow Creek, and

Hensel units. 18

Middle and Upper Hammett Shale

Cl ays tone-Mudstone (LlthqfaclesIV)and Interbedded Limestone ancT Dolomite (lithofacies V)

Description

Immediately overlying the basal sand unit of the

Hammett Shale are two fine-grained lithofacies which thicken from 0 to 55 feet east-south-eastward away from the Llano Uplift {Figs. 1, 4). The lower contact of the cl ays tone-mud stone (Fades IV) with the basal sand (Facies

III) is continuously gradational from sandstone into mud stone. The fossi1iferous mudstone is typically burrowed or laminated and contains sporadic, very thin, lenticular interbeds of lime packstone and cross-bedded sandstone.

This unit grades upward into homogeneous claystone.

Lithofacies IV grades upward into an interbedded limestone and dolomite sequence (Lithofacies V). The beds of this facies are thin and highly irregular; limestone interbeds are sometimes brecciated (Fig. 6 G) and dolomite layers are contorted and contain flow-aligned bioclasts

(F1g. 6 H). The limestone 1s typically clayey-silty (3-25% insoluble residue content) lime packstone, and less com monly wackestone, which contains oysters, recrystallized mollusks, and echinolds (Fig. 7 A, 8, C). Dolomite wacke stone and mudstone (F1g, 7 D) contain more insoluble resi dues (12-45%) but lack the recrystallized mollusks found Figure 7. Photographs: Lithofacies V-V11.

A. Mollusk-oyster Hme packstone (Fades V); note that shells are disarticulated but unabraded; tightly packed and broken valves along base of slab near contact with dolomite.

B-C. Mollusk-oyster 11me packstones; recrystal1ized mollusks and mlcrospar-pseudospar matrix.

D. Contact between limestone (at base) and dolomite wackestone with bloclasts showing flow alignment parallel to limestone contact (Facies V).

E. Thin dolomltlc lime packstone beds from uppermost Facies V; current oriented oyster valves.

F. Photo looking up Into oyster dolomite packstone bed (Facies VI); large oysters, most of which are disarticulated, unabraded, and oriented convex- valve up.

G. Slab of Fades VI oyster dolomite packstone.

H. Fades VII (Carbonate Shoal Environment); two Irregular thick beds; current oriented convex-up valves 1n upper bed. Hthology 1s mostly arena ceous lime packstone. 19

Figure 7. 20

in the limestone. Shells in both the limestone and dolo mite are normally disarticulated and unsorted, and 1f the bioclasts are broken, they are angular or subangular. Only the uppermost 2-4 feet of Facies V displays evidence

{abraded, oriented, and sorted mollusk fragments) of cur rent deposition (Fig. 7 E).

The thickness and frequency of dolomite interbeds rich in insoluble residue decreases upward with a resulting increase in the number of limestone units. Thus, there is a net decrease 1n the amount of silt and clay 1n the Ham mett Shale upward to the Cow Creek Limestone. The diagenetic significance of the Insoluble residue contents and clay types in Facies V carbonates will be discussed later

(p. 6 1, Appendlx C ) .

Offshore, Open Embayment Environment (IV, V)

Interpretat i on

The strati graph 1c position of Facies IV and V be tween the basal sandstone of the Hammett Shale and the re gressive carbonate shoal unit (Facies VII) of the Cow

Creek Limestone Indicates that the claystone and mudstone of Fades IV, and the lower part of Facies V represent the furthest offshore deposits (1n the study area) of the transgresslve phase. Lithofacies IV was deposited under low-energy conditions as Indicated by Its fine grain size, unsorted, angular bioclasts, and bioturbation features 21

(bioclast fragmentation and sediment homogenization prob ably result from Intense b1oturba11 on; Stanton and Warme,

1971). Clay and mud was deposited in a carbonate-deficient offshore environment or, alternatively, on a current-free bottom existing closer to shore than the depositional site of Facies V. If the first hypothesis is correct, the shift from clastic to carbonate deposition resulted from a simple decrease in supply of terrigenous detritus to the deposi tional area, whereas if the latter Is true, a decrease in clastic supply plus further transgression 1s necessary.

Evidence against continued transgression is the ab sence of a repeated Facies IV (claystone and mudstone) in the overlying upper Hammett-Cow Creek regressive sequence.

Dolomite containing a large amount of Insoluble residue de creases in frequency upward through Fades V, also suggest ing a progressive decrease in the supply of detritus to the offshore area. These two observations Indicate that

Facies IV elastics do not represent a nearshore equivalent of carbonate Fades V, but Instead reflect conditions 1n the source area and bottom current activity within the study area during their deposition. Fades V limestone and dolomite was deposited when the supply of fine detritus to the quiet offshore environment became negligible in com parison to carbonate sedimentation rates.

The stratlgraphl c position of Fades V and Its asso ciation with overlying, regressive oyster bank and carbonate 22

shoal deposits (Facies VI, VII) are the criteria used for

Identifying 1t as a deeper, open embayment deposit. The

presence of abundant 11me mud 1n the limestones, the mud-

supported fabric of the dolomites, and the angular, un

sorted nature of the bioclasts are all indicative of depo

sition 1n a low energy environment (Folk, 1962; Dunham,

1962). The fact that dolomite 1s mudstone or wackestone,

and limestone 1s typically packstone suggests that lime

mud accumulation rates, relative to shell production rates,

were greater for dolomite than for limestone; this d iffe r

ence, however, may be the result of diagenesis whereby ara-

gonltic allochems, originally present within a grain-

supported sediment, were obliterated by dolomite replace ment.

The thin interbeds of Insoluble-poor limestone and

insoluble-r1ch dolomite are best explained by evoking the

action of selective binding and trapping agents (i.e ., algal mats, sea grasses [Scoffln, 1970; Neumann and others,

1970]). Slight topographic depressions or patches of sea grasses or algae could have baffled currents and trapped

both terrigenous and 11me muds (dolomites). In non vegetated or slightly higher areas, very low velocity cur rents may have prevented terrigenous mud from settling out while allowing lime mud produced 1n situ to accumulate. Oyster Dolomite Packstone (Lithofacies VI)

Desc r1pt1 on

This uppermost lithofacies of the Hammett Shale occurs throughout the study area. The contacts with overlying and underlying strata are sharp, and the unit itself is one thin bed {1-2 feet thick) composed of silty, clayey, coarsely crystalline (60p-120p) dolomite and packed oyster valves (F1g. 7 F, G). The oysters are large, unsorted, and disarticulated but unbroken; many are oriented convex valve up. Irregularly shaped clusters of two or more en crusted oysters are commonly found within this rock unit.

Oyster Bed Environment (VI)

Interpretat1on

Lithofacies VI 1s the result of deposition in an environment of high oyster productivity which existed just seaward of the carbonate shoals (superjacent Fades VII).

The bedded nature of this facies, its strat1graphic thick ness, and absence of oysters 1n growth position and topo graphic relief eliminate the possibility of this being an oyster reef deposit. Instead 1t represents an oyster bio- strome; oysters were only slightly removed from growth position, possibly by storm waves or tidal currents, as 1s evidenced by the presence of oyster clusters, the lack of abraded valves, and the convex valve up orientation. 24

Cow Creek Limestone

Thick Bedded, Sandy Lime Packstone (Lithofacies VII)

Description

Throughout most of the study area this unit ranges between 3 and 7 feet thick (Fig. 7 H); it is absent in the pinchout area (F1g * 4) and thickens down depositlonal dip to as much as 13 feet. The upper contact Is sharp or gra dational (interbedded), and at outcrops 4, 6, and 10

(Figs. 1, 4} the lowermost beds of superjacent Lithofacies

VIII display shallow. Initial dips (Fig. 8 A), Indicating that some depositlonal relief existed on the top of Litho facies VII.

This lithofacies consists of Irregular thin to thick beds of dolomltlc 11 me packstone and subordinate lime gralnstone (F1g. 8 B). Lenses of well sorted, convex-up mollusk valves (F1gs. 7 H, 8 C) and graded units (F1g. 8 D) are commonly present. Cross-bedding and other current- formed sedimentary structures are absent, but the unit typically displays burrows and other bloturbatlon features

(Fig. 8 E, F).

The Hme packstone (F1g. 8 G) contains a high per centage (403* of the grain population) of fine to very-fine angular quartz sand, and poorly sorted, angular mollusks

( 303*) and oysters (27%); echlnolds, detrltal dolomite, and feldspar are present 1n small amounts (Appendix B). The Figure 8. Photographs: Lithofacies VII.

A. Lowermost beds of Facies VIII displaying Initial dips (toward left side of photo); the beachward dip on these beds indicates some deposltional relief on underlying shoal unit. B. Poorly std., arenaceous mol 1 usk-oyster lime pack stone; most bioclasts are angular; intraclast 1n left center.

C. Current oriented robust convex-up pelecypod valves 1n uppermost bed of Facies VII.

D. Coarse to fine gradation upward through Facies V; large mollusk valves at base.

E. Bioturbated poorly sorted, arenaceous mollusk- oyster lime packstone.

F. Burrows on upper surface of Facies VII.

G. Arenaceous ( v .f .-f.g r.) poorly sorted mollusk lime packstone; most bloclasts are rounded; dolomltic.

H. Well sorted, well rounded coarse gr. mollusk lime grainstone. 25

Figure 8. 26

lime packstone matrix 1s composed of 30% dolomicrlte, 70% mlcrite and microspar, and minor calclte cement; normally, the packstone Is bloturbated (Mg. 8 E). In contrast to the lime packstone, lime gralnstone contains well sorted, well rounded bloclasts and little or no detritus (Fig. 8 H).

The lime gralnstones typically are not bloturbated.

Carbonate Shoal Environment (VII)

Interpretation

The lime packstone and lime gralnstone units of

Lithofacies VII are thought to have been deposited in a carbonate shoal environment which separated the deeper, open embayment from shallower, nearshore environments.

The beachward dipping lowermost strata of Lithofacies VIII manifest the depositlonal topography of these shoals, and the thickness of the shoal deposit at each outcrop may be taken as the approximate topographic relief developed on

It during Its genesis.

Facies IV, V, and VI contain lit t le or no sand size detritus, whereas sand 1s an Important component of Litho facies VII through XI; on the other hand, these overlying upper units contain only minimal amounts of s ilt and clay, which 1s abundant in underlying facies. The small topo graphic difference between sea-bottom and shoal-top seems to have been sufficient to trap fine sand 1n the nearshore environments. Normal outgoing tidal currents were 27

apparently unable to transport the fine sand sediment over the shoal area, whereas occasional storm waves could have picked up oysters and other biotypes living farther o ff shore and carried them beachward over this shoal area.

The occurrence of zones of well sorted, convex-up bivalves, graded sequences and well sorted, well rounded bioclasts in the lime grainstones all Indicate that deposi tion was Influenced by currents or waves. Extensive bioturbation suggests that depositlonal rates were suffi ciently slow enough for the Infauna to disrupt and destroy most of the current-formed structures. The abundance of lime packstone in this facies may be the result of current baffling and sediment stabilization by sea grasses on the shoal (Jlndrlch, 1969), or low velocity tidal currents.

Ball (1967) found that lime wackestone and packstone bars, separated by lime gralnstone floored channels, develop where tidal current velocities are small, whereas the oppo site situation may be true where tidal currents are stronger. Thus, the carbonate shoal unit may well repre sent a tidal bar belt (Ball, 1967), but because outcrops are limited In lateral extent, diagnostic features such as accretion beds and channels were not seen. 28

Fossi1iferous Quartz, Arenlte and Pel 1et1ferous Lime Packstone (Lithofacies V III)

Descri pt1on

This lithofacies is present at all outcrops except

those in the northwesternmost part of the study area and

is thickest (20') in the vicinity of Cow Creek (loc. 17,

Fig. 1). It consists of thin beds of laminated or homo geneous, fossi11ferous fine to very-fine grained quartz arenlte (F1g. 9 A, B, C) interbedded with resistant con cretions, nodules, and lenticular, nodular beds (Fig. 9 D,

E, F). Nodules and concretions sometimes merge to form

beds, and lenticular beds can sometimes be traced laterally

into zones of nodules; these units are made up of arena ceous, pelleted lime packstone (Fig. 9 G). Laminated beds are most common 1n the upper and middle parts of the sec

tion and occur only rarely In the lower part. A transition zone of undulatory beds, containing both coarse and fine quartz sand, occurs where the overlying festoon cross

bedded unit 1s not scoured Into the top of this facies.

Angular, well-sorted, fine to very-fine grained detritus makes up 55% of all grains (carbonate plus clas tic) in the quartz arenltes; of this 55%, 16% Is detrltal dolom1te--the highest percent of detrltal dolomite 1n any of the lithofacies studied. Allochems are medium to very coarse grained, and are typically angular and unsorted

(Fig. 9 C) except In the laminated zones. Oysters and Figure 9. Photographs: Lithofacies V III.

A. Laminated calc., foss111ferous v . f . -f.gr. qtz. arenlte 1n uppermost part of Facies VIII.

B-C. Fossi1Iferous v .f .-f.gr. qtz. arenlte; poorly std., angular-rded mollusk fragments; recrystal lized calclte mud matrix and cement.

D-E. Lenticular, Irregular beds (D) and branched nodules made up of arenaceous, foss111ferous, pelleted lime packstone.

F. Diffuse contact of pelleted lime packstone nodule with dolomltlzed, arenaceous matrix.

G. Arenaceous (v.f.-f.gr. qtz.), fossillferous pelleted lime packstone.

H. Irregular, branched pelleted lime packstone nodules which probably represent crustacean burrows. n v iue 9. Figure 29 30

other mollusks (numerous gastropods) are the most abundant bioclasts (55% and 35%, respectively, of all carbonate grains); the remaining carbonate grains are echlnolds, intraclasts, and pellets (Appendix B). The matrix 1n the arenite is primarily composed of recrystallized lime mud, but it may contain replacement dolomite. Units rich in detrital dolomite are replaced by finely crystalline

(16p -6 4 m ) dolomite to such an extent that the original depositlonal textures and fabrics are obliterated. Nodules are, at most, only slightly dolomitlzed.

Nodules are Irregularly branched (Fig. 9 H) into vertical to horizontal components which truncate laminated matrix sediment. Concretions are large (up to two feet in diameter), flattened ellipsoidal or spheroidal masses

(Fig. 10 A); draped laminations 1n overlying sediments in dicate differential compaction around their upper surfaces.

The contact between a nodule and Its surrounding matrix 1s typically gradational. Where the surrounding matrix 1s dolomite, the periphery of the nodule 1s replaced. If the nodule Is enclosed 1n a quartz arenite unit, quartz-rich recrystallized calclte separates pellets at the nodule margin, but Inward toward the center, the calclte matrix contains progressively less quartz.

Large (2 mm) round, sand-free fecal pellets, now recrystallized to 10u-30p calclte, make up an average of

20% of the allochems in the lime packstone. Oysters and Figure 10. Photographs: Lithofacies V111 - X.

A. Zone of large el 11psoldal-spheroidal pelleted lime packstone concretions; dlagenetlcally enlarged cemented burrows or sediment volcano mounds.

B-C. Large scale festoon (trough) crossbeds of Fades IX (upper offshore environment).

D-E. Poorly sorted arenaceous mollusk oyster lime packstone-gralnstone of upper offshore festoon cross-bed section (Facies IX).

F-G. Thick, evenly bedded accretion units of beach foreshore environment (Facies X). Uppermost part of accretion beds dip and pinch out toward beach backshore 1n G.

H, Arenaceous mollusk lime gralnstone (Facies X). 31

''<*V ■ . ■ ■"

1 mm

Figure 10. 32

other mollusks occur 1n the same proportions as 1n the

arenite, but clastic grains are less abundant (Appendix B); coarse calcite makes up the 1ntergranular matrix. The or ganisms that produced the fecal pellets selectively in gested only the organic-rich carbonate muds while pushing aside nondigest1ble sand-sized detritus. Swinchatt (1967) and Klein (1965) also note organisms which selectively ate only fine grained sediments.

Inner Carbonate Shoal and Restricted Lagoon (V III)

I nterpretat1on

The stratigraphlc position of this facies between beach and shoal units suggests that 1t was deposited 1n a shallow, low energy lagoon or on the shoreward side of the carbonate shoal. This inference 1s further substantiated by the composition and texture of component grains and matrix (well sorted fine sand; angular, unsorted bioclasts; carbonate mud matrix) and sedimentary structures (lamina tions, homogeneous beds, and burrows). Kendall and Skip- with (1969) report similar sediment compositions and tex tures from the Persian Gulf Trucial coast lagoons. As shown by Imbrle and Buchanan (1965) and Swinchatt (1967), the homogenization of sediments 1n quiet water environments is the result of complete reworking by burrowing organisms.

The poorly sorted, laminated beds 1n the upper part of this unit represent the lower energy offshore facies 33

equivalent of the festoon cross-beds (Facies IX), which

were deposited nearer to shore under higher energy condi

tions (uppermost lower flow regime; Harmes and Fahnstock,

1965; Vischer, 1965). Laminated beds are not intensely

bioturbated as are underlying beds; this may be an indica

tion of rapid deposition during storm activity. Any sedi

ments or sedimentary structures deposited above these sedi

ments during normal conditions were removed by dune scour

(basal scour surfaces of the overlying festoon cross-beds)

during subsequent storms or facies progradation.

The branching nodules that truncate laminations and

contain pellets (F1g. 9 E, G, H) are Interpreted as crusta

cean burrows because they bear a striking resemblance to

those described by Shinn (1968) from Florida Bay and the

Bahamian Platform, and Stanton and Warme (1971) from the

Eocene of Texas. The large ellipsoidal and spherical con

cretions are inferred to represent d1agenetlcally enlarged

burrows or pelletlferous sediment volcanos formed at ex

current openings of burrows (Shinn, 1968). The nodules and

concretions underwent early 11th1fIcatlon; differential compaction (draped laminations) took place around them, and, although surrounding sediments are sometimes com

pletely dolomltlzed, the nodules and concretions contain only minor dlagenetlc dolomite. The reasons for this

11th1f1catlon are presented 1n Appendix D1. Nichols (1966)

suggests a burrowing origin for an analogous Lower 34

Carboniferous nodular section 1n Wales.

Lenticular, Irregular pelleted lime packstone beds

(Fig. 9 D) which are extensively bloturbated and contain

less detritus than arenlte beds, are thought to have origi

nated through the redistribution by currents of burrow mounds and other sediments formed through burrowing ac

tivity during times of low detrltal Influx. The Irregular nature of the bedding contacts could be due to compaction, or original bottom Irregularities on the sea floor.

Festoon Cross-Bedded Lime Packstone (Lithofacles IX) anJ Accretlon Bedded Lime firalnstone (Lithofacles X)

Descr1pti on

Lithofacles IX 1s characterlzed by large festoon

(trough) cross-stratificatlon (Fig. 10 B, C) and 1s made up of arenaceous, coarse to very coarse grained lime grainstone (F1g. 10 D, E) and bloturbated, coarse to medium grained lime packstone. No generalization can be made con cerning the sorting and rounding of these sediments except that very coarse lime gralnstones are normally the most angular and least well sorted. Clastic detritus consti tutes a smaller percentage (26%) of the grain population than 1n the underlying facies, and oysters and mollusks are again the dominant bioclasts (Appendix B). Festoon cross-bed orientations Indicate deposition by westward flowing currents (Stricklin and Smith, 1968; Stricklin and 35

others, 1971). A sharp or gradational contact separates

this unit from the overlying accretion bedded deposits

(L i thofac i es X ).

The festoon cross-bedded unit (Lithofacies IX) main

tains a constant thickness throughout the study area, whereas the section of accretion beds thickens down deposi

tion a 1 dip (Fig. 4).

Overlying accretion beds (Lithofacles X) strike east-west (Fig. 1, this paper; Stricklin and Smith, 1968) and have initial southward dips ranging from 4° to 14°

(Fig. 10, F, G; F1g. 11 A) and sediment size and sorting

Increases from toe to crest of individual units. Burrowed, finer grained 11me gralnstone Is common 1n the uppermost portion of each bed. The detrltal content 1s slightly less than that 1n Lithofacles IX, and mollusks and oysters are again the most abundant allochems. A stratigraphlcally thinner set of accretion beds, associated with small chan nels (F1g. 10 G; F1g. 11 B), is rarely present at the top of this sequence; these beds display dip directions oppo site (northward) those of the underlying unit.

Upper Offshore (IX). Foreshore, and Backshore (Xj Environments

Interpretation

Stricklin and Smith (1968) recognized the upper Cow

Creek Limestone as representing a regressive beach sequence. Figure 11. Photographs: Lithofacles X and XI.

A. Well sorted, well rounded mollusk lime gralnstone (Facies X).

B. Channel cut into beach backshore at top of beach foreshore accretion beds.

C-D. Tightly packed nodules (Facies XI) of caliche origin at top of beach sequence. Irregular, pock marked upper beach surface in D.

E. Distorted dolomite layer (Facies XII) on top of nodular section; taken as evidence of expansion of nodular section during cal1ch1f1cat1on.

F. Dolomite and mlcrlte cemented beach Hme gralnstone from lowermost nodules (Facies XI); grains are bored and encrusted with m1cr1te-dolom1cr1te.

G. Dolom1t1c 11me wackestone; fracture filled with spar cement and Infiltrated vadose silt.

H. Fractures 1n lime wackestone which cut through matrix and circumscribe Individual grains ( "c1rcumgranular cracks”). 36

Figure 11 37

The upper offshore zone of the conceptual beach model

(Bernard and others, 1962; Ball, 1967) is characterized by the presence of large-scale festoon (trough) cross strati f 1cat1on . This cross-bedding type results from the migration of dunes, a bed form that develops 1n the middle to upper part of the lower flow regime (Harms and Fahn- stock, 1965; Simons and others, 1965). Longshore currents paralleling the strandllne give rise to festoon cross bedding and ripple trains oriented perpendicular to the dip of the foreshore beds (Ball, 1967; Potter, 1967).

Each accretion unit represents a single foreshore deposltlonal event 1n which the beach was extended seaward.

Sediment was washed up and deposited on the gently sloping foreshore by breaking waves. These sediments do not 11e at their angle of repose (as they do 1n a gravity con trolled avalanche deposit) because the sediments were de posited as a traction load from high tangential flow velocity currents set up by waves breaking in the beach swash zone (Imbrle and Buchanan, 1965). Due to the coarse sediment size, laminations, characteristic of the swash zone (Allen, 1963) are almost invariably absent or d if f i cult to d1stlngulsh.

The water depth 1n which the upper offshore sequence was deposited 1s estimated by the thickness of the fore shore unit (I.e ., for a 20' thick foreshore section, off shore dunes were forming 1n 20 feet of water). The 38

distance from shore to the longshore currents can be ap proximated by calculating {water depth) / ( tangent of accre tion bed dip angle). At Cow Creek, the foreshore unit 1s ten feet thick and dips range from 6°-14°; the distance to the upper offshore dune area was 60'-14Ql , and the water depth was approximately 10 feet.

The northward dipping accretion unit at the beach top represents either the landward dipping portion of a beach berm or washover fan (F1 g. 10 G) that was deposited by a storm which cut channels through the beach berm and splayed sediment into the backshore area.

Brecclated Lime Wackestone (Llthofades XI)

Desc r1pt1on

Field relations suggest that nodular limestone

(Lithofacles XI) 1s only sporadically present at the top of the beach; at location 17 (Cow Creek, F1g. 1), for example, the nodules are present In the outcrop, but are absent in a core drilled only a few hundred feet away.

This tightly packed nodular zone (Fig. 11 C) rests on an irregular, pock-marked upper beach surface (F1g. 11 D) and where this facies 1s well developed 1t attains a maximum thickness of five feet. Although the contact 1s distinct

1n the field, no sharp contact between nodules and under lying beach material can be recognized 1n thin sections.

Overlying beds are often distorted and follow the 39

irregularities developed on the upper surface of Lithofa cles XI (Fig. 11 E) .

Dolomicrlte and micr1te-cemented gralnstone at the base of this facies grade upward into wackestone (Fig. 11

F , G, H), which contains progressively fewer floating oyster and quartz grains. Bioclasts in both deposits are extensively bored and breccia cracks filled with in f il trated calcareous s ilt and sparry calclte cut through grains and grumose-pel1eted appearing matrix; often, cracks circumscribe individual grains and then continue through the matrix (Fig. 11 G, H).

"Marine" Fung-Algal Caliche Environment (XI)

Interpretat1on

All the above mentioned properties, plus the charac ter of the overlying facies, suggest that the nodular zone represents a paleocaliche which developed by the alteration of upper beach material 1n the backshore zone while this area was s till influenced by mixed marine-fresh waters.

The deposit is therefore termed "marine" caliche to distin guish 1t from the caliche paleosols which are present 1n the overlying alluvial plain fades. The origin of the mlcrlte and dolomicrlte cements and grain borings 1s at tributed to the activity of 1nterpart1cu1 ate fungi and algae, and precipitation from ascending and descending vadose waters. E. A. Shinn and R. K. Matthews (personal 40

communications, 1971) report similar soil horizons forming

in near strandline positions from the Persian Gulf and Bar bados. Logan, Read, and Davies (1970) find numerous cal crete (caliche) nodular zones displaying parallel rock fabrics and dlagenetlc textures in coastal Pleistocene de posits from Sharks Bay, Australia.

Two easily recognized structural properties typify caliche soil horizons: volumetric expansion of the se quence, and the presence of packed. Irregular nodules.

Buckling and bowing of beds, and fracturing 1n caliche sequences are caused by desiccation and expansion resulting from the continual addition of calcium carbonate to the section (Reeves, 1970; Young, 1964; Blank and Tynes, 1965).

Gile and Hawley (1966) and Reeves (1970) note that nodules, formed through the solut1on-redepos1tion of calcium car bonate, characterize mature caliche profiles. The lami nated crust of the High Plains caliches (Reeves, 1970) and the Florida Keys (Multer and Hoffmelster, 1968) did not develop on the top of the Cow Creek beach; the reason for its absence 1s considered on page 127.

When this caliche deposit 1s interpreted 1n an environmental-paleogeographlc context, the existence of a significant strat1graphlc hiatus at the base of the caliche

(F1g. 4), as postulated by Lozo and Stricklin (1965),

Stricklin and Smith (1968), and Stricklin and others (1971) becomes untenable. It 1s more logically concluded that the 41

Hensel Sand is a regressive fluvial deposit rather than

transgressive estuarine or fluvial (?). The absence of a

hiatus 1s also substantiated by the presence of caliche

clasts, with fabrics identical to those of the nodular

zone, reworked into the underlying Cow Creek beach beds.

If transgression over an exposed beach top had occurred, a

zone of caliche lag deposits or reworked beach clasts would

be present at the base of the Hensel. Such a deposit, how

ever, has not been found.

Thinly Bedded Dolomite (Lithofacles XII)

Descri pti on

This highly localized dolomite facies is developed

at only five outcrops (F1g. 4). It Is never more than 7

feet thick and tends to thicken and thin over the topog

raphy developed on the beach top (F1 g. 11 D, E). The thin,

irregular dolomite mudstone beds, and thinly bedded dolomltic lime wackestone are rich 1n quartz silt and clay.

Bioclastic debris in the 11me wackestone consists of algae, ostracods, and small gastropods {F1g. 12 A). At locations

3, 13, and 17, black or 1ron-sta1ned breccia clasts

(Mg. 12 B) are found in association with this deposit, and the 11me wackestone Itself displays the expansion- desiccation features noted in Lithofacles XI (Fig. 11 G, H; p. ). Dolomite beds, composed of coarse dolom1crospar, also exhibit these features, along with irregularly shaped, Figure 12. Photographs: Lithofacles XII and XIII.

A. Algal, ostracod lime wackestone from supratldal pond {Fac1es XII).

B. Brecciated dolomite-1ime mudstone associated with Facies XII and lowermost part of Facies XIII; breccia clasts are black (pyrlte) or iron-stained; interpreted as forming along margins of supratidal hypersaline lakes. C-D. Storm deposited layers (white in C) from supra tldal Facies XII thinly bedded dolomite.

E. Possible rootlet casts In coarsely crystalline dolomite (XII).

F-G. Festoon cross-bedded pebbly, Immature subarkosic ss. (Facies XIII) and massive, graded, lenticular bodies of pebbly ss. and dolomite pebble con glomerate; alluvial channel fills .

H. Vertical lime mudstone caliche nodules 1n red mud stone of alluvial floodplaln origin (Facies XIII). 42

Figure 12. 43

more finely crystalline nodular zones (Fig. 12 C, D) and

possible rootlet casts (F1g. 12 E).

Supratldal Marsh-Lake Environment (XII)

Interpretati on

The lime wackestones are Inferred to represent

hypersaline lake deposits on the basis of their deposl-

tional texture, restricted fossil content (algae, ostra-

cods, and small mollusks) and stratlgraphlc position. The

lake sediments display the caliche fabrics of Facies XI,

plus they are found associated with caliche breccia clasts

(F1g. 12 B). Ward and others (1970) report blackened

brecciated caliches along the margins of coastal hyper

saline lakes 1n Mexico. Ephemeral hypersaline lakes exist behind coastal beach ridges, dunes, and 1n supratldal areas throughout the Carrlbean (Deffeyes and others, 1965;

Shinn and others, 1969), Western Australia (Logan and others, 1970), Turkey (Muller and Irion, 1969), and many other locations.

The dolomite mudstone beds lack algal structures,

laminations, fenestral fabrics, desiccation cracks and flat pebbles which are considered to be diagnostic of supratldal environments (Shinn and others, 1969; Shinn,

1968; Kendall and Sk1pw1th, 1968). However, It 1s here

Interpreted as a supratldal marsh deposit because: 1 ) Its strati graphic position above, and thus shoreward from, the 44

beach; 2 ) association with hypersaline lake deposits; and

3) the presence of probable root casts (F1g. 12 C) and light colored, nodular, storm deposited layers (Shinn and others, 1969; this paper, F1g. 12 A). The absence of many supratldal sedimentary structures Is attributed to dolo mite recrystal 1 1 zatlon and distortion caused by the expansion-heaving of the underlying nodular zone. This latter observation also suggests that the nodular zone de veloped during and after deposition of this dolomite unit.

A thick, more well-defined supratldal sequence is lacking due to rapid progradation and high energy coastline condi tions, which caused the displacement of most marine car bonate muds to deeper offshore waters. With this lack of abundant near-shore lime muds, the supratldal environment was sediment starved, allowing the beach top sediments to be ca 11 c h 1 f 1 ed .

Hensel Sandstone

Red Mudstone-Sandstone (Lithofacles XIII)

Descrlptlon

Lithofacles XIII Is over 80 feet thick at the Cow

Creek plnchout; 1t thins rapidly to less than 25 feet at

Cow Creek, a distance of 12 miles directly perpendicular to deposltlonal strike (F1g. 1). The best developed len ticular channel section (F1g. 12 F, G) occurs 10 to 15 45

feet above the beach sequence at Cow Creek, but 1s found progressively higher 1n the section toward the Llano. It is separated from the beach top by red, maroon, and green mudstone containing vertical lime mudstone nodules

(Fig. 12 H) and lenses. Channel fills are rarely encoun tered within a few feet of the beach top and, where pres ent, they never cut Into the backshore or foreshore sedi ments.

The channel deposits are composed of subarkose and dolomite pebble conglomerates (F1g. 12 G). Pebbles are well rounded; sands are poorly sorted and angular, and the feldspars are extremely fresh. In complete sections, mas sive sandstone and conglomerate beds, with sharp scoured lower contacts, grade upward Into festoon cross-bedded sandstone. The mean grain size and scale of cross-bedding decreases upward. In most sequences, however, channels are superposed, and younger channels cut Into the tops of older channel deposits leaving only the lower parts of the channel f i l l . Sand and pebbles 1n these units are some times cemented with calcite, and reworked fragments of these cemented clasts are often found in the channel fills . 46

Alluvial Plain Environment (XIII)

Interpretat 1 on

The red mudstone and vertically arranged lime mud

stone nodules are Interpreted as being alluvial floodplain

deposits (Nagtegaal, 1969). The types and distribution of

sedimentary structures, vertical variation of grain size

within the lenticular sandstone and conglomerate section,

mineralogy, and the association of this unit with red mud

stones Indicate that 1 t represents alluvial channel and

1nter-channel deposits (Vischer, 1965).

The nodules and calcite cemented sands are caliche

paleosols formed on the floodplain and at the tops of

fille d , abandoned channels during slow rates of deposition

(Ruhle, 1967; Nagtegaal, 1969). Cal1ch1f1ed alluvial sedi

ments are common throughout the arid to semlarld south

western United States (for example, see Reeves, 1970; G11e

and Hawley, 1966). The nodular and often pipelike charac

ter of caliches formed 1n fine grained sediments (Reeves,

1970) Is a diagnostic criterion used for the recognition

of caliche paleosol horizons. Nagegaal (1969) finds simi

lar nodular caliche zones 1n red mudstones from the Permian

of Spain, and Castle (1969) reports nodular caliches from

Texas Trinity Division rocks to the northwest of the pres

ent study area. 47

Pa 1eoc11matology

Evidence concerning the climate which existed dur ing Middle Trinity time is obtained from the presence of paleocal1ches , and the clay mineralogy of Lithofacies XI-

XIII. Further documentation 1s offered by the unweathered nature of the feldspars 1n the Hensel alluvial sands and the existence of evaporltes in the basal superjacent Glen

Rose Limestone.

The existence of a seasonal, hot arid climate 1s suggested by the caliche paleosols which occur at the top of the Cow Creek beach, and within the alluvial plain se quence. Caliches form under a wide range of climates, but are normally absent 1 n continually hot and arid dry re gions because of the lack of sufficient downward percolat ing water (Reeves, 1970). Instead, the favorable environ ment for the formation of caliche Is one In which the

Infiltration rate, plus the temperature range and amount of precipitation throughout the year, are balanced such that leaching by downward percolating waters 1 s followed rapidly by an upward capillary removal of water through evaporation. Arkley (cited 1n Reeves, 1970) states that

"one would expect caliche at even higher rainfalls than

25" 1n hotter climates, and at lower rainfalls, provided the precipitation 1 s concentrated 1n a cool period, fol lowed by a dry, hot summer as 1n Mediterranean climates."

According to Rutte (1968), 1n a study on the eastern coast 48

of Spain* caliche nodules form where the annual precipita

tion 1 s approximately 2 0 ”; in this situation high tempera tures and the concentration of rain 1 n one season are the

factors controlling the caliche development. Thus, a sea

sonal ar1d-semi arid climate seems to be ideal for caliche formation, assuming that sufficient carbonate is available.

The clay mineralogy of Hensel mudstone-cal1ches, and the "marine" caliche at the beach top 1 s also Indica tive of ar 1 d-semiar1d conditions; clay analyses of these units are summarized 1n Table 1. It was found that 1111te

(35.67 wt. %) and smectite (16.39%) are the dominant clay minerals; chlorite makes up between 0% and 48% (average

15.37%) of the clay fraction, and kaollnlte generally con stitutes between 3% and 10% (5.41% average) of a sample.

This clay suite could represent the group of clays which was derived from the Llano source area. However, this hypothesis requires the corollary that no clay altera tions took place on the Hensel Alluvial plain; furthermore, smectite would have to be transformed to other clay types upon its entrance Into the offshore marine environment

(smectite 1s absent from marine Hammett sediments; see

Appendix C). While this latter transformation undoubtedly took place (Grim, 1968), 1t does not seem reasonable to assume that no clay alteration occurred on the Hensel allu vial plain. It 1s more logically concluded, therefore, that the Hensel clays represent a suite of source area 49

TABLE 1

Clay Mineral Analysis of "Marine" Caliche and Hensel Alluvial Plain Sediments <2u fraction

X Mixed Samel e In so 1 . Smec tlte Laye red Kaoli ni te Chlorite Quartz No. Res. 111 1 te

-25* 4.44 8.70 40.32 8.48 20.90 21 , 60 -26* 6.65 3.73 27.66 9. 88 20.02 38. 73 -28* 3.92 18.15 23.85 14.19 23. 54 20. 27 -29* 12.90 13. 36 56. 23 6. 51 11 . 97 1 1 . 93 -17 + 10.84 7. 57 42. 68 8.04 12.32 29. 38 -18 + 20.28 9. 91 37. 98 7.06 14.33 30.71 -19(1 ) + 12.10 0. 00 23. 79 16.85 26. 13 33. 23 -19(2)+ 15.90 0.00 27. 29 11. 72 18.17 42.82 -23(1 )♦ 35.08 11.14 37. 40 0.00 34.75 1 4.06 -23(2)0 24.98 19. 07 42. 47 0. 00 24.95 1 3. 52 0-1 + 77.35 12. 39 28. 09 0. 00 48.30 1 1 . 22 7-57* 6 .80 0.00 73. 56 1. 20 5.74 1 9. 39 7-58* 9.54 0. 00 70. 30 0.00 10. 36 1 9. 34 7-59* 27.54 15.41 34.96 2 . 62 11 .24 35. 77 7-60* 14.12 27 .24 22. 09 3. 94 0.00 46. 79 7-61* 3.20 17.81 31 . 27 2. 43 8.06 40. 45 7-62* 1 .00 18.49 41 .86 2 .23 a. 30 29. 12 7-63* 5.80 12.79 28. 37 5. 69 10.73 42. 42 7-64* 9.00 15.41 35. 27 3.03 5.71 40. 55 7-65* 9.20 17.15 27. 90 6.48 14. 77 33. 70 7-66* 10.34 16.54 25.37 2.77 6. 30 49. 00 7-67 + 90.38 12.96 33.43 7.49 14.81 31 . 32 7-69o 23.28 19.62 44.10 4. 66 1 3.81 1 7, 81 7-70 + 46.18 26.33 33. 31 0.00 11.55 28.83 7-71 + 15.44 33.36 35. 78 1 . 68 1 2. 69 10. 64 7-720 59 .98 21 . 67 49. 23 0.00 9. 97 19.13 7-730 ---- 23. 26 34. 78 2.71 9 . 06 30. 20 8-77 + 99.00 30.4 7 13.13 10 . 88 21 . 33 24. 19 8-78o 41 .92 39.48 0. 00 14.11 26.18 20. 33 8-80(1 ) + 85. 38 27 .20 42.11 5. 70 9.89 15.10 8-80(2 )♦ 80.85 28.80 41 . 29 7.26 10.71 10. 36 lean 16.39 35.67 5.41 IS. 37 26.84 itandard 10.17 14.41 4.61 9.64 1 1 . 53 Dev 1atlon

* "Marine" caliche o Alluvial plain caliche + Alluvial plain mudstone 50

clays that essentially attained equilibrium with the chemi cal conditions that existed 1 n their environment of deposi tion.

According to Alexander and others (1939), Knox (in

Grim, 1968), and Winters and Simonson (1951), this suite

( i 1 1 1 te-smectite) characterizes soils formed under hot, arid conditions. Parry and Reeves (1968) and Arlstarain

(1971) report that soils formed on the southern High Plains of Texas and eastern New Mexico are rich in 1111te and smectite. Thus, even though there may have existed a wet, cooler season, the conditions which prevailed throughout most of the year, and during deposition of the Hensel Sand, must have approached a desert type climate.

Two other facts lend further substantiation to the existence of a hot, dry climate. The firs t of these 1s the overall freshness of potassium feldspars in alluvial channel fills . Texturally Immature arkoslc sediments con taining unweathered feldspars are indicative of rapid de position In hot arid, or cold climates (Folk, 1968).

Secondly, Forgotson (1956) reports anhydrite at the base of the overlying Glen Rose Limestone some distance to the north. This deposit 1s a sure Indication of a dry climate, as evaporltes only form, and are preserved, 1 n areas of low rainfall (Kinsman, 1969). 51

Summary and Pa 1eogeoqraphlc Reconstruction

The petrologic properties and environmental Inter pretations of Middle Trinity Lithofacles in the study area are summarized 1n Table 2; 1n this chart the prime charac teristics used in defining each fades are capitalized.

The reconstructed pa 1eogeography and lateral facies rela tionships are represented in Figure 5.

The early pa 1eogeographlc setting in which Middle

Trinity deposition was Initiated was an active proximal source area flanked to the east by an upper Sycamore allu vial pia1n-terrace environment (Lithofacles la, b).

Strandline deposits, consisting of a well defined beach unit (Llthofacies II) and an intertidal-subtldal fosslHf- erous sandstone (Lithofacles III) which overlie a bored dolomite pebble zone, mark the transition from subaerlally deposited Sycamore Conglomerate to superjacent marine

Hammett Shale. These units, along with the claystone- mustone (IV) and lowermost carbonates (V), were deposited in low-energy offshore environments during the regional westward marine transgression onto the Llano Uplift. This transgression took place during regional subsidence and denudation of the Llano source, as 1s evidenced by the de crease of insoluble residue content upward through carbo nate Facies V, and the absence of a repeated clastic

Facies IV in the overlying regressive sequence. COM O K I Lt*«ST0ftl MNNITT SMK | S'UROftE CQ«*.0*fMT£ I I I V II V 11( VI ]v v r-- _ lal**'L L ia 1 * t- it * r C 1 t i 1 0 i Z CLATITOHC- “■ M(fSTORE H | CMS f *ft o R n « l a t RIDILI I1 l l i l i R001 llltIFC FO tftft# l H | l n e ifte F RRERItt * RiOitv Tlt - lttR tT L L U J I M T I I r I a X o r E t i C H «*t«* p 1 n t«r*ftl«t*M t a o T i d u tTt tM O O 0TSMR t f t R D TI | H I - 001- UM HC llOOfO THICK RUOSTORI ORI I IT R tO M CftftflftRftrlI* 0*001* b m OORRTZ u i i 001 s o f _____ Fftdot Summary of Chart

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53 ‘j4

The upper part of the interbedded limestone-

dolomite Facies (V), and all facies except the uppermost

Hensel Sandstone, were laid down during a regional east

ward transgression. The regression was effected by re

newed uplift in the Llano area (resulting in a new supply

of coarse terrigenous detritus) accompanied by a slight

downwarplng of the shelf. The regressive sequence did not,

however, develop due to a lowering of sea level, but

rather due to sedimentary facies progradation under nearly

static sealevel conditions. Shoreward from the relatively current-free bottom

on which Facies V was deposited was an area of prolific

oyster growth (Facies VI). Landward toward the shallower

environments, shoaling areas composed of carbonate sand

and mud (Facies VII) developed and acted as an effective

barrier to the seaward transport of fine terrigenous sand.

Behind these shoals, and 1n front of the regressive lime

gralnstone beach complex of festoon crossbeds (IX) and

accretion beds (X), existed a restricted lagoon in which

foss1 1 1 ferous quartz arenltes and pellet 1 1me packstones were deposited. "Marine" fung-algal caliches (XI) and

supratldal flats (XII) occasionally developed in the beach

backshore environment. During the gradual eastward facies

progradation, older marine units were covered by the Hen

sel red mudstone and alluvial channel sequence (X III).

After the conversion of approximately thirty miles 55

of marine shelf to alluvial plain, the source area again became denleted and as subsidence continued, marine trans gression over the Hensel alluvial surface began. Red mud stone grades upward into Intercalated Lower Glen Rose fos- siliferous sandstone and arenaceous dolomite (Facies XIV) which appear to represent Intertidal and supratidal de posits. With this, the deposltional history of the Hammett

Shale, Cow Creek Limestone, and Hensel Sandstone was con cluded.

This interpretation of Middle Trinity depositional history discounts the need for a lowering of sea level at the end of Sycamore time to explain the sporadic caliche paleosols (Lithofacles lb), and presumed unconformity at the top of the Sycamore (Lozo and Stricklin, 1956; Strick lin and others, 1971). This soil more likely formed as the result of a slackening 1 n the rate of detrital Influx during the early Hammett Shale transgress!ve phase. The explanation also eliminates the significant stratigraphlc hiatus previously inferred to be present at the top of the

Cow Creek (Lozo and Stricklin, 1956; Stricklin and Smith,

1968). Again, the caliche paleosol that developed here represents localized areas in which weathering proceeded more rapidly than deposition. The present author believes that the presence of paleosols within a stratigraphlc se quence 1s not sufficient evidence for hypothesizing regionally "significant unconformities." Indeed, the only 56

"significant unconformities" in the Middle Trinity are the fossi1iferous sandstone zones at the top of the Sycamore and Hensel which mark the transition from regressive allu vial to transgress!ve marine intertidal sedimentation.

Reconstruction of the depositional environments and paleogeography of the rocks studied in this area is not considered an end in itself. Rather, this reconstruction provides an environmental basis for interpreting the syn- genetic and early diagenetlc modifications of the Cow

Creek-Hammett Shale sequence 1 n terms of the diagenetlc controls active In subsea carbonate environments, and the early pa 1eogeohydro 1 ogy of the region. CARBONATE DIAGENESIS

General

Glnsburg (1957) pointed out that early diagenesis

(processes acting at or slightly below the sed 1ment-water

Interface [Emery and Rlttenberg, 1952]) "may not obscure the sedimentary environment, and it may leave a record of the conditions of deposition, which, in some cases, 1 s just as clear as clues presented by purely deposltlonal fea tures . "

As Purdy so aptly Indicates (1968), the carbonate diagenetlc researcher should be concerned more " 1 n differ entiating subsea, subaerial, and deep subsurface diagenetlc processes and effects" rather than 1 n placing an early or late time connotation on these processes, as these terms carry different meanings to different individuals. Purdy

(1968) states that "the type of alteration a given lithology experiences 1s determined by Its post-deposit 1 onal environ ment, while the degree of alteration 1 t experiences is de termined by its residence time 1n that environment." The diagenetlc modifications and fabrics delineated 1n the Cow

Creek Limestone and Hammett Shale are all considered 1n terms of diagenetlc or deposltlonal environments.

The code and terminology proposed by Folk (1965) for

57 58

describing diagenetlc carbonates 1s used for most of the carbonate grain and matr1x-cement dlagenetic fabrics. The detailed code for diagenetlc calclte from Folk is pre sented 1n Appendix D 6 . An explanatory resume of pertinent dlagenetic terminology used throughout the text is pre sented below.

Neomorphism, neomorphic calcite (Folk, 1965)— "a collective term for both Inversion and recrystal 1 1 zation, or where the exact process [of transformation or original mineralogy] is not known." Neomorphic calclte is the cal cite formed through neomorphism.

Inversion, Inverted (Folk, 1965)--the process by which aragonite 1 s transformed into calclte 1 n the solid state, but 1n the presence of fluids.

Recrystal 11zation (Folk, 1 965)--the change 1n the size, shape, or orientation of crystals without an atten dant change 1 n their mineralogy. M1cr1te, microspar, pseudospar (Folk, 1962, 1965)-- neomorphlc calclte size terms: micrlte, microspar,

5m-30m; pseudospar, >30y. If mineralogy is dolomite, each

1 s prefixed with dolo-. Spar (Folk, 1962)--clear, precipitated calclte cement with equant, bladed, or fibrous morphology.

Moldic void and moldic cavity f111--a void formed by dissolution, and occluding calclte in the void, respec tively. 59

Microcrystal11ne (mlcrlte) grain envelope (Bathurst,

1964)--the peripheral replacement of shell material by a fine-grained mosaic of carbonate crystals.

The following aspects of carbonate diagenesis, and the distribution of resultant diagenetlc modifications, were studied in the analysis of Hammett-Cow Creek lime stones and dolomites:

Grain Diagenesis M1crocrystalline Grain Envelopes (mineralogy and morpho1ogy) Grain M1cr1t1zatlon Grain Do 1om111za11 on Grain Invers 1on/Recrysta111za11on-Gra1n Dissolution Matrix Diagenesis Mud Matrix Stabilization Mineralogy (calclte or dolomite) Size (mlcrlte, microspar, pseudospar) Cement Diagenesis Mi neralogy Morpho1ogy Size and Substrate Distribution of Ferroan and Iron-Free Calcltes

Statistical Analysis *

The point count frequency of each grain type, grain diagenetlc modification, and matrlx-cement textures plus the derived ratios, from 167 thin sections of the Cow

Creek Limestone were analyzed according to a least squares analysis of variance 1 n a completely randomized design.

Samples of Hammett Shale carbonates and the Cow Creek 60

"marine" fung-algal caliche were not amenable to the point count method used and thus were not Included 1 n this statistical analysls .

Each inferred environment of the four Cow Creek

Limestone facies, combined with its representation at 15 outcrops, was designated as a treatment; the lagoonal en vironment (facies) VIII was split into two parts because the sediments originated through different deposltlonal processes which operated within the same environment. Thus, the five environments (facies) that were analyzed are:

Shoal (VII); Lagoonal Nodules (V illa ), Other Lagoonal Units

(Vlllb), Beach Upper Offshore (IX), and Beach Offshore (X).

Least squares means and F-values for each variable

1 n each outcrop, environment (fad es ), and outcrop by en vironment (facies) Interaction were calculated 1 n this design. The least squares means (175 points counted per thin section) for the environment (facies) analysis are presented 1n the text; significance at the .95 and .99 levels of confidence are Indicated by (*) and (**), respectively, for each dlagenetic feature displaying a significant variation among fades. The analysis of vari ance tables, and least squares means for the outcrop and outcrop by environment (fades) analyses, are given 1 n

Appendix 8 along with a brief discussion of the dlagenetic modifications which show significant variation 1 n the out crop, or outcrop by environment (fades) analysis. 61

The probability model that this statistical analysis follows is:

y = U + 0 + F + 0*F + e where,

y = the frequency of occurrence of a given variable

u = the overall mean

0 = the effect of the Individual outcrop as a devia tion, NID (O.ctq), from the overall mean

F = the effect of the Individual facies as a deviation from the overall mean, NID (0,Op)

OxF = the effect of the facies at a given outcrop as a deviation from the overall mean, NID (0 ,cJqxF)

e = error term

Hammett Shale Diagenesis m

Clay Mineralogy, Diagenesis, Dolomite Injection

Descri ption

Some differences between Fades II limestone and dolomite are the direct result of slight variations be tween their environments of deposition (p. 2 0 ) but others, discussed here, are of dlagenetic origin. Limestones in the Hammett are 11me packstone composed of recrysta111 zed lime mud matrix and angular, unsorted bioclasts (F1g. 13 A).

Constituent grains are mostly oysters and other mollusks, but a few echlnold plates and serpulld worm tubes are also present. Most mollusks other than oysters were originally Figure 13. Hammett Shale limestone and dolomite.

A. Recrystallized mollusk-oyster Hme packstone; bioclasts are angular and unsorted; minor fine qtz. sand.

B. Loafy microspar (NE2 ) grading to pseudospar; recrysta 111 zed mollusk fragment on top and right.

C. Fibrous and bladed microspar (NBO-NFO) continuity with oyster fragments; mlcrospar-pseudospar matrix.

C. Contact between dolom1crospar (dark) 1n contact with pseudospar (light); contact Is gradational on left side and sharp on right side.

E. Fractured oyster fragment separating clayey dolomlcrospar (top and lower left) from fractured mlcrltlc limestone; fractures below oyster fragment are filled with calclte spar.

F. Fractured mlcrlte lense between Injected dolomite; fractures are filled with dolomlcrospar.

G-H. Compactlonal flow aligned bioclasts In clayey, silty dolomite; bioclasts aligned parallel to contact with limestone (top of G, bottom of H). 62

Figure 13. 63

composed of aragonite and have undergone Inversion and re crystal 11zation. Oyster shells rarely show signs of m1cr1- tization and micr1 te-enveloped grains, although present, are not as common as in the overlying Cow Creek units.

The matrix of these lime packstones is iron-free and ranges from fine {5-16u) through coarse microspar

(NEg), which sometimes grades Into pseudospar (>30p;

NE3 _4 ) (Fig. 13 B); pseudopelleted fabrics in the micro spar are common. The Individual crystals are most commonly equant in shape and have serrate edges, but frequently the curved edged loafy forms described by Folk (1965) make up the entire matrix. Sometimes radially arranged blades or fibers of microspar, 1 n optical continuity with oyster fragment ultrastructure (NB 2 _3 0 -NF2 _3 0 ; Fig. 13 C) or fringing quartz grains (Chanda, 1963, 1967; Folk, 1959), give the Impression of a first generation bladed spar cement. The neomorphic growth of microspar has pushed the included terrigenous clays Into 1 ntercrystal 1 1 ne positions.

An excellent discussion on the origin of microspar Is given 1n Folk (1965).

The junction between a lime packstone and a dolo mite 1 s either a sharp contact or a zone 1 n which micro spar 1 s gradually replaced by finer dolom 1crospar

(Fig. 13 D). Concave mollusk valves at the contact of a limestone and dolomite are generally fractured and fla t tened, and the limestone as a whole may be Intensely 64

fractured (F1g. 13 E, F).

Hammett Shale dolomites are dissimilar to the lime

stones in that they are wackestones and never packstones; angular and unsorted oyster and echinold fragments make up

the bioclastic debris (F1g. 13 G), but recrystal1ized mol-

lusk fragments, which are common 1 n the limestone, are almost always absent. The possibility exists that no

original aragonltlc mollusks were present In the dolomit-

1zed sediments, but 1 t seems more plausible that they are absent as a result of having been obliterated during dolomltlzatlon. The allochems are typically iron-free, and are often replaced at their margins by a matrix consisting of subhedral and euhedral ferroan-dolomlte crystals 2 0 p to

60y in diameter. In contrast to the limestone, grains 1n the dolomite show compactlonal flow alignment, particularly near the contacts between a dolomite and limestone

(Fig. 13 G, H), breakage (Fig. 14 A), and pressure solu tion (Fig. 14 B) .

In general, the more dolomite a sample contains, the greater the percentage of insoluble residue (F1g. IS); the reason for this difference in Insoluble residue con tent has already been presented (p. 22). Although no size analyses were done on these residues, only minor detrltal silt and very fine sand was noted 1 n thin sections; It 1s assumed, therefore, that most of the detritus falls within the fine s ilt and clay size grades. The clay analyses of Figure 14. Hammett Shale dolomite; m lcrlte envelopes.

A. Grain breakage 1n Injected, csely. crystalline clayey, silty dolomite wackestone.

B. Interpenetrating oyster fragments (middle and bottom) showing mlcrostylol1 tic pressure solution contact.

C. Very thin mlcrlte envelopes rimming bioclasts in mollusk packstone of shoal facies. D-E. Thick mlcrlte envelopes on large oyster (D) and moldic cavity Infilling (E); distinct borings 10u-15y in diameter penetrate the grains.

F. Encrusting type microcrystalline grain envelope; note sharp contact of this envelope with grain and its extension outside the grain boundary. Two small grains are encased 1n this envelope type in upper left center.

G. Microcrystal 11ne carbonate crust overlapping first generation spar cement rinds.

H. Irregular crusts enclosing grains and occluding pore space; crust development gives rise to rounded, sinuous pores. 65 % InsotoabL Rm h I im 0 6 7 D- ‘ * ------— -— — — Figure is. Insoluble residue content content residue is. Insoluble Figure ------J i III' I i — i— 1 i ------ooie n oul Fraction Soluble in % Dolomite ' ------r o f f o Hamnett Shale limestone and dolomite. and limestone Shale Hamnett 1.7 22.1)(x - ♦ * ♦ 100 67

these carbonates and underlying shale are presented and discussed in Appendix C.

Pi scussi on

It is well known that fine grained carbonate muds* rather than sands, are selectively dolomitized (Murray,

1960; Murray and Lucia, 1967; Lucia, 1962), probably be cause they are more reactive due to fine grain size and large surface area to volume ratio. Because no evidence exists that suggests there was any great difference in grain size 3nd carbonate mineralogy between the insoluble rich and insoluble poor muds at the onset of dolomitization, some other factor must have controlled the selective nature of the replacement. It is suggested here that the amount of clay-sized detritus contained in the original

Hammett Shale carbonate muds was instrumental in determin ing which sediments would later be preferentially replaced by dolomite. Only a few studies have been done on clay- dolomite relationships; Murray (1960) and Fisher (1968) note that percent dolomite is directly proportional to the insoluble residue content, as it does in this study; other studies (Zenger, 1965; Goldich and Parmalee, 1947) show either an inverse relationship or none at all.

Lime muds with a relatively high clay content do not consolidate as rapidly as do those containing less clay (Chillingar and others, 1967). As a result of 68

preferential consolidation, the Hammett Shale clay-rich carbonate muds might have retained higher porosities and permeabilities longer than the muds containing less clay.

Because of this porosity-permeabi1ity contrast between the two sediment types, dolomitizing fluids were allowed more rapid access into, and consequently achieved more efficient replacement of the clay-rich lime muds. Shinn (1968) has shown that slight differences in the porosity and permea b ility of supratidal sediments control to a large degree which sediments will be dolomitized.

Clay minerals might also have played an active role in the do 1omitization process. Clays may enter directly into the chemical reactions resulting in dolomitization, act as ion exchange centers, or serve as catalysts for nucleation and growth of dolomite crystals (Kahle, 1965).

However, the ambiguous nature of the clay-ca1cite-dolomite relationships (see Appendix C) leaves the unanswered ques tion of whether the clays entered into the dolomite chemi cal reactions at all, and if they did, whether chlorite was transformed into i 1 1 ite or vice versa during the re placement process.

Fine grained detrital dolomite was also present in the sediments. Because sand-sized detrital dolomite is sorted with quartz grains and bioclasts in the beach, lagoon, and shoal units, it is logical to assume that fine silt and possible clay-sized detrital dolomite was 69

transported into the quiet offshore environments where it was deposited along with other fine detritus. Detrital dolomite clasts can act as nucleation centers for diage-

netic dolomite; with seed crystals, the kinetics necessary

to start the reaction are lowered and do 1omiti2ation takes place under conditions which normally would not allow dolo mite growth (Lindholm, 1969).

Folk (1965, 1969) has suggested that because micro spar typically occurs in clayey limestones, or in lime stones associated with shale beds, the clay content plays a key role in the formation of microspar, either because of the clay type present, or because of its control on cer tain unknown chemical reactions in the diagenetic process.

This neomorphism takes place shortly after deposition in the presence of subsaline waters because it is most common in marine or brackish water limestones, and rare in fresh water limestones (Folk, 1965). Neomorphism (recrystalliza tion to microspar and pseudospar) of Hammett limestone probably took place nearly contemporaneously with dolomitization of the associated units (the fact that dolomites contain iron and the limestones normally do not is probably the result of the rapid 1 ithification of limestone matrix before reduced, iron-bearing dolomitizing waters entered the sed iments).

As the overlying sediments accumulated on the Ham mett carbonates the semi-fluid clay-rich lime muds were 70

squeezed into, and mixed with, the consolidated clay-poor lime muds (as indicated by grain breakage and flow struc tures in the dolomite and fractures in the limestone

[Figs. 13 E-H; 14 A, B]). Whether do!omitization was initiated during or slightly after this injection, while a contrast in the degree of consolidation still existed, is unknown. It is definite, however, that replacement was early, because dolomitized intraclasts from this facies are found reworked into the overlying beach sediments.

Inasmuch as dolomitization took place during early diagenesis and just after burial beneath the overlying beach sequence, the writer believes that this replacement and limestone recrystallization occurred when the sedi ments were infiltrated by mixed fresh and saline waters.

This mixing might have taken place in the zone of diffusion which separates the fresh water lense from saline inter stitial waters of marine origin (Hanshaw and others, 1971).

Details on the chemical nature of the waters effecting dolomitization and limestone recrystallization will be pre sented in the construction of the regional diagenetic model for the Hammett-Cow Creek sequence. 71

Cow Creek Limestone Diagenesisjy i i n — — ------—

Grain Diagenesis

Micrite Envelopes

Genera 1

Grains in the Cow Creek Limestone may be rimmed with microcrystalline carbonate in two distinct manners.

In the firs t type, the original shell material is replaced

by micrite or ferroan-dolomlte; in the second type, micrite or ferroan dolomicrite encrust the grains.

The inner boundary of firs t rim type with the in verted shell material or moldic cavity f i ll is typically gradational, but may also be quite sharp. The envelope is either homogeneous or is composed of distinct, microcrys talline carbonate filled tubes throughout its entire thick ness, or at the margin between the rim and the shell mate

rial (Fig. 14 C-E). Rim thickness may be very thin (10-20 microns) or almost completely replace an entire sand-sized g ra i n .

Envelopes of this nature have been reported from

Recent carbonate environments by many workers (Ginsburg,

1957; Bathurst, 1964, 1966; Swinchatt, 1969; Winland, 1968;

Kendall and Skipwith, 1969; Gelubic, 1969), who ascribe its origin to the Infilling of perforate blue-green algae f i l a ments. Diameters of the individual algal borings 72

(filaments) range from 4 to 12 microns in diameter whereas the lengths of individual filaments ranges upwards to 50 microns. Boring sizes in grains from the Cow Creek Lime stone fall within this range, except for those associated with the second type of micrite envelope.

Even though the fabric produced by these boring a l gae is constant, the type of microcrystalline carbonate plugging the perforations is not. Bathurst's (1966) analy sis indicated that these infillings were composed of ara gonite, as did the study by Kendall and Skipwith (1969).

Winland (1968), however, reports high Mg calcite micrite envelopes. Friedman (1964) suggested an originally arago- n i ti c compos ition for micrite enveloping grains in Pleisto cene limestones. Microcrystalline aragonite or high Mg calcite envelopes invert rapidly to low Mg calcite, rather than dissolving, upon exposure to fresh water (Purdy, 1968); they do not undergo dissolution, and are preserved around the molds of leached grains, either because of their iso tropic nature (Purdy, 1968), or because of rapid nucleation and high, fluid supersaturation conditions within the rim

(Winland, 1968). Moore (personal communication, 1971) sug gests that only high Mg calcite rims are preserved, and that aragonite rims are probably lost through dissolution and are not recorded in ancient carbonates.

In the second type of "evelope," iron-free micrite or ferroan dolomicrite distinctly coats the grains 73

(Fig. 14 F). Ward (1970) has also reported these types of envelopes from Pleistocene eolianites in Yucatan. Uncom monly, but significantly, the crust will overlap a first generation (bladed) cement (Fig. 14 G). The contact of a crust with a grain is always sharp and its outer boundary with cement or matrix is typically abrupt, but it can be highly irregular and does not necessarily reflect the shape of the grain or grains, upon which it is developed

(Fig. 14 H). Commonly, branched borings which range from

15 to over 150 microns in diameter (Fig. 16 A, B) extend from the crust into the grain; borings merge with crusts and they appear to be one and the same, both mineralogi- cally and texturally (Fig. 16 C, D). Borings without at tendant crusts are rarely filled with a green to iron oxide stained mineral, possibly a phosphate (very low to no bi refringence), but whose identity is uncertain. In a few instances, a dolomitized or iron-stained plexus of calcite filled tubules (or cement separated filaments) is developed in voids adjacent to encrusted and bored grains (Fig. 16 E).

These borings and crusts are interpreted as the hyphae and fruiting bodies of endolithic fungi on the basis of the extremely large diameters, branching nature, and morphologies of the borings, which bear striking resemb lance to photomicrographs published by Kohlmeyer (1969), and the encrusting nature of the grain envelopes. Within certain species of marine blue-green algae, and apparently Figure 16. Mlcrlte envelopes and m1cr1t1zed grains.

A-B. Large, branched (up to 1 50p 1n d1a.) fungal borings in oyster fragments; borings are filled with dolomlcrlte.

C-D. Encrusting dolomlcrtte envelopes with extending, branched grain borings; borings cannot be separated from crusts on a textural or mineralogical basis. Crust 1n C binds grains (upper) and overlaps first generation bladed cement on grain underside.

E. Void plexus; calclte filled "cells" are separated from each other by dolom 1cr1te ; plexus represents either calclte filled tubules or loosely packed filaments separated by calclte cement.

F-G. M1cr1tized oyster fragments; shell structure 1s s till apparent In F, but very d ifficult to discern or completely obliterated 1n G.

H. Recrysta11ized-1nverted grain displaying relict shell structure as discontinuous microcrystal1 1 ne lines; grain was probably m1cr1 t 1zed firs t and then recrys ta 111 zed. *91 3an6xd

VZ 75 also fungi, thalli and hyphae, respectively, specialized

into parts which bore Into a substrate and parts which en

crust substrates (Kohlmeyer, 1969).

When crusts become extremely well developed they

bridge intergranular pores and bind grains. These features

can be considered "quasi-crust" or true microcrystal 1 ine

cements, but because they are a typical associate of the marine caliches, they are best discussed in that section.

Distribution of Grains with Micrite or Dolomlcrite Envelopes

High F values indicate significant interval varia

tion for micrite envelopes, dolomlcrite envelopes, and

(micrite envelopes + dolomlcrite envelopes)/Total allo-

chems (Appendix B; Table 3).

Micrite enveloped grains are not appreciably abun

dant in any facies, but they are notably sparse 1n the

nodules and other lagoonal beds. The F-value calculated

for the distribution of dolomltized envelopes is large

(Appendix B) and the lowest and highest frequencies of

dolomlcrite enveloped grains occur in the lagoonal units

and foreshore facies, respectively.

To eliminate the Influence of allochem frequency as

a control Imposed on the number of mlcrlte and dolomlcrite

envelopes recorded for each facies, values were calculated

for micrite envelopes/a 11ochems, dolomlcrite envelopes/ allochems, and (mlcrlte + dolomlcrite envelopes)/a 11ochems.

These three ratios Indicate that envelopes are relatively 76

Table 3. Distribution of Grains with Mlcrlte or Dolomlcrite Envelopes

Laaoon Upper Fore Facies Shoal Other offs hore shore Nodules Un1 ts ^ -^ ^ * V a r 1 abl e

Mlcrlte Rims (**) 8 . 52 2.61 2.99 7.55 7. 54

Dolomlcrite R1msl* ) 9.62 5.76 4.81 6.09 13.21

Mlcrlte R1ms .16 .04 .13 .13 . 1 5 Total Allochems

Dolomlcrite R1ms .23 .12 .13 .13 .24 Total Allochems

Mlcrlte R1ms + Dolo- mlcrlte Rims f**) .38 .16 .20 .26 .40 Total Allochems 77 least numerous in the nodules and other lagoonal beds, whereas they are plentiful in both the shoal and foreshore beds. Dolomlcrite envelopes are more abundant than micrite envelopes in all facies.

Interpretation of Grain Envelope Distribution

Irrespective of the present-day mineralogy of the filled borings, the distribution of enveloped grains sug gests that the perforating algae and fungi were least ef fective 1n the lagoonal environment; rather, they preferred the beach foreshore or shoal environments. A prime factor to consider 1n the development of grain envelopes is the

Intensity of light necessary for algal growth (Swlnchatt,

1969), but 1t 1s highly unlikely that the depth or the water turbidity of the lagoon would be such that 1t would blot out most available sunlight above the sediment sur face. Work on Recent sediments (Swlnchatt, 1969; Bathurst,

1966; Kendall and Sklpwlth, 1969) shows that grain enve

lopes develop in a diversity of shallow marine carbonate env 1ronments.

Possible controls to consider are the stability of

grains within an environment, the relative rates of depo

sition among environments, and the amount of 11me mud matrix 1n the sediment. If grains are constantly being moved about, abrasion would continually remove the bored

peripheries of the shell and no envelope would develop.

Moreover, if deposltlonal rates are too rapid, as 1s the 78

case for thin, storm deposited units, Included grains

would not be exposed at the sediment Interface for long

enough periods of time necessary for algae to extensively

bore the grains. Even 1n the light of these firs t two

restraints, algae (or fungi) could still conceivably mi

grate Into the sediment and bore grains if the pore space

environment was not Inimical to their existence. A number

of factors could therefore regulate grain envelope develop ment and preservation.

The fact that envelopes are most prolific 1n the

beach foreshore sediments Indicates to the author that

grain stabilization and absence o' matrix are the two most

Important controls. The greatest amount of wave energy

was exerted on the beach foreshore and depositlonal rates

varied from slow to rapid; grains were continually abraded

and moved about until they were tossed onto a berm. Here

they became stabilized and remained near the sediment sur

face for a period of time sufficiently long enough for algae and/or fungi to filte r through the matrix-free sedi ment pores and bore, or in some cases bind, the constitu

ent grains. Bored grains are less frequently encountered

1n the upper offshore sediments because grains were con

tinuously abraded and transported before becoming burled

(Moore, personal communication, 1971, finds the same enve lope distribution 1n Recent Grand Cayman beaches). The lagoonal sediments possibly supported only a small boring 79

algae population because of their* high mud content and

their Intermittently rapid deposition. Additionally, even

though the shoal sediments contain appreciable quantities

of mud, lime gralnstones and sparry lime packstones are

commonly present. Perhaps the reason for the great abun

dance of envelopes within this facies is the result of

slow depositlonal rates which allowed grains to remain

near the sedlment-water Interface long enough to become

perforated by algae.

The distribution of the different mlneralogic types

of envelopes closely follows the pattern of dolomltlzatlon within the Cow Creek Limestone. Encrusting dolomlcrlte

rims are most abundant 1n the foreshore Interval and are

normally associated with the "marine 11 caliche.

The hypothesis offered for the dolomltlzatlon of

grain crusts and envelopes In the "marine" fung-algal caliche (p. 124) could be evoked to explain the large num

ber of ferroan dojomlcrite envelopes 1n the shoal unit.

Of significance, however, 1s the observation that the matrix of the shoal facies contains abundant ferroan dolo mite (Table 6 ). It Is suggested that grain envelopes, along with the mud matrix of the shoal packstones, were

probably selectively dolomltlzed Irrespective of original mineralogy because they were composed of fine crystals and were therefore highly susceptible to dolom1t1zatlon (Moore,

1960; Moore and Lucia, 1967). Alternatively, they may 80

have been originally composed of high Mg calclte and later converted to dolomite (Friedman and Buchblnder, 1970; Land,

1 970).

Grain M1cr1t1zation

General

Grain mlcr1t1zat1on 1s a destructive diagenetlc process whereby the ultrastructure of an allochem 1s a l tered to a homogeneous microcrystall1ne fabric (Fig. 16 F,

G). This degeneration of Internal structure has been blamed on bacteria or algae fillin g , 1954), decomposition of boring algae (Newell and others, 1960) and Infilling with microcrystalline aragonite of boring algal tubules

(Bathurst, 1966; Kendall and Sk1pw1th, 1969). Purdy

(1968), 1n his comprehensive analysis of this alteration, concludes that 1t 1s caused by the decomposition of grain organic matter, whether distributed as organic matrix be tween component crystals or In peripheral algal borings.

The breakdown of organic compounds within the particle may set up a micro-environment 1n which the original carbonate is dissolved and microcrystal11 ne carbonate 1s precipi tated 1n Its place. Alteration occurs while the grains lie at or slightly below the sedlment-water Interface.

Work done by Purdy's students (cited In Purdy,

1968) indicates that a change In grain mineralogy does not take place during recrystal 1 1 zat1on to m1cr1te-s 1zed 81

carbonate. This may well be true for grains homogenized through the decomposition of parent organic matter, but 1t does not seem to hold for allochems mlcritlzed by boring algae or fungi (Winland, 1968; also see Grain Envelopes section and “Mari ne 11 Cal i che) .

Certain shell types are more easily micritized than others (Purdy, 1968). Differences In susceptibility may be related to different crystal sizes and crystal fabrics composing the grains, or slight differences 1n the types and decomposition rates of organlcs within the shell mate rial (Purdy, 1968, pp. 191, 196).

Di strlbutlon

The F-values calculated for mlcritlzed grains (N^) and micritized grains {N

Even upon cursory examination of the point counted 82

Table 4. Distribution of Mlcritlzed Grains

Laqoon Facies Upper Fore Shoal Nodules offshore shore Variable

Mlcritlzed Grains (**) 11 .50 12.30 11.98 18.01 16.64

Mlcritlzed Grains f**A Oysters + Unknowns1 ' .54 .44 .50 .62 . 66 83

thin sections 1t 1s apparent that most mlcritlzed grains are oyster fragments. The apparent absence of this early alteration process 1n what were aragonite mollusks 1s at tributable to a combination of three factors. First, in verted mollusks in which the original shell structure can still be Identified as nacreous or cross-1amel 1a r, seem ingly resisted m1cr1tlzatlon. Secondly, later neomorphic recrystal 11zation (N^), during which large calcite crys tals are formed out of smaller aragonite crystals, prob ably resulted 1 n the masking of any previous micrltlzatlon textures except for where neomorphic crystal enlargement was Incomplete (F1g. 16 H). The third factor to consider here 1s that many aragonite mollusk fragments, whether mlcritlzed or not, underwent gross dissolution, thus de stroying any record of a previous dlagenetic modification.

Interpretation

The distribution of recognizable mlcritlzed grains and their high concentration 1n the upper offshore and foreshore lime grainstones 1 s 1n accordance with distribu tional patterns noted 1n Recent carbonate environments.

Purdy (1968) found 1n his investigations of the Bahamian

Platform and coastal carbonate environments of British

Honduras that mlcr1t1zat1on of sand-sized carbonate grains proceeds rapidly on the sea floor, but is hampered 1n car bonate mud environments. He hypothesizes (p. 196) that 64

mud Inhibits bacterial action, either because 1t Is Inhos

pitable (as compared to sand-$1zed deposits) to certain

bacterial types, or because 1t represses the life processes

of bacteria present. Hence, by reduction of bacterial ac

tivity, which 1s thought to be instrumental in the forma

tion of cryptocrystal 1 1 ne carbonate, micr 11 1 zation is

slowed down or stopped 1n low-energy deposits. Moore

(personal communication, 1970) has fourd that more micri

tized grains occur 1n Grand Cayman beaches than in adjacent offshore sediments.

Extensive grain mlcrltlzatlon characteristically ac

companies the development of the "marine" fung-algal caliche and ultimately results 1 n such a complete homoge

nization of fabrics that the bioclasts "disappear" into a micrlte cement background. The formation of the marine caliche, and grain mlcr 1t 1zat1on within 1t, are thought to

be mostly related to microbiological activity at the beach

top and will be covered under the "marine" caliche heading.

Grain Recrystal11zat1on-Gra1n D1ssolut1on/Reprec1 pita11 on

General

Inversion 1s used here 1n the sense of Folk (1965) whereby aragonite 1 s transformed to cald te 1n the solid state (with fluids present) without the formation of an obvious Intervening void stage. Recrystal 11zat1on effects a change 1n size, morphology, or orientation of the 85

previous crystal types and typically accompanies the inver

sion process. Two other methods by which stabilization of marine carbonate grains may be accomplished are replace ment or dissolution with later mold infilling by low Mg

calcite or other carbonates stable outside the marine en

vironment. Recrystal 11zatlon as used in this section car

ries with It the inference that 1t occurred concomittantly» or shortly after, aragonite inversion to calcite.

Resultant crystal shapes, sizes, and boundaries, and crystal fabrics arising from the recrystal11zat1on-

inversion processes show all degrees of gradation among and within Individual mollusk fragments, even though sev eral distinct fabrics can be delineated. Many of these

types are similar to those Illustrated by Wilson (1967).

Easiest to recognize as recrystallized fabrics are those grains which contain yellowish tan, occasionally pleochrolc, remnant shell organic material. This type 1s most commonly composed of large (larger than approximately one- third of the shell width), straight or Irregular sided equant crystals (F1g. 17 A-D) which may coarsen towards the center of the shell. Less frequently shells are made up of pervasive or small peripheral prismatic crystals

(F1g. 17 E); only rarely developed are fabrics of homo geneous small equant crystals. Some fabrics of bioclasts have organic Inclusions on one side, whereas crystals on the other side are clear and the shell Itself gradually 86

disappears Into surrounding cement (Fig. 17 F).

When no Inclusions are preserved, it Is often d if ficult or Impossible to determine whether the bioclast represents a case of recrystal 1 1 zatlon or moldic cavity

infilling. The only identifiable recrystal1ized bioclasts which do not contain organic Inclusions are those: a) In which shell structure is defined by lines of finer crys tals (F1g. 17 G), and b) which are composed of "dirty," small, equant or prismatic crystals with Irregular boun daries and random size distribution (F1g. 17 H). Both of these types may represent Inverted grains which in itia lly underwent micr1t1zat1on in the marine environment. A method used in Identifying other recrystal 11 zed bioclasts is presented 1 n Appendix D3.

Other marine carbonate grains have been stabilized by aragonite dissolution, followed by precipitation of low

Mg calcite 1n the resultant moldic voids. Ideally, these solution cavity 1nf11l 1ngs consist of a bladed crust de veloped on the Inner shell margin, succeeded by straight edged equant crystals towards the center of the shell

(F1g. 18 A, B). Well developed moldic cavity Infilling is readily distinguished from recrystallized fabrics using the guidelines of Bathurst (1958, 1964), but they are more d ifficult to determine when the precipitated crystals con tain inclusions or have curved or irregular boundaries. Figure 17. Recrystal 11 zed-1nverted grains.

A-B. Recrystal 11 zed grains (originally aragonite) with relict shell structure; large straight sided equant crystals except at shell periphery. Shell has yellow-yellowish brown pleochroism due to included organic matter.

C. Recrystallized grain displaying relict shell struc ture; large equant crystals have irregular, rounded edges; grain 1s pleochrolc.

D. Yellowish-brown pleochrolc recrystallized grain made up of stra1ght-1rregular sided large equant and bladed (at grain periphery) crystals. Crystals are very cloudy due to organic Inclusions.

E. Recrystal 11 zed grain (upper half of photo) made up completely of large, prismatic, serrate prismatic crystals which meet along a central line; shell structure visible 1n grain In lower half of photo.

F. Recrystallized, pleochrolc grains (middle and top) which just disappear Into surrounding cement (which 1s probably recrystal 1 1zed Itself).

G. Relict shell structure In recrystall1 zed, originally mlcritlzed grain; homogeneous fabric of small Irregular calcite crystals.

H. Cloudy, loafy equant crystals In recrystallized gra1ns. 87

t mm

i m m i m m

i mm Figure 17. Figure 18. Moldic cavity fills , solution features and matrix types.

A-B. Examples of moldic cavity Infilling displaying Inward gradation from fine blades to coarse equant mosaic (B is xN).

C-D. Tightly packed quartz and oyster fragment residue from beach foreshore Interval at location 13. Grains display breakage and Interpenetration. No origi nally aragonltlc grains are present and cement 1s sparse. Interpreted as a Cretaceous solution feature.

E. Matrix of coarse loafy, equant pseudospar in oyster wackestone of lagoonal sequence; a few quartz grains. This probably was an oyster-mollusk packstone but the aragonltlc mollusks were obliterated during recrystall 1zat1on.

F. Coarsely crystalline dolomite of lagoonal beds.

G. Recrystall1zed grain (left center to lower left) 1n optical continuity with pseudospar matrix.

H. Fine sand size detrltal dolomite 1n dolomitized lagoonal sandstone; note that the rhombs are broken--a sign of their detrltal origin. Figure 18. 89

01strlbut 1 on

Because 1t 1s sometimes Impossible to determine whether a bioclast 1s recrystall1zed or a moldic cavity infillin g, least squares means were calculated for "indetermlnates" in addition to Interpretable recrystal 11 zed- inverted (N ^) and d1ssolved-reprec1p1tated (Ps) mollusk fragments; the Inclusion of the "Indeterminate" category did not affect the calculated recrystal 1 1zed grain and grain d1ssolut1on-reprec 1p1tatlon means 1 n any way that would warrant a different Interpretation of their distri butions.

The proportions "dlssolved-reprec1p1tated grains/ total mollusks" and "d1ssolved-reprec 1p1tated grains +

Indeterminate grains /total nollusks" were calculated 1n order to eliminate the Influence of the number of mollusks counted per thin section as a constraint on the number of d1ssolved-reprec 1p1tated grains recorded for each facies.

A highly significant vertical variation 1s indicated by large F-values for each of the above variables (Table 5;

Append!x B).

Least square means for dlssolved-reprec1p1tated grains (P£) and d1ssolved-reprec1pitated grains + indetermlnates are considerably higher 1n both the shoal and foreshore beds, and are lowest 1n the nodules and nodular beds. The ratio "d 1 ssolved-repreclp 1tated grains {Ps)/ total mollusks" Indicates that the leaching of aragonite Table 5. Distribution of Recrystal 11 zed-Inverted (N^) Grains and Moldic Cavity Filled Grains

Shoal offshore

Moldic cavity filled grains (**) 8.17 1.43 3.86 3.62 9.81

Moldic cavity filled + Indeterm1nate/**v 12.51 3.04 5.37 6.51 13.75 grains ' '

Recrystallized Grains (**) 9.65 9.02 5.91 13.36 11.87

Recrystallized + Indeterminate Gra1ns(**) 14.09 10.60 7.41 16.23 15.91

Moldic cavity filled grains /*** Mollusks ' * .28 .04 .17 .23 .42

Moldic cavity filled grains + /**\ Indeterminate Grains 1 Mol 1usks .50 .17 .31 .35 .58 91

grains became increasingly more efficient from the base of

the lagoonal unit to the top of the beach; however, ara gonite grain dissolution 1n the shoal deposits was compar able to that 1n the beach foreshore unit. Conversely, recrysta 1 1 ized- 1nverted grains (N^) and recrystal 1ized-

inverted grains (N^) + 1ndetermlnates are the most abun dant in the upper offshore and least numerous in the la goonal sequence. Unfortunately the proportion recrystal 11 zed-1nverted grains (N^)/total mollusks was not calculated 1 n the least squares analysis, but a manual cal culation of arithmetic means Indicates that most mollusks were stabilized by recrystal 1 1 zat1on-1nvers 1 on 1n the nodules and upper offshore units. The smallest proportion of mollusks to undergo recrystal 1 1zat1on occurs 1n the foreshore and shoal deposits.

Interpretation

Marine carbonate minerals can undergo stabilization to low Mg calcite by either d1ssolut1 on-reprec1p1tatIon or inversion (most likely through mlcrosolut1on- microdeposlt1on).

Whether grain dissolution and grain recrystalHzatlon-

1nvers 1on takes place depends upon the degree of saturation of dlagenetlc waters with respect to c a ld te and aragonite

(Purdy, 1968). Therefore, by noting the distribution of grains that have undergone either of these stabilization 92

processes, one can gain some Insight Into the nature of early dlagenetlc fluids and their distribution 1n the beach sequence. Reworked clasts containing truncated re crystallized and leached grains are present in the beach beds and Indicate that both stabilization processes af fected the grains early 1 n the dlagenetlc history of the sequence.

One significant divergence from the Increase 1n mold1c-cav1 ty -f 111ed mollusks upward Into the beach 1s the anomalously high amount of leaching which took place 1n the dolomltlzed shoal facies. This large amount of disso lution, however, 1s expected 1f the dolomlt 1zat1on model proposed by Murray (1960) 1s used to explain the do1om1t1- zation of this facies; as outlined by Murray, carbonate must necessarily be added to the system 1n order to accom modate the growth of the denser dolomite and also, to occlude the porosity that existed within the space now taken up by dolomite rhombs. Because of their higher solu bility, aragonltlc bioclasts, rather than calcltlc bio clasts, were dissolved and acted as an Immediate source for the carbonate 1ons needed for effecting dolom 1tizatlon.

To date, 1t 1s the consensus of carbonate geochem ists (Chave, 1962; Land, 1967; Purdy, 1968), that aragonite

Inverts to calcite in solutions nearly saturated with re spect to aragonite under surface temperatures and pres sures. Conversely, aragonite dissolves when it Is 1n 93

contact with water unsaturated with respect to calcite* or water saturated with respect to calcite but undersaturated with respect to aragonite.

Purdy (1968) and Matthews (1968) have presented generalized fresh-water dlagenetlc models from which the location of certain dlagenetlc modifications (solution,

Inversion, and cementation) within the ground-water system can easily be predicted. Both models predict that solu tion of unstable marine carbonates should take place near the fresh water recharge zone. Similarities stop here, however, as Purdy relates Inversion to the low vadose and phreatlc zones and explains cementation as a result of surface evaporation, which results in the upward capillary migration of water, loss of COg, and precipitation. Mat thews relates carbonate alteration processes to the flow rates through the aquifer; local cementation can take place In the vadose zone 1f flow velocities permit, whereas widespread cementation only occurs further down the aqui fer .

The Increased numbers of mo Id 1c-cav1ty-f111ed grains and attendant decrease of recrystallized grains to wards the beach top suggests that Interstitial fluids 1n the beach sediments became more saturated with respect to aragonite downward from the sediment surface. Local hum mocks on the beach top, possibly slightly burled beneath a thin veneer of supratldal mud or alluvium, acted as 94

recharge areas to local fresh ground water lenses. Rain water charged with carbon dioxide fell on these areas and dissolved mollusk fragments as 1t percolated down through the vadose zone; eventually these waters became essen tially saturated with respect to aragonite. At this point* presumably at or near the ground-water table* gross disso lution of grains ceased and the Inversion process pre- domlna ted.

Recrystal 11 zed-1nverted grains (N^J and moldlc- cavity-filled grains (Ps) from beds In and below the lower foreshore-upper offshore zones are composed of ferroan and/or Iron-free calcite whereas grains from superjacent units are almost Invariably made up of iron-free calcite.

Iron distribution may therefore be a clue to the location of the ground-water table (Evamy, 1969) which existed early 1n the dlagenetlc history of the Cow Creek beach sequence.

Even though the scheme presented above generally holds true* the actual chemical and physical situation 1 s more complicated. First, more leaching of grains took place 1n sequences beneath marine caliches (an explanation for this 1s given 1n the "marine" caliche section, p. 124).

Secondly* very sparse data suggest that more grain Inver sion took place 1 n foreshore deposits overlain by supratidal marsh and lake sediments (except where these are also extensively cal 1ch1 f 1ed). More grain Inversion 95

occurred 1n these areas because a vadose zone was non existent below lakes, and was shallow or absent In the salt marshes of the supratidal flat (Butler, 1970). Waters in these areas were heated at the surface and equilibrated with aragonite before discharging through the underlying beach sediments.

Three southern outcrops (1oc. 13, 14, 16 in F1g. 1) show complete leaching of aragonite from the upper portion of the lagoonal sequence and part of the beach foreshore units. The Immediate result of this total removal of ara gonltlc grains was the weakening of structural framework of the units to such an extent that they collapsed. Though no bed distortion or collapse features can be seen on the outcrop, thin sections from these units are composed of a tightly packed oyster fragment residuum. L ittle or no cement separates the grains, and grain breakage and Inter penetration, plus microstylol 1 t 1c contacts are common

(Fig. 18 C, D). The cement which 1s present normally In cludes, or 1s separated by, m1cr1t 1c wisps which probably are the remains of completely crushed mlcrlte envelopes.

It 1s Improbable that the dissolution took place after ex tensive cementation; If abundant cement was present before leaching occurred, 1t would have offered some rigidity to the beds and they would not have collapsed. 96

Intergranular Diagenesis

Matrix Diaqenesis

General

Only a brief Introductory statement concerning the matrix and Its diagenesis is warranted here because the

environmental processes controlling carbonate mud d is tri

bution and various aspects of dolomlt1zatlon have been discussed previously In the sections on the diagenesis of micrlte envelopes, and "marine" caliches.

Marine carbonate muds may originate In four dis

tinct manners: Inorganic phys1ochemical precipitation; microbial activity; abrasion of coarser grained bloclastic debris; and 1 n situ disintegration of certain carbonate secreting algae (Folk, 1965). It 1s d ifficu lt to discern which of these four modes of mud formation is responsible for the formation of Recent deposits of carbonate mud; moreover, with subsequent stabilization of carbonate mud to calcite or dolomite 1t becomes impossible. Further more, the original mineralogies (assemblages) of ancient carbonate muds are lost forever upon diagenesis and llth i- ficatlon.

Bathurst (1958) hypothesized that the 11thtficatlon of unconsolidated muds to hard, crystalline mlcr1te-s 1zed calcite comes about through solution and overgrowth pre cipitation on existent nuclei; dry state grain growth 97

processes, driven by grain surface tension energies, are responsible for the formation of coarser mosaics. Folk

(1965) disagrees with Bathurst's envlsuaHzed grain growth processes and Invokes solid state neomorphism, in the presence of Interstitial fluids, as the process which con verts mlcrlte to coarser grain microspar and pseudospar.

The Interested reader 1s referred to Folk (1965) for a pragmatic discourse on the processes and complexities of the conversion of carbonate mud to mlcrlte, and mlcrlte to microspar and pseudospar. The causes and controls which function 1n the dolomlt 1zat1on of a carbonate mud are well covered by Murray (1960), Murray and Lucia (1967), Weyl

(1960), Lucia (1962), and Llndholm (1969); these are dis cussed 1n other sections of this text.

D1strlbutlon

Table 6 shows that mlcrlte, fine microspar (NEj_2)» and fine dolomlcrospar (RE^) (except 1n the nodules) show a significant Increase 1n abundance from the beach beds continually downward Into the shoal iac 1es; mlcrlte and fine microspar Is rare 1n the beach foreshore unit. Dolo- micrlte (RE<|) 1s most abundant 1n the shoal unit and 1s least frequent 1n the upper offshore beds. Most matrix recrysta 1 1 1 zatlon to microspar and pseudospar (NE3 5 ) took place In the nodular units, whereas the Intercalated la goonal beds suffered the most replacement by coarse ------Upper Foreshore Shoal Variable "0

Mlcrlte (NEj;<4n) and Fine Microspar 30.54 11.76 11.59 4.15 0.86 (NE2; 4-16m)

Coarse Microspar to Fine Pseudospar 11.45 30.67 11.71 6.86 0.08 (N E 3 ; 16-64m) ( }

Coarse Pseudospar (NE^_g; >64y) (**) 1.87 12.67 3.31 2.35 0.00

Oolomlcrite (RE^; <4y) (**) 10.39 1.93 2.33 1.52 2.06

Fine Dolomlcrospar (RE2; 4-16u){**) 8.52 2.77 4.29 3.36 1.48

Coarse Dolomlcrospar to Coarse Dolospar 1.79 2.92 5.77 .11 0.00 (RE3_5; 16-64p; >64y)

Z Calcite Matrix (HE) 4.68 1.59 3.30 1 .42 ,21 Z Dolomite Matrix (RE)

Z Matrix 3.24 6.03 .79 .67 .05 Z Cement 1 }

V> Co 99

dolomite (RE 3 _5 )- Th* ratio ENE/ERE Indicates that, ex cept for the nodular unit, there 1s a gradual decrease 1n dolomite frequency, relative to that of total calcite matrix, from the foreshore sequence downward Into the shoal unit; the matrix, relative to total cement (E matrix/

E cement), shows a significant Increase along the same line. The F-values for each variable are given 1n

Appendix B.

I nterpretat1on

As could be predicted from the deposltlonal model, mud 1s concentrated 1 n the lower energy environments (la goon and shoal) that existed seaward of the beach zone.

The question arises as :o why the lagoonal beds contain coarse calcite (NE^_ 5 ) dolomite (RE 3 _g) (F1g. 18 E, F), whereas the shoal unit Is composed of fine calcite (NE-^g) and dolomite (R E ^ _ 2 )• The nodules 1n the lagoonal sequence are composed of recrystallized grains and matrix and contain little or no dolomite. Furthermore, dolomite replaces microspar ex posed at the peripheries of the nodules encased 1n dolomltized beds; rarely, remnant patches of fossl 1 1 ferous quartz arenlte are found 1n beds that are completely dolomltlzed.

Almost all mollusks In the quarts arenlte and lime packstone beds are recrystal 11 zed (N^) (Figs. 17 D; 18 G).

As expressed 1n the grain recrysta111zatlon-gra1n 100

d1ssolut1on-reprec 1p1ta t 1on section* the Inversion process takes place 1n waters essentially saturated with respect to aragonite (Purdy* 1968). It 1s suggested here that 1n the Cow Creek sequence these waters attained aragonite saturation through water mixing or ion diffusion in the zone of diffusion separating fresh ground waters from interstitial marine waters in and beneath the beach se quence. Folk (1965) states that the recrystal 11zatlon process may be related 1n some way to water salinity be cause microspar most commonly occurs In brackish (i.e .. mixed) water and marine deposits. Thus* both grain Inver sion and matrix recrystal 11 zat1on may take place 1n the same chemical environment. Much microspar Is 1n optical continuity with recrystallized grains in the lagoonal se quence; because of this relation, inversion and recrystal lization of one of these members could have Induced, or set up nucleatlon sites for, the recrystal 1 1 zatlon of the other. This relationship, however, could be purely coin cidental and related to a much later stage of advanced recrystal 1Izatlon (I.e ., during burial). Exactly why most of the mlcrlte 1n the shoal unit did not recrystal 11ze to microspar 1s a perplexing question. In light of the dis cussion above, this phenomenon may be related to the rela tive absence of recrystallized mollusks.

The replacement of certain lagoonal beds by coarse dolomite was possibly Induced by the presence of detrltal 101

dolomite. Dolomi11 zed beds contain more recognizable detrital dolomite (Fig, 18 H) than do the pelleted lime packstone nodules and recrystall1 zed beds (Amsbury, 1962, also noted detrital dolomite in the Cow Creek units, but did not correlate Its presence to the do 1omi1 1zation of the sediments). Lindholm (1969) suggests that the presence of detrital dolomite 1n a lime mud alleviates the kinetic problem of nucleatlon, unless, of course, the nuclei are coated. He also points out that 1n limestone In itia lly containing 4 percent detrital dolomite, all that 1s neces sary to convert the rock completely to dolomite is a three-fold Increase in size (I.e ., a 27-fold increase 1n volume) through dlagenetlc overgrowth. It seems logical to assume, therefore, that certain beds within the lagoonal sequence were more amenable to dolom 1 1 1za1 1 on than others because of the presence of numerous detrital dolomite seeds. Overgrowth took place on some of these seeds, as

Indicated by the presence of nuclei (which are size sorted with the included quartz sand) within rhombs that are up to 1 mm (RE^_g) in size. Dolomite undoubtedly did nucleate at preferable sites within the lime mud, because finer dolom1crospar (RE2 ) 1s also common.

Only minor detrital dolomite was detected 1n the shoal unit. This suggests that only a small amount of 1t was deposited 1n these beds, or the dolomite which did reach the shoal area was 1n the mlcrlte and fine microspar 102

size ranges. This latter suggestion seems to be more

logical because other terrigenous detritus displays a

gradual fining downwards through the sequence (towards

deeper offshore environments). Although the presumed

detrital dolomite was very fine grained, this is not a

sufficient explanation for the fine grained nature of the diagenetlc dolomite. Again, as for the mlcrlte 1n the

shoal beds, the writer 1s at a loss to explain why the

replacement dolomite Is not as coarse as 1 t 1s 1n the over-

lying and underlying dolomite units.

Dolom1t1zat1on of the lagoonal, shoal and Hammett deposits was early enough to allow dolomltlzed llthoclasts

to be reworked Into the overlying beach sediments. Once

again, as for the case of grain and matrix recrystal 11za-

tlon, the author calls on a water mixing mechanlsm--th 1s

time to effect the dolomi11za11 on of these units. Fresh

regional waters, originating In the Llano Uplift, and draining through the Paleozoic dolomites, could have acted

as one source for the magnesium necessary to dolomltlze

the 11me muds. Mixing of these waters with marine Inter

stitial waters, both 1 n subsea regional discharge areas,

and more widely 1n the zone of diffusion separating the

fresh and salt waters beneath the beach beds (Kohout,

1960), would supply additional magnesium to the system.

Mixing would also raise the activity coefficient of the magnesium Ion (and decrease complexlng) so that smaller 1 03

molal concentrations of magnesium (I.e ., than 1n waters with high Ionic strengths [Oeffeyes and others, 1965]) would be needed to cause do 1om111zatlon. Thermodynamic calculations and field evidence from Florida aquifers

(Back and Hanshaw, 1966, 1970; Hanshaw and others, 1971) indicate that limestone can be replaced by dolomite in fresh water systems which have Mg/Cail. Hanshaw and others

(1971) hypothesize that dolomit1zat1on can readily take place 1n the zone of diffusion which separates meteoric fresh water lenses from interstitial salt waters 1n coastal aquifers. The working feasibility of this dolomitiza11 on model and the attempt to construct an overall dlagenetlc model for the Cow Creek Llmestone-Hammett Shale Intervals will be presented 1n the conclusions and Implications of this study.

Carbonate Cements

General

The carbonate cements encountered in the Cow Creek beds can be subdivided In itia lly Into two genetic cate gories: those originating by pure physlochemlcal precipi tation from fresh, marine or mixed waters, and those which are associated with, or are the result of, algal or fungal g rowth.

This second group 1s either composed of mlcrlte or dolomlcrlte and 1s restricted to the uppermost beach 104

foreshore beds (Facies X) and the "marine" fung-algal caliche (Facies XI). Because these are genetically related to the marine caliches* they will be discussed under that topic (p. 124). The first of these types are the spar cements of Folk (1962). These cements are classified on the basis of mineralogy (aragonite, high Mg calclte, low

Mg calclte), shape, size, orientation, and the substrate upon which they develop.

The mineralogy of carbonate cements depends on the chemistry of the precipitating fluids. Low Mg calclte cements are precipitated from fresh waters 1n the vadose and phreatlc zones (Friedman, 1964; Folk, 1965) and they are by far the most common types found 1n the Cow Creek

Limestone and 1n other carbonates. On the other hand, cements composed of aragonite or high Mg calclte (or stabilized cements originally composed of these two miner als) precipitate from marine or sea-fresh water admixtures in carbonate sands on the sea floor, 1n reefs, or in the intertidal (beach) zone (Schmaltz, 1971; Land, 1970;

Friedman, 1968; Shinn, 1969; Taylor and Illing , 1969).

The shape and size of a spar cement 1s dependent upon its original mineralogy, Its substrate, pore size, and the rate and time span of the precipitation process.

Low Mg calclte cements are usually either bladed (PB) or equant (PE), and rarely fibrous (PF) 1n shape (F1 g. 19 A-E;

Bathurst, 1958; Friedman, 1964). Bladed cement, arranged Figure 19. Carbonate cements.

A. Bladed spar cement (PB 5 O) in optical continuity with oyster fragment; xN.

B. Monocrysta11ine„ optically continuous eguant spar /«,- « \ --(veioped on echinoid fragment (upper

C. Equant cement mosaic (PE 3 ) In well sorted, well rounded lime gralnstone.

D. Bladed first generation spar (PB 4 C) on recrystal lized mollusk fragment; xN.

E. B1aded-f1brous (PB3C-PF3 C) on m1cr1t1zed oysters; thin mlcrite envelope on moldlc cavity fillin g on right. F. Recrystal 11 zed-1nverted fibrous first generation spar (PF3 ) and mlcrite cement (PE*i), both of Intertidal origin.

G. Large mlcrosta1 act 111c bladed cements (PBd.6 C-PB4 _6 0 ) 1n recrystallized mollusk lime grainstone; bladed cements are always larger on grain undersides. xN

H. Beachrock clast (truncated grains and cements) with PF3 and PEi cements. 105

Figure 19. 106

perpendicular to the grain surface (F1g. 19 A, D)f 1s nor mally the firs t cement type which forms 1n a pore. Equant cement occludes pore centers and typically displays no orientation. Both of these cements may be 1n optical con tinuity with a grain; optically continuous bladed cement

(PBO) may develop on foliated or prismatic shell structures

(F1g. 19 A) and monocrystalline overgrowths of equant cement (PEOm) normally precipitate on echlnoderm fragments

(F1g. 19 B). Aragonite cements have a fibrous or "mlcrltlc" habit (F1g. 19 F) whereas high Mg calclte cements are gen erally rhombohedral or "mlcrltlc" (C. H. Moore, personal communication, 1971). The state of the art concerning sub marine and Intertidal beach rock cements 1s excellently summarized 1n the 1969 Sedlmentology (vol. 12), and the book, Carbonate Cements, edited by Brlcker (1971).

Cement fabric can be Indicative of vadose or phreatlc zone precipitation, such as 1n the case of Isopachous

(phreatlc) cement (F1g. 19 F; Land, 1970), and mlcrostalac- tltlc (vadose) cement (Fig. 19 G; Taylor and 1111ng, 1969).

Microstalactltic cement precipitates from water which clings by surface tension to grain undersides, whereas the formation of a rind of Isopachous cement necessitates equal rates of precipitation from all sides of a grain.

Unfortunately, mlcrostalact1tic cements are rare, and iso pachous cement can form 1n the vadose as well as 1n the phreatlc zone (Ward, 1970). 107

One other easily utilized criterion which may be of some aid In determining carbonate cementation environments is the Iron content of the cements as determined by stain ing (Evamy, 1969). Calcites containing ferrous iron are presumably precipitated in the phreatic zone where inter stitial water characteristically has a low Eh. Vadose waters are normally oxidized and a greater proportion of iron present Is in the form of ferric Iron, which will not substitute for calcium because of the difference in valence and ionic radius.

However, this simple criterion does not always with stand the rigorous tests Imposed by nature. Cements pre cipitated from 1ron-def 1c1ent phreatlc waters may not con tain sufficient ferrous Iron to be detected by staining^ furthermore, phreatlc waters may be oxidized, or vadose waters may be reduced, depending on the prevailing climate, the amount of vegetative cover 1n the ground water recharge area, and the presence of minerals which may readily lower the Eh during their oxidation. However, when ferroan ce ments are juxtaposed with iron-free cements, either within a pore, a thin section, or throughout a stratlgraphic se quence, more assurance can be placed 1n the Interpretation that those with ferrous iron were precipitated under phre atlc conditions, and those without ferrous iron were formed 1n the vadose zone. 108

01str1but1on

Most cement types and cement ratios show highly sig nificant Interval variations (Appendix B). The least squares means for most fibrous and bladed cements (PF's and PB's), and all random equant cements (PE's), increase upward from the shoal to the foreshore (Table 7). Mono crystalline overgrowths (PEO^s) are most common in the upper offshore beds, whereas optically continuous equant cements (PEO's) decrease 1n abundance downward from the foreshore sequence. The ratios shown 1n Table 7 were de rived 1n order to determine the cement types or cement groups relative to others. It should be noted that throughout most of the sequence:

1) Total cement Increases upward, as Indicated by the

means for the individual cement types and E matrix/

l cements.

2) Fibrous and bladed cements show an upward Increase

in frequency relative to carbonate grains

[Z allochems/Z (PF + PBO + PBC)2_4].

3) Fibrous and bladed cements show an upward Increase

In frequency relative to equant cements

[E (PE + PEO + PE0m)2_6/I (PF + PBO + PBC ) 2- 4^

4. Bladed cement crystals are coarser 1n the beach

foreshore Interval than In the upper offshore

Interval [z (PB2) + PBgC + PF’s) / PB3 4 C] Table 7. Cement Distributions

Lagoon Facies Upper Shoal Foreshore Nodules Other Units offshore Variable

FIBROUS AND BLADED CEMENTS PFO's + PFC’s (4-250p) .01 .00 .00 .01 .22 PBO1s (4-250u) 1.00 .97 ? N BO .23? NBO 1.15 2.10 PB2C (4-16u) (**) 2.87 .00 1.927NBC, 7.93 14.07

PB3C (16-64y) (**) 1.70 1.75? NBC. 1.8 6 ?NBC. 5.98 12.67 .05 .28 .42 PB4C (64-250U) (**) .13 ? NBC4 .00 tpb2o + PBC + PF (**) .79 .22 .46 1.44 1.22 EPB3 . 4C

EQUANT CEMENTS Monocrystal 11ne {64-25Oy) {**) .25 .60 ? N EO .46 ? N EO 1.85 .84 PE4°m m4 m4 (.25-4 mm) (**) .19 .367NE0 .65 ? NEO 3.13 2,17 PE5-6°m m5-6 m5-6

Optically Continuous

PE30 (16-64u) (**) 1.22 3.48?N EO 3 3.67?NE0; 4.19 1.95

PE40 (64-250m) (**) .47 .997NE0. 2 . 90?NEo] 2.92 4.41 PE5_60 (.25-4 mm) (**) .02 .747NE0 .437NE0 1.26 2.21 5-6 5-6 Table 7 (continued)

Facies Lagoon Shoal Upper Nodules Other Units offshore Foreshore Variable

Random

PE2 (4-1 fly) 6.02 6.61?NE. 7 .05?NE. 9.18 9.82

PE3 (16-64y) (**) 9.25 12.92?NE! 18.46 ? NE• 29.40 31.19

PE4 (64-250u) (**) 4.24 8.097NE] 11.22?NE^ 14.85 18.06

PE5_6 (.25-4 mm) (**) .13 .607NE .63 ? NE 1.38 1.59 5-6 5-6 PE, ,0 + PE, , f c * J______( i t i t \ 3.28 4.93 7.97 3.85 2.68 £(PE + PEG +>E0m)4J *

I Matrlx/I Cement (**) 3.24 6.03 .79 .67 .05

Other Ratios £ Allochems tJ.x f(PF + PBO + PBC) 2. 4 1 ' 19.16 6.72 13.69 7.25 4.11

£(PE + PEO + PE0m) 2_6 4.56 6.54 11,88 6.14 4.54 eT?F + F IS + PBC) 2-4

£ PEO .16 .74 .62 .73 1.99 n r (**)

£ Clastics w/PF or PB , _, .001 .01?NB .007?NB .04 .13 110 m a s tic s------(**) Iron-Content (**} 2.96 4.22 3.90 4.33 4.85 I l l

5) The proportion ( Z elastics with PB or PF)/

Z elastics Indicates that more detrital grains were

bound by PB or PF In the foreshore unit than 1n any

other.

6 ) There Is an Increase 1n coarsely crystalline equant

cement frequency relative to that of small equant

cement from the lagoonal Interval Into the fore

shore unit

7) Ferroan calclte content of thin sections decreases

upward (a larger number Indicates less ferroan

calc 1te ).

I nterpreta 1 1on

General.--Sediment size, grain sorting, and mud winnowing all increase upward from the offshore through foreshore units of the beach sequence. Thus, pore size and cleanliness would also Increase upward, and therefore one would expect the size of Individual cement crystals and the total cement volume to Increase upward. As pre dicted, these relationships are borne out 1n the Cow Creek sequence: cement becomes more abundant upward (z matrix/

Z cement) and the coarsely crystalline cements are most common In the beach foreshore unit [(PE 2 _30 + P2 3 E)/

Z (PE + PEO + pE0m>4_6; Z (PB20 + PB2C + PF1S) / PB3 _^C].

Coarse, monocrystalline cements (P 6 0m) are most common 11 2

in the upper offshore unit because these beds are coarse grained, poorly sorted, and contain an abundance of echi-

noid fragments.

Three significant aspects of Cow Creek cementation are not forecast by the sedimentary process-response beach model. First among these 1s the sporadic occurrence of

Intertidal beachrock cements. Secondly, fibrous and bladed cements are the largest and most abundant in the foreshore beds. Thirdly, progressively less ferroan cal clte cement, and more Iron-free cement, occurs higher in the sequence.

The "cements" coded ?N 1n the cement distribution chart may be either recrystallized matrix or recrystal lized cements, or both; these are discussed briefly 1n

Appendix D4.

Intertidal Cementation.--Stabilized fibrous and ml crltlc marine beachrock cements, which were originally composed of aragonite or high Mg calclte, are found mostly

1n lime gralnstone clasts 1n the foreshore sequence; they are developed 1n situ 1n the foreshore accretion beds at location 14 1n the main Cow Creek beach, and at location 7

1n the lower "Hammett" beach (F1g. 4). The fibrous cements noted at location 14 and 1n the lime gralnstone clasts

(Fig. 19 F, H) show signs of recrystal 11zatlon (serrate edges), and occur as equal thickness "onion skin" rinds

(Land, 1970) on grains; mlcrltlc cements develop as crusts 11 3

on these rinds or completely occlude the pore space remain

ing after Initial cementation by the fibrous cements.

Both of these cement types mimic the Recent submarine and beachrock cements reported on by innumerable workers in the past three years (Shinn, 1969; Taylor and I I 11ng, 1969;

Carbonate Cements, 1971), and because the fibrous cement forms an equally thick crust on grains, 1t is Interpreted as having been precipitated 1n the marine phreatlc zone

(Land, 1970). Allen (1970) has also Identified similar cements 1n another Lower Cretaceous beach sequence of

Central Texas.

The "Hammett" beach at location 7 displays the most spectacular cement fabrics noted 1n the entire sequence.

Here, rudaceous coqulnas, composed almost completely of recrystallized aragonite mollusks, are bound with bladed and fibrous cements that range upwards to 2 mm 1n length

(F1g. 20 A, B) . These blades typically widen outwards from their bases on mollusk fragments, and blades commonly contain black (pyrltlc?) traces of relict fibrous cements.

A mlcrostalactltic arrangement (In which the cement fringes are thicker on grain undersides) 1s common (Fig. 19 G);

Taylor and 1111ng (1969) have reported similar cements from carbonate beaches 1n the Persian Gulf; here, as also

Inferred for location 7, the microstalactltic nature of the cements results through the precipitation of aragonite from water droplets left clinging to the undersides of Figure 20. Carbonate cements and "marine" fung-algal caliche.

A. Bladed mlcrosta1act1t 1c cement (PBg.gC) in lime gralnstone from "Hammett" beach foreshore.

B. Bladed (recrystallized fibrous) cement with relict fibrous traces (parallel black lines) and black (pyrltlc?) lines paralleling the edge of the bio clast 1t 1s developed upon.

C. Dolomite s ilt perched on first generation bladed cement.

D. Dolomicrite-m1cr1te matrix of "marine" caliche 1 1me wackestone.

E. Caliche dolomltlc lime packstone with bored and m1cr1t 1 zed grains; pore space 1 s almost entirely occluded by m 1cr1 te-s 1ze cements.

F. Caliche dolomltlc lime packstone-wackestone with vaguely visible micrltlzed and partially dolomltlzed grains; all aragonltlc grains have been removed by leaching.

G. Breccia fractures 1n sandy dolomite mudstone; fractures filled with qtz. sand-slit and m1cr1t 1c- dolom1cr1t 1c vadose s ilt.

H. Dolomltlc lime wackestone with micrltlzed grain (right) and oyster surrounded by c 1rcumgranular crac k. *02 ajnb-tj

frll 115

grains during low tide; thus, the fibrous mlcrostalactltic

cements are the only cements 1n the Cow Creek that can

unequivocally be said to have formed 1n the marine vadose

env 1ronment.

Recrystal11zed isopachous fibrous cements, resemb

ling those found at location 14, are sporadically present

and contain black pyrltic traces which define their origi

nal morphologies (Fig. 20 B); these cement fringes may

also display thin black or mlcrltlc lines which parallel

the shell fragment on which they are developed. Taylor

and 1111ng (1 968) note alternating fibrous and mlcrltlc

bands In submarine cements and suggest that the mlcrltlc

bands represent zones of arrested fibrous cement growth,

during which time the outermost fringe of the fibrous band

1s altered to, or coated by, a crust of microcrystal 1 1 ne

high Mg calclte. Besides these crusts. Infiltrated micro-

crystalline calclte can be seen perched on the fibrous

cements (Fig. 20 C). Infiltration may have taken place

either during exposure 1n the beach vadose zone or during

resubmergence and burial beneath the shoal beds that were

deposited on the top of this beach.

The absence of supersaturated marine Interstitial

waters and the proper microflora, or very rapid beach ac

cretion are possible explanations for why Intertidal marine vadose and phreatlc beachrock formation did not

take place more often. On the other hand, evidences for 116

aragonite and high Mg calclte cements may either have been

removed by subsequent diagenesis, or they just cannot be

recognized. Because some of the fabrics of the "marine"

fung-algal caliches bear a striking resemblance to those of beachrocks, the distinct possibility arises that some of the "marine" fung-algal caliches could have originally

been Intertidal beachrocks, but were modified later by desiccation, expansion, dissolution, and the like.

Bladed Cements.--One possible hypothesis regarding the increased profusion and size of bladed and fibrous spar 1n the foreshore unit relies essentially on the dis tributions of inverted grains, (N^)# moldlc cavity filled

(Ps) mollusk grains, and mlcrite envelopes, and on the chemical situation set up by the mixing of fresh and marine waters. Solutions permeating downward into the beach sediments began to equilibrate with their new en vironment by rapidly converting microcrystalline grain en velopes to calclte, and concurrently or shortly afterwards dissolving aragonite grains from the foreshore accretion units. These water were probably saturated with respect to calclte, and only later, during further downward and lateral movements, did they reach aragonite saturation and stabilized grains through Inversion.

Although beach interstitial fluids were more satu rated 1n the lower foreshore and underlying units, the ab sence of abundant calclte nucleatlon surfaces (i.e ., 117

inverted envelopes) probably inhibited the precipitation

of excess calcium carbonate as low Mg bladed and fibrous

spar. Pingatore (1970) has noted a similar phenomenon 1n

the Pleistocene of Barbados, where only the inverted por

tions of coral fragments bear calclte cements. Numerous

other workers (Friedman, 1964; Gavlsh and Friedman, 1969;

Matthews, 1967; Harris and Matthews, 1968) have also re

corded abundant Initial cementation from the local disso

lution of aragonltlc grains 1n Pleistocene rocks exposed

to fresh waters.

However, nothing has been said of the complexities and alternatives offered by the consideration of what hap

pens when fresh waters Intermix with saline waters. Be cause saline waters have high 1on1c strengths, the activity concentration ratios of dissolved 1ons are lower than those

In fresh waters; moreover, more complexlng takes place In marine water, and 1t also contains 1ons, magnesium 1n par

ticular, which Inhibit the nucleatlon of low Mg calclte

(Berner, 1966; Blschoff and Fyfe, 1968). Runnells (1969) and others previously have pointed out that the mixing of two dissimilar but carbonate saturated waters results In the dissolution of more carbonate. Vertical differences

1n the sizes and amounts of bladed and fibrous cement may therefore be the sole result of less Inhibition by magne sium (Berner, 1966) and organic matter (Suess, 1970) in the upper part of the bearh where fresh waters were 118

influenced less by salt water mixing. Vertical differences in Ionic strengths may have also played significant roles in determining where most precipitation took place, but the influence of these two factors cannot be assessed at this time. Indeed, the sporadic occurrence of bladed low Mg calcite cement on detrital grains (which do not act as nu cleatlon sites) in the uppermost foreshore interval sug gests the existence of highly supersaturated conditions

(Taylor and Illing, 1969) and easier or more rapid nuclea tlon of calclte than In the underlying sequence.

Ferroan Cements. --Typical!y, bladed first generation cements are iron-free and only in rare instances are they composed of ferroan calclte. Equant cements, which occlude the remaining pore space, may be either ferroan or Iron- free calclte; ferroan calclte cements are essentially con fined to beds below the lower part of the foreshore unit, but Individual cement crystals sometimes display ferroan and Iron-free calclte growth zones throughout the sequence.

This does not, however, alter the overall cementation pat tern. The association of Iron oxides with Iron-free ce ments in the foreshore beds 1s significant when one con templates the Eh that prevailed during the precipitation of these last stage ferroan and Iron-free equant cements.

Iron concentrations were high 1n the ground waters of Llano recharge area (Hennlngsen, 1962). These waters eventually made up the regional ground water flow system 119

and therefore another explanation, other than Iron-free water, must be given for the distribution of Iron-free

cements. Final cementation by ferroan calcltes and Iron-

free caldtes came about as fresh vadose (?) and regional

phreatlc waters passed through the nearshore carbonate de

posits. Because ferroan caldtes are rare 1n the fore

shore units, two conceivable explanations present them

selves.

One possibility Is that the fresh water table was

be!ow sea level (Water Table I, F1g. 25); Iron-free cal cltes along with ferric hydroxides, would then be precipi

tated 1n the oxidizing vadose zone (I.e ., most of the

thickness of the foreshore beds) and ferroan-caldte would

precipitate from the reduced waters of the phreatlc zone

(Evamy, 1969). The second hypothesis suggests that the

fresh water table was above sea level (I.e ., located some

place 1n the alluvial plain sediments). This placement

necessitates a change 1n either pH or Eh within the

phreatic zone to bring about the formation of the two

cement types.

Recent fresh water tables can sink below sea level

in arid 1ow-ly 1 ng coastal plains, but the most spectacular

examples of this phenomenon occur where water 1s a r t i f i

cially removed from aquifers faster than 1t can be replen

ished. For example, 1n Orange and Los Angeles counties,

California, unconfined water surfaces are below sea level 120

and slope Inland (Owen, 1967; Milne, 1967); 1n Tunisia,

the water tables of coastal aquifers capped by semlper-

meable caliches are 20-30 meters below sea level (Dutcher

and Thomas, 1966). On the south coast of Jamaica, a

coastal water table drops naturally to three feet below

sea level during the dry season (Wozab and Williams, 1967).

Utilizing these examples as a basis, and the hypothesized

arid climate during Cow Creek deposition, the regional

post-strandl1ne ground waters occupying the Cow Creek-

Hammett sediments could have been depressed 10-15 feet

below sea level (i.e ., the depth below beach tops where

ferroan calclte cements become predominant). This could

be accomplished by extreme evaporative withdrawal of waters through the alluvial plain sediments at rates

faster than the infiltration rates of recharge waters from

the Llano area. Earlier cementation of beach sediments,

along with the development of semipermeable marine ca

liches, possibly obstructed the vertical and lateral in

flux of fresh waters and the lateral landward seepage of marine fluids. As discussed previously (p. 9 l). the In crease upward through the sequence In the number of ara gonite grains that underwent d 1sso1ut1on-reprec 1p itat1on, and the concentration of 1nverted-recrysta 111 zed grains below the foreshore sequence, 1s another manifestation of

the position of the early ground water table.

The second hypothesis Involves the location of the 121

regional ground water table above sea level (Water Table II,

Fig. 25). In this situation, no vadose zone would have

existed on a regional scale within the beach, and therefore

a change of either Eh or pH with depth must be called upon

to explain the relationship between the ferroan and non-

ferroan calcite cements.

Only trace amounts (5 molal percent) of iron in the

calclte lattice are needed to induce a stain indicative of

its presence (Evamy, 1969). For purposes of determining whether the requirements necessary for the precipitation

of Fe and Fe-free caldtes are drastically different, 1t

was assumed that the stained caldtes contained less than

5 molal percent Fe. Calculations (presented In Appendix D5),

based on the methods of Garrels and Christ (1965, pp. 48,

90) show that the equilibrium activity product (ideal) of

(Ca gsFe 0 5 )C03 1s 10' 8 ' 36 as compared to 10" 8 ,3 4 at STP

for pure low Mg calclte, and that mFe+^ = 10~ 3 *4mCa+^ 1n

dilute aqueous solutions 1s necessary for the formation of

a 5 molal percent Fe-caldte.

Because of the almost Identical K ‘ s of the two sp calclte species, 1t seems unlikely that pH changes, by controlling the types of carbonate 1ons present 1n solu

tion, Inhibited the formation of one calclte while allow

ing the other to precipitate. Alternatively, vertical

variations 1n pH and/or Eh through the phreatlc zone could

have prohibited the existence of a sufficient amount of 122

ferrous Iron necessary for the precipitation of ferroan calclte. As the pH of a solution Increases, lower Eh's + 2 are required to maintain a high Fe /Total Fe;1f negative

Eh's do not accompany high pH's, the majority of ferrous iron is oxidized and precipitated as insoluble ferric hy droxides (Fig. 21). Because calclte normally only pre cipitates from solutions with a pH >. 7, low Eh conditions are needed to precipitate a ferroan calclte.

Regional ground waters could have existed in a state analogous to that of lacustrine environments (Oborn,

1967). In a model lake, the dissolved oxygen content of water diminishes with Increasing depth; near the water-air

Interface most Iron is present as suspended Iron hydrox ides, but downward, Iron 1s gradually reduced and exists mainly 1n the ferrous state. Even though lake waters are oxygen-def 1c1ent for different reasons {i.e ., organic decay and absence of circulation) than phreatlc waters, both can be oxidized 1n their upper port 1ons--1n ground waters, because of the absence of soil organlcs, and the assimilation of oxygen present 1n the vadose zone. Ground waters would also be expected to have a higher pH near

(10-15 ft) the vadose zone because these waters would con sist primarily of warmed Infiltrated vadose waters lacking

1 n C02; the Increase 1n pH would lower the Eh required to retain abundant ferrous Iron to such a point that only non-ferroan calclte would be precipitated. In the 1 23

♦ 04 -

+ oa -

+ o . « -

FERRIC HYDROXIDE** F«(OH)i

o.o

- o .t -

SIDERITE FaCOa

- 0 .4 -

-ao -

- 0 .0 -

(After Garrels and Christ, 1965) - to t 4 pH 10 It •4 Figure 21. Diagram showing tho relations among tho metastable Iron hydroxides and slderlte at 25 °C and I atmoaphara total pressure. Boundary batwaan solids and Iona at total activity of diaaolvod spacias * 10'*. Total diaaolvad carbonate apaciaa ~ 10'*. Dashed linea are boundaries batwaan fields dominated by the labeled Ion. 124

uppermost phreatlc zone small oscillations 1n the Eh or pH across the boundaries of the ferroan calclte stability field (which approximates that of slderlte) would give rise to the iron zonatlons noted in calcite cements.

The decision on which of the two suggested models is most appropriate for explaining the latest cements of the Cow Creek Limestone 1s left to the reader. The writer is biased towards the existence of a regional beach vadose zone because 1n the hypothesized arid climate, the poten tially small amounts of rainfall 1n the Llano region and on the Hensel alluvial plain would not be sufficient to maintain a water table above sea level. Furthermore, 1f one does prefer the water table to lie 1 n the alluvial sediments, it would stand within 1 0 - 2 0 feet of the land surface throughout most of the study area; this 1s seem ingly Incompatible with the deep water tables expected 1 n an arid, alluvial plain setting.

"Marine" Fung-Algal Caliche

The strat1graph1c position and general properties of the "marine" fung-algal caliche, plus Its stratfgraphlc significance, were presented 1n the llthofacles analysis

(L1thofac1es XI, p. 38).

The nodular caliche sporadically caps the beach foreshore sequence at many localities and, although Its lower contact with beach 1 1 me grainstones 1 s sharp in the 125

field, no distinct contact can be discerned between them

in thin section. Upward from the foreshore lime graln-

stones Into the caliche, progressively less grains are

preserved until all that remains are quartz grains and a

few oyster fragments which float 1 n a m1cr1 te-dolomicrite

matrix (Fig. 20 D). Petrographlc evidence suggests that

this floating grain fabric arose through a combination of

caliche expansion, dissolution of aragonite grains, and

gradual destruction of oyster grains through boring, micri-

tlzatlon, and dolomlcrlte replacement (F1g. 20 E, F).

Expansion (and desiccation) take place whether a

caliche develops by replacement of bedrock (Blank and

Tynes, 1965) or by precipitation of calcium carbonate in

an unconsolidated soli horizon (Reeves, 1970). Expansion

in the Cow Creek nodular caliche 1s manifested by both the

petrographic characteristics, such as Irregular fractures,

vadose s ilt, and c1rcumgranular cracks (Fig. 20 G, H), and

vertical changes in the percent Insoluble residue between

unaltered bedrock and the overlying caliche. Figure 22

depicts the vertical changes 1 n Insoluble residue percen

tages encountered In four "marine" fung-algal caliche sec

tions. In general there exists an upward decrease in

residue content, which reflects along with the petrographlc

evidence, more Intense cal 1ch1 f 1catlon upward from the

beach top; slight fluctuations seen 1 n the Individual graphs are attributed to minor differences in the detrital Top o f Nodulor Caliche Zone

Top of fteach Gramitones / • * -

29.54 % In soluble Residue (Caliches)

Figure 22. Plot of Insoluble Residue Content in "Marine" Fung-Algal Caliches. 921 127

sand content of the original beach beds. The rapid In

crease 1 n percent Insoluble residue at the top of each se

quence 1 s a consequence of sediment Infiltrations from the

overlying supratldal and alluvial plain units.

The fact that the overlying supratldal deposits are distorted (F1g. 11 E) and display cal1ch1f 1cat1on features

(Fig. 23 A) indicates that they were deposited penecontem-

poraneously with nodular caliche soil formation. It should

be pointed out here that the laminated calcrete crust, which often caps a nodular caliche zone (Reeves. 1970). 1s

probably absent as a result of the cover offered by the

alluvial plain and supratldal deposits. Reeves (1970)

states that a laminated crust will not form unless the

nodular zone of a caliche becomes plugged (impervious) or

1s exposed at the surface. Thus, the blanketing effect

offered by the superjacent units during the formation of

the nodular zone probably prevented the ca 11 chif1cat1on

process from culminating In the prec 1p1 tation of a lami

nated crust.

The Cow Creek caliches bear an overall striking re

semblance to profiles from the High Plains described by

Reeves (1970) and Blank and Tynes (1965); therefore. 1t is

nearly certain that similar processes operated 1 n their genesis. The Cow Creek "marine" fung-algal caliches, how

ever. developed 1 n a strandllne environment 1 n which there

existed unstable marine carbonate grains, mixed Figure 23. "Marine" fung-algal caliche fabrics.

A. Fractures and rounded breccia clasts 1n supratldal dolomite facies.

B. Irregular mlcrite-dolom1crite crusts of fungal and algal origin in lime "packstone" (micrite-cemented lime grainstone) from base of "marine caliche."

C. Mlcrite envelopes* crusts, and cements of Holocene strandllne caliche, St. Croix, B.W.I. (photo cour tesy of D. Boleneus). Rounded, Irregular sinuous pore system strikingly similar to that of A and D.

0. Micrite cemented supratldal lake deposit (Fades XII) which has undergone ca11ch1f1cation.

E. Holocene strandllne ("marine" caliche) with thick mlcrite crusts and cements; sinuous pores; St. Croix, B.W.I. (courtesy of 0. Boleneus). 128

F igure 23. 1 29

marine-fresh waters, and a marine microflora. This environ mental setting and sediment substrate was unlike that of the High Plains, and therefore differences do exist between the two caliches. These differences are 1n the types, mineralogy, and origin of some of the microcrystal 1 ine ce ment which makes up a significant portion of the Cow Creek caliche sections.

Blank and Tynes (1965) attribute the origin of the

High Plains caliche cements to the dissolution of bedrock carbonates and localized repreclpi ta t 1on of It as microcrystalline cement. Much of the homogeneous microcrystal-

1 1 ne cement of the mlcrlte-dolomlcrlte cemented lime gralnstones and wackestones probably originated 1 n a similar manner; evidence for this solutlon-depos 1 tlon process 1 s found 1 n the beach sediments underlying the caliche sec tions. Here, more than average originally aragonltlc allochems were leached than 1 n beach sequences not capped by caliches. Infiltrating rain waters, charged with carbon dioxide from the caliche organlcs and the atmosphere, dis solved grains during their downward migration. Because of the extreme evaporation taking place off the caliche sur face most of these vadose waters were sucked upward by capillary action upon near surface evaporation, some of the dissolved carbonate was precipitated as caliche ml crite cements. Continual cycling of waters 1n this manner led to progressively more dissolution and reprec 1 p1 ta t 1 on. 130

However, cementation by this type of mlcrite may have been

effected by a resident caliche microflora (Krumbein, 1968;

Irion and Muller, 1968).

Other mlcrocrystal11ne cements encountered at the

beach top and in the ‘’marine" caliche are dissimilar to those described above 1 n that they are not pervasively

homogeneous. These form mlcrite or dolomicrlte crusts of

variable thickness on the grains they bind (Fig. 23 B);

the contact between a grain and crust may be sharp or gra dational, and the Irregularity of the outer margin of a crust with sparry calclte, which fills the remaining pore

space, may give rise to a highly Irregular petrographlc fabric (F1g. 23 D). Borings, which often extend from the crusts Into adjacent grains, are branched, very sinuous, and range from 1 5y to 150p 1n diameter (Fig. 16 A, B); borings can be composed of either mlcrite or dolomlcrite.

Similar encrusting microcrystalline cements have been reported from Recent Caribbean beachrocks, where they are high Mg calclte or aragonite (C. H. Moore and D. Bo leneus, personal communication, 1970) and nodular or lami nated Pleistocene strandllne caliches (D. Boleneus, per sonal communication, 1971; Multer and Hoffmelster, 1968) where they are composed of low Mg calclte. Cements of these Recent and Pleistocene deposits are compared with those of the Cow Creek “marine" fung-algal caliches In

Figure 23 B-E. 131

The microcrystal 1Ine cement crusts and grain borings are Interpreted as the remains of hyphae, fruiting bodies, and th a 111 of endollthlc fungi and algae. The crusts rep resent the thalll and hyphae of marine fungi and blue- green algae which specialize into encrusting, or binding, portions of the colony (Kohlmeyer, 1969). The branched borings with extremely large diameters bear a striking re semblance to photomicrographs of boring fungi published by

Kohlmeyer (1969) and Taylor (1971), and are therefore at tributed to this microflora. The problem still exists, however, of why the crusts and borings are composed of ferroan dolomlcrlte or Iron-free mlcrite, while the sur rounding sparry calclte 1s invariably Iron-free.

No problem exists as to where the Iron 1n the fer roan dolomite originated; bacteria, as well as algae and fungi, are known to concentrate Iron during their life processes (Oborn, 1960). Moreover, Umonlte stained m 1 - crlte envelopes from other Cow Creek Intervals and pyrl- tlzed envelopes from the Jurassic (Brown, 1966) suggest that certain boring algae may concentrate Iron. The only requisite, then, for Iron to be retained In a carbonate formed In the presence of "ferrugenous" algae or fungi 1s the existence of a reducing (low Eh) or high Ph environ ment, which would convert most of the iron present into the ferrous state.

Different microfloras, some giving rise to ferroan 132

carbonates and others not, could have coexisted side by side (grain by grain) In a prevailing reducing environment; this 1s negated, however, on the basis of associated Iron- free spar cements and grains. It 1s more reasonable to assume that mlcro-reduc1 ng environments existed in the more restricted algal and fungal structures, while high Eh conditions prevailed throughout the rest of the sediment.

Why, though, 1s there a contrast not only in the Iron con tent of the carbonates, but also 1 n their mineralogies?

Malone and Towe (1970) have synthetically produced three minerals by allowing nitrogenous organic materials to decompose under a variety of oxygenated conditions 1 n sea water. Struvlte (NH 4 MgP0 4 *6 ^ 0 ) was formed under com pletely aerated conditions but, under partial aeration, mixtures of monohydrocalc 1 te (CaCOg-HgO) and struvlte pre cipitated. Additionally, (Ca ggMg ^)C0 3 and struvlte precipitated when anaeroblosls existed 1n the sea water.

Purl and Collier (1967) have also demonstrated a microbial precipitation of aragonite from sea water. Furthermore, organic compounds forming strong calcium complexes cause the formation of high Mg calclte, and, with Increasing temperature and magnesium concentration 1 n solution, the magnesium mole percent In the Mg calclte Increases

(Kitano and others, 1969). Gebeleln and Hoffman (1971) found that the blue-green alga, Sch1zothr1x calclcola. concentrates magnesium 1 n Its filaments, which upon death 133

1n a high Ph environment, precipitated 17-20 mole percent

Mg calclte.

Evidently, then, the roles assumed by organlcs in micro-environments are as relevant as the macro environment's phys 1 ochemlstry 1 n the formation of marine carbonate phases. Consequently, It 1s not unreasonable to hypothesize that an algal or fungal boring may 1 n one In stance be the site of aragonite precipitation (Bathurst,

1964) 1f the Eh 1s high, whereas 1f the Eh 1s low, condi tions give rise to high Mg c a ld te (Wlnland, 1968; Malone and Towe, 1970).

Dolom1cr1te envelopes have been reported from an cient rocks (Buchblnder and Friedman, 1970) and preferen tial dolomlt1zat1on of Pleistocene coralline algae 1s noted by Gross (1965) and Land (1970). Land (1970) sug gests that high Mg calclte can stabilize either to low Mg ca ld te or dolomite dependent upon the rate of fresh water flow through the rocks. If a low Eh microenvironment, 1n which ferrous Iron predominates, exists 1 n juxtaposition with a higher Eh pore (or pore fluid) a necessary dictate

1s that there cannot be a free Interchange between the macro- and mlcro-envlronments. Thus, water movements or ion diffusion within the micro-environment (mlcrocrystal- line cement or boring) must be nil; magnesium from the stabilizing high Mg c a ld te 1s retained, and ferrous Iron

1 s Incorporated Into the newly formed dolomite. 134

Conversely, where there 1s a free Interchange between pore

fluids and microfloral cement and borings, magnesium 1 s

removed, most ferrous Iron 1 s oxidized to ferric Iron, and

Iron-free mlcrite results. It Is worthwhile to recall here

that the green mineral 1 n the fungal borings may be phos

phate and could have possibly precipitated as struvlte

under extreme oxidizing conditions (Malone and Towe, 1970).

In conclusion, many factors must be taken Into ac

count when one considers the possible microenvironments,

and resultant minerals, which could exist 1n a caliche (or

beachrock) pore. Among the most obvious are: the type of

resident microflora, the rates of precipitation of unstable

carbonates within algal or fungal structures, the packing

density and size of these structures, the depth of grain

borings, the types of organlcs present, and the quality,

and flow rate, of pore water.

Summary

Hammett Shale

The Hammett Shale carbonates are lime packstones

and dolomite wackestone or mudstone; dolomite beds become

progressively thinner and less frequent upward Into the

Cow Creek Limestone. The limestone 1s made up of

1 nsol ubl e-poor mlcrite and original c a ld tlc and aragonltlc

bioclasts, whereas dolomites consist of 1 nsoluble-rlch matrix and typically lack original aragonltlc bloclasts. 135

Aragonltlc bloclasts 1n the limestone are recrystallized

rather than leached, and grains 1 n both the limestone and dolomite normally are not mlcrltlzed and lack mlcrocrystal-

line envelopes. Sem1-flu1d (now dolomltlzed) Insoluble-

rich lime muds were forcefully Injected Into more rigid

i nsol ubl e-poor lime ,'m 1 cr1 teH muds as evidenced by sharp

fractures 1 n the limestone, along with the compactlonal

flow alignment, grain breakage, and 1 nterpenetra 1 1 on of

grains 1n the dolomite. Injection probably took place during the progradation of the beach over offshore sedi ments; It was also during this time, or shortly afterwards,

that dolom1 t 1 zat1on of the insoluble-rich lime muds took

place.

The statistical analysis of point count frequencies for grain and 1 ntergranular dlagenetlc modifications and textures 1n the Cow Creek Limestone reveal the following:

Grain D1agenes1s

Envelopes and M1cr1tlzatlon

A general upward Increase occurs 1n the number of mlcrltlzed grains and grain envelopes, with maximum devel opment of these 1 n the beach foreshore unit; fungl-bored grains occur at the beach top 1 n association with the

"marine" fung-algal caliche. The lowest means for grains that underwent these modifications are recorded 1n the lagoonal sequence. This distribution suggests that the 136

amount of mud matrix, rata of deposition, and grain stabi

lization are the controlling factors 1 n the formation of

grain mlcrocrystal1 1 ne carbonate; mud that clogs the sedi

ment pores, and rapid sediment deposition, inhibits or

ganic processes on or within grains, and the rate of abra

sion vs. the boring rate of algae and fungi controls ml

crite envelope preservation. Dolom1cr1te envelopes formed

either as a result of selective replacement of microcrys

talline carbonate In the shoal, or the redistribution of

magnesium 1n stabilized high Mg calclte at the beach top.

Recrystallized and Dissolution

Leached grains, which are now represented by moldlc

cavity f ills , are most abundant In the beach foreshore

Interval; downward, progressively more aragonltlc grains

were stabilized by 1nvers 1on-recrystal 1 Izatlon except 1 n

the dolomltlzed shoal strata, In which 50% of the origi

nally aragonltlc grains underwent dissolution. Grain

leaching was most efficient 1n the beach foreshore as a

result of downward percolating undersaturated meteoric

waters reaching equilibrium with respect to carbonate.

Once equilibrium was attained, and meteoric waters mixed with marine phreatlc waters, grain stabilization by

inverslon-recrystal1izatlon became effective. The leach

ing of grains 1 n the shoal unit supplied the extra car

bonate necessary for matrix dolomltlzatlon. 137

Intergranular Diagenesls

Matrix Diagenesis

Recrystallized and replaced lime mud matrix 1s es sentially absent from the beach upper offshore and fore shore units. Mlcrite and fine dolomite constitute the matrix of the shoal beds, whereas microspar, pseudospar, and medium-coarse dolomite are the prevalent matrix types

1 n the lagoonal sequence. Again, dolom 1t 1zat1on and re crystal 1 1 zatlon are Inferred to have taken place 1 n the mlxed-water or 1on diffusion zone which separated the strandllne and regional fresh ground water lenses from marine Interstitial fluids. The presence of abundant In verted aragonite grains 1 n the lagoonal sediments, and detrltal dolomite 1 n the shoal and lagoonal sequences, are thought to have acted as activation sites for matrix re crystal 1 1 zatlon and dolom1 1 1 zat1 on, respectively.

Early 111h1fIcatlon took place 1n the nodules, can nonball concretions and nodular beds of the lagoonal sec tion (Appendix Dl). This 11th1fIcatlon was Induced by the decomposition of organlcs within the fecal pellet rich lime mud sediment; calcium, and possibly ammonium, re leased from the decaying organic materials (Berner, 1968) set up local supersaturated conditions In which calcium carbonate was precipitated. Some of these centers of pre cipitation continued to expand outward and resulted 1 n the 138

formation of cannonball concretions. L1th1f1cat1on was

early as is made evident by differential compaction around

nodules; 1 t was also pre-dolom1t 1 zat1 on as indicated by minor replacement of the microspar and pseudospar matrix

of the nodules.

Cement Diagenesis

Cement distribution 1n the Cow Creek Limestone 1s predictable from the conceptual deposltlonal model. Total cement, cement size, and frequency of f 1brous-bladed ce ments all increase upward into the foreshore beach beds.

Intraclasts of beachrock, bound by fibrous and mlcrltlc cements, are commonly encountered, but only two outcrops contain sands that were cemented 1 n the Intertidal zone.

Mlcrite and dolomlcrlte cements, probably of organic o ri gin, bind the uppermost beach sands that are overlain by the "marine" fung-algal caliche.

The decrease In frequency of bladed sparry cements downward from the foreshore Into upper offshore sediments is attributed to either the absence of nucleatlon sites, or magnesium or organic Inhibition of low Mg calclte nu cleatlon; the higher 1 on1c strength (and therefore possible undersaturation) of mixed fresh-saline waters, 1 n the beach phreatlc zone may have also played a role. The In crease In the amount of ferroan-ca 1 d t e downward through the sequence may be a clue to the approximate position of 139

the early ground water table.

The homogeneous mlcrocrystal11ne cements of the

"marine" fung-algal caliche originated through the dissolu tion of aragonite grains, and redistribution of this car bonate as cement; other more diagnostic "marine" caliche cements are algal and fungal 1n origin. The decomposition of encrusting algal and fungal tubules 1n confined, anaero bic microenvironments resulted 1 n the precipitation of high

Mg calclte, which was later converted to dolomite; decompo sition in aerated microenvironments gave rise to low Mg calc1 te .

Comparison with Other Beaches

The above dlagenetic modifications and trends com pare favorably with those described from other Recent and ancient beach sequences. Moore (personal communication,

1971) reports that mlcrltlzed and enveloped grains are more numerous In beach foreshore vs. offshore sediments In

Grand Cayman, B.W.I.; Perkins and Halsey (1971) note that grains bored by fungi and algae are concentrated 1 n both

Recent and Pleistocene strandllne deposits. Allen (1970) and Moore and Allen (1971) Identify the same distribu tional pattern of mlcrite envelopes In a study of another

Lower Cretaceous carbonate beach 1n Texas. Needless to say, the Intertidal cement types noted 1n the Cow Creek have also been reported from Innumerable Holocene 140

carbonate beach sequences throughout the tropical areas of the world (Moore, 1970; Shinn, 1969; Land, 1970; Taylor and Illlng, 1969; and others).

The distribution of subaerlally Induced dlagenetic changes 1n the Cow Creek beach and near-offshore lithified sands and muds are similar to those encountered 1 n the other studies of ancient carbonate beach strata. Again,

1n Allen's (1970) and Moore's and Allen's (1971) beach syntheses, 1 t was found that: a) the frequency of leached

(moldlc cavity filled ) allochems Increased upward through the sequence; b) grain Inversion fabrics predominate in the upper-lower offshore Intervals; c) both equant and bladed cements are better developed In the foreshore unit; d) Iron-free cements bind the foreshore sediments while ferroan-calc 1 tes are found In the offshore sequence; and e) caliches formed 1n the backshore areas. In contrast to these findings, Land (1970) concludes that In the Pleisto cene Belmont beach sequence of Bermuda, grain Inversion and precipitation of abundant coarse cement occurred 1 n the meteoric phreatlc zone of the beach, whereas only minor, fine calclte spar was precipitated 1 n the meteoric vadose zone; apparently, grains did not undergo dissolution

1n either environment.

Regressive carbonate beach sequences are character ized by a unique vertical succession of sedimentary fades which develop 1n a unique geographic position. It has 141

also been shown that these sequences display markedly d if ferent dlagenetic modifications from base to top. The en vironments of deposition of these sediments Impose certain controls on the syngenesis and later diagenesis of the sequence. However, possibly even more Important 1n the gross diagenesis of these carbonate sequences, 1 s the ver tical arrangement of the sedimentary facies, and the posi tioning of these facies In respect to both the local strandllne hydrochemical flow system, and the more wide spread, regional post-depo$1tional flow system. It 1s these Interpretive aspects of the Hammett-Cow Creek study that are treated 1 n the concluding chapter. GENERAL DIAGENETIC-PALEOHYDROLOGIC CONCEPTUAL MODEL FOR MIDDLE TRINITY TIME

General Paleohydrology and Dlagenetic Environments

The early dlagenetic modifications suffered by the

Cow Creek Limestone and Hammett Shale necessarily occurred

1 n one of the hydrochemical dlagenetic environments given in the following chart.

I . Ma r1ne Grain mlcrltizatlon and A. Vadose Zone mlcrite envelopes develop. B. Phreatlc Zone Possible precipitation of high Mg calclte and ara gonite.

II, M1xed-Mar1ne or Fresh Grain mlcr1t1zation and Waters saturated with mlcrite envelopes develop. respect to dolomite (?) (Moore, per. comm., and/or calclte 1971); low Mg cald te and A. Vadose Zone dolomite form through pore B. Phreatlc Zone space precipitation, re placement, or Inversion.

III. Mixed Marine or Fresh Dissolution of marine and Waters undersaturated non-marine carbonates. with respect to calclte and/or dolomite A. Vadose Zone B. Phreatlc Zone

It 1s a relatively simple matter for the carbonate petrologlst working on Recent sediments and rocks to d if ferentiate between the results of processes acting under chemical circumstances I, II* III* but It 1s more

142 1 43

d ifficu lt to delineate the modifications which are peculiar to each A and B environment. Needless to say, the task of precisely defining "early" dlagenetic realms for ancient carbonates 1s often complicated by the loss of original or pre-existing mineralogies, textures, and fabrics. The Cow

Creek Limestone ahd Hammett Shale, however, still retain dlagenetic features which may offer an estimate of what happened to the sediments before deep burial.

The major dlagenetic modifications and their trends are one basis on which the construction of a dlagenetic- paleohydrologlc model rests. These modifications and trends must then be synthesized and explained within the context of known submarine diagenesis and Recent ground water flow systems 1 n beaches and 1ow-ly 1 ng coastal prov1 nces.

Subsea D1agenes1s

The subsea dlagenetic alterations to the Cow Creek and Hammett sediments include grain mlcr1tlzat1on, grain envelopes, Intertidal cementation, and 1 1th1 fIcation of lagoonal nodules and nodular beds. The development of a generalized model explaining the presence or absence of these modifications 1 n an ancient carbonate sequence 1 s best left to those researchers studying Recent carbonate sediments for which the pertinent parameters (chemical, microbiological, and sedlmentologlcal can be more readily 144

identified and analyzed. It can be stated that submarine alterations to carbonate sediments are Induced primarily by organics (grain mlcr1 t 1zat1onf grain envelopes, nodule

1 1 thificatlon, and some Intertidal cementation); some intertidal cementation (fibrous and rhombohedra 1 cements) may be pure physlochemlcal precipitates, but organic con trols on pore microenvironments may be responsible for the chemical conditions effecting the precipitation. Deposi- tional rates, the amount of carbonate mud 1n a sediment, grain stabilization, and amount of abrasion are thought to be secondary controls which determine whether submarine diagenesis occurs, and 1f 1t does, whether or not a record of 1t 1 s preserved. More detailed discussions of each modification have been given 1 n the Individual sections concerned with submarine modifications.

Subaerial D1agenes1s

The specific dlagenetic data that are to be used 1n the formulation of the subaerlal dlagenetic model are:

1. The concentration of dolomite in the lower part of

the sequence and 1n certain lagoonal beds.

2. The Increase 1n percent leached grains, along with

the increase 1 n first generation bladed cements up

ward through the nearshore facies.

3. The concentration of recrystal 11zed fabrics below

the beach units. 145

4. The abundance of ferroan caldtes beneath the

beach sequence and their rarity within It.

5. The cal1ch1f1cat1on of the beach top.

Ground water systems which occupied the Hensel-Cow

Creek-Hammett sediments during Middle Trinity time existed on both the regional and local scales. Eventually, the meteoric waters from both of these systems met and mixed with marine Interstitial fluids 1n the Cow Creek Limestone and Hammett Shale. This mixing (or diffusion) of marine and fresh waters was an Important factor which controlled the distribution of early dlagenetic processes and pro ducts. Kohout (1960) has demonstrated 1n the field that the diffusion zone, which separates the fresh-water lens from marine Interstitial waters, 1 s a gradient 1n which sea water 1s gradually diluted. This dilution sets up a cyclic flow of waters wherein seawater 1 s continually drawn landward to replace diluted, less dense brackish waters which move upward and seaward through the sediments

(Cahill, 1967; Kohout, 1960) (Figs. 24, 25). Water in the zone between the fresh and salt water bodies is diluted either as a result of 1on1c diffusion between the two water masses, or mechanical water mixing caused by advances and regressions of the fresh-salt water contact during long or short term changes 1 n sea level or fresh-water discharge rates.

The subaerial dlagenetic history of the Cow Creek 1 46

Limestone and Hammett Shale can be divided Into two phases based on the magnitude (local and regional) of the fresh water flow systems (Toth, 1963) in which the diagenetic modifications took place.

The local fresh-water flow system paralleled the

Cow Creek beach and extended for only a short distance be hind it. The fresh water lenses 1n this system were re charged by rain water that fell on the topographically higher areas of the beach and backshore zones; the contacts of these lenses with saline pore waters were characterized by mechanical water mixing or 1on1c diffusion (Kohout,

1960). This system, plus the dlagenetic processes and modifications that took place within 1t, is termed the

"Cow Creek Strandllne Phase." The regional ground water system extended to the

Llano hinterland and occupied the Middle Trinity sequence which underlaid the Hensel alluvial plain; this system, and the alterations effected by 1 t, 1 s referred to as the

"Post Cow Creek Strandllne Phase." The regional system was certainly recharged by meteoric ground waters which flowed through the Paleozoic carbonates of the Llano Up l i f t and Infiltrated downward through the Hensel Sand. 147

Cow Creek Strandllne Phase

The strandllne flow system and dlagenetic model for

the Cow Creek Beach 1s diagrammed 1n Figure 24. Topo

graphically higher beach ridges and backshore washover

fans acted as recharge areas for local fresh-water lenses

existing beneath them; because these areas underwent ca-

11 ch1 ficatlon, 1 t is inferred that they were, at most,

only sparsely vegetated. Rain water percolating into the

beach sediments gradually reached carbonate saturation

condition by dissolution of aragonite grains. During

periods of rainfall, some water continued percolating

downward through the oxidizing vadose zone and was added

to the local ground-water reservoir (Fig. 24), while in

dry seasons water was drawn upward as a result of capil

lary action induced by evaporation on the beach ridge sur

face. During this upward movement, the vadose waters were

heated, CQg driven off, and the excess carbonate from

leached grains precipitated as iron-free bladed and micro crystalline cements 1 n the beach and caliche, respectively

(Fig. 24).

Infiltrating vadose meteoric water which arrived at the ground water table Intermingled with mixed fresh- marine phreatlc waters (F1g. 24). The phreatlc waters were almost saturated with respect to aragonite and caused grains to Invert and recrystalllze rather than dissolve

(Purdy, 1968). The chemistry of these mixed waters could -300-1200 fw»

Marin*" Funa-algal Supra tidal t*®l

Fmh-Muwd

® HjOZona •j SI-

and Mixing

1 ARAGONITE GRAIN DISSOLUTION: PPT. Of MICRITE - DGEAMUCRITE CEMENTS; ZONE OP HIGH FUNGAL - ALGAL ACTIVITY

2 ARAGONITE GRAIN DISSOLUTION: PPT. OF BLADED, LON MG CALCITE CEMENTS (PB) AND HOLDIC CAVITY FILLINGS (PJ)

] SAME AS 2 BUT TO LESSER DEGREES: MORE GRAINS STABILIZED BY INVERSION IN^ THAN DISSOLUTION - REPRECIPITATIOM (P.)

4 HEOMORPHISA (N1l OF ARAGONITE GRAINS; MINOR PRECIPITATION OP BLADED LON MG CALC ITE cm nrrs AND eouart CEMENTS (PE)

J DOLOM1TIZATIOM

4 NICRITIZATION AND FUNG - ALGAL HICRITE ENVELOPE FORMATION

7 PRECIPITATION or INTERTIDAL ARAGONITE AND HIGH MG CALC ITE CEMENTS

Figure 24. COW CREEK STRANDLINE DIAGENETIC PHASE -t* CD 1 49

have caused not only grain Inversion, but also matrix In

version and less precipitation of calcite cement. Berner

(1966) and Garrels and Christ (1965, p. 106) show that

MgC^ solutions increase the solubility of calcite; Berner

also indicates that magnesium in sea water inhibits inver

sion of aragonite to calcite. Magnesium concentration might have been high enough to slow, not stop, the inter

stitial precipitation of low Mg calcite 1n these mixed

waters, but at the same time 1t was sufficiently low to

allow inversion (but not solution) to proceed. Less mag

nesium existed 1n vadose meteoric solutions, and for this

reason, plus the reasons given above, solution and pre

cipitation processes acted more freely.

The concentration of magnesium by beach microflora

(Gebeleln and Hoffman, 1971), and the upward withdrawal of

phreatic solutions during dry seasons could have supplied

sufficient magnesium to effect the partial dolom1tization

of the "marine" fung-algal caliche. This 1s not to say

that penecontemporaneous dolom1t1zat1on took place in

hypersaline solutions at the beach top; rather, it is

thought that the magnesium supplied was Incorporated into

high Mg calcites which were forming 1n low Eh pore micro

environments (see p. 124; also Malone and Towe, 1970).

Iron-free low Mg calcite cements were precipitated

locally as a result of calcium and carbonate being added

to the water during Inversion and grain dissolution 1 50

(Friedman, 1964; Pingatore, 1970). Whether the phreatlc zone in this strandline setting was an oxidizing or reduc ing environment 1s not known; Iron-free cements might have formed under any Eh in this situation if there was only a small amount of total Iron in the marine carbonate sands and locally mixed strandline waters (Krauskopf, 1967;

Billings and Ragland, 1969). Ferroan calcite cements have been reported from Holocene carbonates (Colley and Davies,

1969; Moore, personal communication, 1971), but in both of these occurrences, associated basic volcanic elastics probably supplied the iron In the carbonate cements.

Conditions were much different under adjoining supratidal marsh surfaces In comparison to those existing in the intertidal zone and beach ridges. Ground water in the supratidal areas was saline or m1xed-fresh-saline and the water table stood very close to the land surface

(Fig. 24). Water was supplied to these low-lying backshore areas by storm overwash and evaporative pumping

(Hsu, 1969).

Supratidal lakes contained lower salinity waters than did the marsh areas and therefore the lacustrine sediments contain only sparse dolomite (Shinn and others,

1969); however, because evaporation was more intense on the muddy marsh surface, magnesium was sufficiently con centrated here to bring about dolom1tlzation. During periods of extreme aridity the lakes and marshes dried up, 151

and the deposits were cemented and brecclated through ca-

1ich if1ca11 on processes (Ward and Folk, 1 970).

The abundant organic matter 1n the marsh and lake sediments lower the Eh of the water in the supratidal en vironment. This reduced and hypersaline water refluxed downward into the underlying beach sediments and mixed with iron-bearing water which had drained through the featheredges of the alluvial plain redbeds (Fig. 24).

Consequently, these waters precipitated ferroan calcite cement and inverted grains (rather than leached) in the foreshore sediments Immediately subjacent to the supra tidal deposits. As these waters continued to move seaward they mixed with waters beneath the beach ridges where they were oxidized, or precipitated a minor amount of ferroan calcite cement.

The subaerial strandline phase of diagenesis af fected only the upper portion of the Cow Creek-Hammett sequence because the local meteoric water lenses were of limited extent 1n both the horizontal and vertical dimen sions. According to the Ghyben-Herzberg principle, for every foot of fresh water above sea level, the thickness of the fresh water lens floating on salt water of ocean water density 1s about forty feet. Because of the gradual decrease in grain size and sorting (and thus permeability) from the beach downward into the offshore sediments, and because of tidal fluctuations and water mixing in the zone 1 52

of diffusion, the fresh water lens 1s expected to be some what less than predicted (Kohout, 1960; Back and Hanshaw,

1970). Thus, assuming a water table at or near sea level

(i.e ., 0-1 foot above sea level), the fresh water lens in

the Cow Creek beach only extended, and Induced subaerial diagenesis, to approximately 20 feet below sea level.

Therefore, the diagenesis affecting the facies subjacent

to the beach foreshore interval must have been the result of processes acting In a more extensive ground water body.

This aspect of Cow Creek-Hammett diagenesis, the post-Cow

Creek strandline phase, 1s considered next.

Post-Cow Creek Strandline Phase

The chemical relationships and flow patterns for the strandline phase of dlagenesis are necessarily over simplified because of the lack of published data on simi

lar Recent systems. It serves its purpose, however, 1n

pointing out the fact that the various aspects of early

subaerial diagenesis in a beach sequence are intricately related, and can be explained 1n terms of localized beach ground waters equilibrating with surrounding mineralogies.

The simplified regional flow system (post-Cow Creek

Strandline phase), contrasts with the strandline system, firs t, 1n having originated 1n carbonate rocks other than those under concern, and secondly, 1n affecting the lower units of the Cow Creek sequence. Owing to the former, 1 53

assumptions must be made regarding the quality of the fresh water passing through the rocks of the Llr.no recharge area:

1. The pH of waters entering the exposed Paleozoic sedimentary rocks was only slightly acidic (pH 5-6). Due to the arid to sem 1 -ar1d climate, vegetation was sparse and soil veneers were relatively free of decaying organics; infiltrating waters were probably 1n near equilibrium with atmospheric ant^ thus only slightly acidic* As these waters percolated downward, and laterally (Fig. 24) Paleo zoic limestone and dolomites, and possibly the Sycamore

Conglomerate, underwent dissolution; as a result the pH of the solutions gradually rose.

2. The lack of vegetation and soil organics, plus the probably rugged terrain (I.e ., the presence of cobbles and pebbles in the Hensel alluvial channels) in the re charge area, resulted 1n the oxidation of vadose and near surface phreatic waters. The Eh of these waters was gradually lowered as dissolved oxygen was used up during the oxidation and hydrolysis of minerals containing fer- + 2 rous iron and other reduced ions or atoms (i.e ., Mn ,

Cu+^) (Garrels and Christ, 1965). By the time these re gional ground waters entered the Cow Creek and Hammett

Units they undoubtedly had a negative Eh. S till, oxidized ground waters may have been present 1n the beach sequence as a result of localized Infiltration through the Hensel alluvial plain sediments. 1 54

3. Paleozoic carbonates 1n the Llano region are predominantly dolomite. Waters passing through these dolo mites should have contained calcium and magnesium in the same proportion (1:1) as it exists in pure dolomite (Meis- ler and Becher, 1966). It is doubtful, however, that down- gradlent waters could reprec 1p1tate dolomite, because dis solution of limestones and other calcium sources in the recharge area would lower the Mg/Ca below the level neces sary for dolomite replacement or precipitation. These waters probably became supersaturated with respect to cal cite during their seaward flow; calcite was precipitated and the Mg/Ca of the solutions rose as a result of the removal of calcium. This chemical change apparently takes place 1n the Florida aquifers (Back and Hanshaw, 1966) where ground waters first become saturated with calcite and then, further down the aquifer, Mg/Ca increases to levels greater than one and the solutions are supersatu rated with respect to dolomite. Back and Hanshaw (1966) and Hanshaw and others (1971) report that dolomitlzatlon can be effected by ground waters with a Mg/Ca^l .

Ground water in the Cow Creek-Hammett units could have existed under confined (artesian) or unconfined (water table) aquifer conditions. The clay and mudstone beds in the lowermost (Hammett) and uppermost (Hensel) parts of the sequence probably impeded the flow of water, but it cannot be stated that they acted as completely Impermeable 155

barriers because intercalated sands, thicker and more abun dant near the embayment margins (Fig. 4), probably allowed

the passage of some water. At the very least, the clay-

stone and mudstone beds acted as semipermeable membranes

through which select ions diffused; this, however, is a complicating factor in the development of a simplified model and will not be delved into further. It 1s logical to conclude, therefore, that the regional flow system operated under nonconfined, water table conditions or, alternatively, under very leaky (partially-confined) arte sian circumstances.

The post-Cow Creek Strandline hydrochemical system, and attendant diagenetlc alterations, are depicted 1n

Figure 25. The following discussion 1s limited to the de velopment of a dolomit 1zat1on-recrysta 111zatlon model for

the Hammett and Cow Creek units (the final cementation of the Cow Creek beds that 1s thought to have taken place in this system has already been discussed, p. 118). Because of the arid climate, it is assumed that salt water Intruded

inland for some distance, but exactly how far is not known.

A large circulating cell of salt water, separated from the fresh water by a brackish water zone, existed as 1n the strandline flow system. Circulation within this cell was slower than 1n the local beach cells because of the lower permeabilities of the fine-graln offshore sediments.

As the waters from the dolomittc recharge area 5 20 miles

Mg/Ca £ l Mg/Ca « 1 Mg/Ca 21

Lower Po U o i o k Carbonates

Sycamore Conglomerate I PPT. OF IRON FREE CALCITI CEMENTS ! PPT. OF FERROAN CALCITE lEMLNTS 3 DOLOMITIZATION 4 MEONORPHISM OF MATRIX AND ALUXTHEMS

Figure 25. POST COW CREEK STRANDLINE DIAGENETIC PHASE

on O 1 57

entered the region of salt and fresh water mixing, major chemical adjustments began to take place. Marine inter stitial waters in the semi-compacted lime muds (the ones that would be dolomitized) of the Hammett and lower Cow

Creek shoal beds started to undergo dilution with fresh waters (Fig. 25). The activity coefficients of magnesium and calcium in the marginal marine interstitial waters rose; coneurrent 1y, the activity coefficients of these ions in the adjacent "dolomitic" fresh waters were being lowered. Due to water intermingling (or 1on diffusion) activities of magnesium and calcium would decrease with respect to the unmixed interstitial marine water.

Assume that calcium and magnesium concentrations 1n the Trinity Group fresh waters were 100 ppm and 50 ppm

(molal Mg/Ca * .833) respectively (Hennlngsen, 1962), and, for normal, marine water, Ca * 400 ppm and Mg = 1300 ppm

(molal Mg/Ca=5/1) (Krouskopf, 1967). With these figures, marine Interstitial fluids could be diluted eighteen times over with fresh ground waters before the mixture would fall below the Mg/Ca - 1 necessary for dolomit1zatIon to proceed in carbonate saturated fresh waters (Back and Hanshaw,

1966). Some coastal ground waters of Yucatan, for example, are supersaturated by a factor of 10 with respect to dolo mite (Back and Hanshaw, 1970). Land (1970) presents carbon and oxygen Isotope data for dolomltized Pleistocene car bonates from the north coast of Jamaica which are 1 58

Indicative of replacement under meteoric conditions.

Lateral pulsations of the ground-water lens (caused

by changes 1n sea-level or climate) would shift the brack

ish water zone back and forth and act as a mechanism by which magnesium would constantly be supplied to the dolo- mitizlng system. The presence of clays and detrital dolo mite in the sediment could catalyze the dolomitlzing reac

tions in the clay-rich, noncompacted lime muds; replace ment did not take place 1n the Insoluble poor lime muds

because they had already compacted and lost most of their

porosity and permeability. Extra carbonate, needed for

the replacing dolomite, was derived from locally dissolv

ing shell fragments and brought 1n by mixed waters (Moore,

1960).

As the mixed, less dense waters were forced upwards

(F1g. 25) through the sequence, recrystallization of both allochems and matrix took place in the shoal and lagoonal

beds because of the same basic mechanism, supersaturation with respect to calcite, presented in the strandline d 1a-

genetic phase (p. 147). Some selective dolomitization occurred after matrix recrystal 11zat1on 1n units contain

ing abundant detrital dolomite. Fluids were still satu

rated with respect to dolomite, but, either because of the

kinetics of nucleatlon or the large size of the crystals

1n the recrystal 11 zed lime muds (Murray, 1960), only cer

tain beds were dolomltized. Detrital dolomite acted as 1 59

activation (nucleatlon) sites for the growth of dolomite from solutions, which by now were only slightly super saturated with respect to dolomite. The state of super saturation was probably insufficient to effect spontaneous replacement because the kinetic energy necessary for nu cleatlon was not high enough. Thus, only beds containing very numerous seeds (detrital dolomite) underwent signifi cant dolomite replacement. CONCLUSIONS AND IMPLICATIONS

The Cow Creek Limestone has long been recognized as a prograding beach complex. However, the detailed analy sis of its constituent lithosomes, and those of the super jacent Hensel Sand and subjacent Hammett Shale, has led to a more precise definition of the environments represented and a better understanding of their interrelationships.

As in similar stratigraphlc analyses, once the depositlonal regimes are deduced from petrologic evidences, and these

1n turn are viewed in relation to their stratigraphic framework, the construction of a depositlonal model is feasible. Ultimately, the sequdnce's depositlonal history can be interpreted on a sound basis.

1. The upper surface of the Sycamore Conglomerate was

covered by overlapping basal Hammett sandstones,

claystones, and clay-rich carbonates. These lltholo-

gies were deposited during a marine transgression

and represent the detritus derived from the denuded

Llano source area.

2. As clastic influx continued to diminish, carbonate

environments came Into existence 1n the quiet open

marine areas. Effective biological trapping

mechanisms were responsible for the deposition of

160 161

the Intercalated clay-rich lime wackestones and

clay-poor lime packstones. The lime wackestones

were dolomitized soon after deposition.

3. Synchronous with the deposition of these deeper,

quiet water lime muds, a sequence of shallow, near

shore marine carbonates and alluvial plain sedi

ments began prograding eastward.

4. The shallow offshore to strandline carbonate units

consist of dolomitized oyster packstones, deposited

1n an oyster bank environment; the tidal shoal se

quence 1s composed of burrowed arenaceous 11me pack

stones and sorted lime gralnstones; lagoonal and

inner shoal units are coarsely crystalline dolo

mites, recrystallIzed fossl 1 1 ferous quartz arenite,

and pellet 1 1me packstone nodules and nodular beds;

oyster-mol 1usk lime gralnstones ana packstones make

up the beach foreshore accretion beds and upper

offshore festoon cross-beds.

5. The uppermost foreshore sediments underwent callchi-

flcation soon after deposition while the foreshore

was s till under the influence of marine waters.

6 . Occasionally, supratidal salt marshes and hyper

saline (?) lakes developed 1n the backshore areas

of the beach. Some of these dolomitized supratidal 162

sediments and lake deposits were cal 1ch1f 1ed con

temporaneously with the alteration of the uppermost

foreshore units.

7. Red mudstones and channel sandstones and conglomer

ates, deposited on the Hensel alluvial plain,

covered the backshore and beach sediments as the

strandline continued to advance seaward.

8 . Ultimately, sediment progradation slowed to the

point where 1t could not keep pace with shelf sub

sidence, and marine transgression ensued. The upper

part of the continental Hensel deposits are 1n

fades with overlapping basal Glen Rose 1ntert1dal-

supratidal dolomites.

9. The caliches in the upper beach and 1n the alluvial

sequence, plus the abundance of smectite 1n alluvial

plain sediments Indicate that the Middle Trinity

climate in this area was sem1 -ar1d to arid.

In light of the depositlonal model, the upper

Sycamore to Lower Glen Rose Interval Is interpreted

as representing a regressive progradat 1onal package

of continental (Hensel) and marine (Cow Creek-

Hammett) sediments bounded by marine transgress 1ve

"unconformities." Previously, a regional uncon

formity, representing a significant time hiatus, 163

was Inferred to be present at the base of the marine

caliche. Deductions based on the diagenetic and

sed1mentol og 1 ca 1 data of this study negate the pres

ence of this unconformity, thus making the Hensel

deposits a facies equivalent of the Cow Creek Beach.

The application of this depositlonal model to other

Texas Lower Cretaceous sequences would lead to a better understanding of the Interrelationships between represented environments, and more logical pa 1eogeograph 1c reconstruc tions and depositlonal histories.

The dlagenetlc Interpretations given below are a partial verification of the "early" 1 1 1h1fIcation of car bonates, but more Importantly, they suggest that many diagenetlc modifications are related one to another. These findings are also taken as a test of the hypothesis that shallow, carbonate diagenesis 1s Intrinsically controlled by both depositlonal processes functioning within Indi vidual llthotopes, and early, nearsurface marine, mixed marine, and fresh water systems. The analyses of Allen

(1970) and Moore and Allen (1971) also lend substantiation to this hypothesis, and the dlagenetlc history of the Cow

Creek Limestone 1s 1n Itself a further confirmation. Veri fication of this hypothesis from other carbonate sequences

1s necessary; It must be understood, however, that such studies must not only be studies 1n diagenesis but also studies of regional paleogeographles and depositlonal 1 64

hi stori es,

1. The occurrence of mlcrltlzed grain envelopes and

nodules reflects the varying effect of organics and

microflora within the deposltional environment.

Subsea grain micrltlzatlon and envelope development

proceeded most efficiently within the well-sorted

beach sands. The absence of mud within a sediment,

plus the rate of deposition, are controlling fac

tors 1n the distribution of the bacteria, algae,

and fungi responsible for these grain modifications;

sediment stabilization and Intensity of abrasion may

determine whether mlcrlte envelopes ever form, or

are preserved. Moore (personal communication,

1970) and Allen (1970) have also found that these

two alterations are most pronounced 1n beach vs.

offshore equivalent sediments.

2. Cement distribution patterns are 1n part predictable

from the beach depositlonal model, but substrate

type, water movements, and water chemistry must

also be taken Into account.

3. The early marine-fresh water flow and chemical sys

tems operating within a beach and Its backshore

subenvironments (lake, salt marsh, caliche) are

Intimately related to topography, prevailing climate,

and the textural and mlneraloglcal attributes of the sediments. Because topography and sediment d is tri

bution are controlled by depositlonal processes, much of the early hydrochemical reg 1me--and the

strandline dlagenetlc phase--1s also. The Increase

upwards 1n the beach 1n frequency of moldic cavity

filled (Ps) aragonitlc grains and first generation

bladed cements bears a cause-effect relationship which 1s set up by the local hydrology and the chemical equilibration of rain water in the fresh water vadose zone. In the phreatlc zone of the beach, or underneath the supratidal marsh surface, mixed (?) or supersaturated waters, al-ong with pos

sible ion Inhibition, caused aragonitlc grains to

Invert and recrystal 1 1ze while curtailing cement

prec 1 pi tation .

The marine caliche formed on slight topographic

highs 1n the beach backshore through cementation

from rising vadose waters, and fungal and algal

activity. The packstone to wackestone gradation

upward through the caliche came about through desiccation, grain dissolution, and grain m icrlti-

zatlon. The contrast between the ferroan dolomlcrite cements and Iron-free mlcrlte and spar cements

1s the result of precipitation of high and low mag

nesium calcltes within Interstitial microenviron ments. 166

5. Semi-fluid clayed lime muds were Injected Into com

pacted clay-poor offshore lime muds due to loading

caused by progradation of the nearshore sequence

over them.

6 . Mixing of marine Interstitial waters with waters of

the regional flow system caused the selective dolo-

mitlzatlon of the semi- 1 1 th 1f 1ed clayey offshore

11me muds, during the post-Cow Creek strandline dla

genetlc phase. Lateral migrations of the zone of

diffusion led to a situation 1n which the magnesium

necessary for dolomit 1zat1on was continually sup

plied to the sediments.

7. During the upward flow of the less dense mixed

waters Inversion and recrystal 1 1zat1on of shallower

nearshore carbonate muds took place. These waters

were deficient 1n magnesium as a result of the

dolom11 1zat1 on of Hammett carbonates and, therefore,

only the lagoonal beds containing high percentages

of detrital dolomite seeds were dolomitized.

8 . Final calcite cementation of the beach sediments

took place under either reducing phreatlc and oxi

dizing vadose conditions, or 1n ground waters which

were oxidized near the water table and reduced at

some depth (10-20 ft) below sea level. The former

circumstance calls for a ground-water table below 167

sea level to explain the Iron-free (vadose) calcite cements in the upper part of the foreshore beds and the ferroan (phreatlc) cements at greater depths.

The subsea landward sloping ground-water table which existed in this situation 1s d ifficu lt to envision, even though similar hydrologic conditions have oc casionally been reported 1n Recent arid coastal areas. In the context of the Cow Creek-Hensel pa 1eogeography and climate, the landward sloping salt water or ground water table, rather than above sea level water table, 1s favored to account for the distribution of iron-free and ferroan cements encou ntered.

Validation of the explanations given for the dlagenetlc alterations studied may be obtained from carbon-oxygen Isotope studies of the dolomites and recrysta 11 ized lime muds; microprobe traverses of the cements for Fe+^, Sr+^, Mg+^, Na+^ might also refine the Inferences concerning their precipita tion environments. REFERENCES

Alexander. L. T., Hendricks, S, B., and Nelson, R. A., 1939, Minerals present in soil colloids, II, Esti mation in some representative soils: Soil Science, v. 48, p. 273-279.

Allen, J. R. L., 1963, Internal sedimentation structures of well-washed sandstones in relation to flow con ditions: Nature, v. 200, p. 326-327.

Allen, S. H., 1970, Stratigraphy and diagenesis of car bonate beach complexes, central Texas: Master's Thesis, L.S.U., 176 pp.

Amsbury, D. L., 1967, Caliche soil profiles in Lower Cre taceous rocks of central Texas (abst.): GSA Program 1967 Annual Meetings, New Orleans, La., p. 4.

Amsbury, D. L., 1962, Detrital dolomite in Central Texas: Jour. Sedimentary Petrology, v. 32, p. 5-14.

Arlstaraln, L. F., 1971, Clay minerals in caliche deposits of Eastern New Mexico: Jour, of Geology, v. 79, p. 75-90.

Back, W., Cherry, R. N., and Hanshaw, B. B., 1966, Chemi cal equilibrium between the water and minerals of a carbonate aquifer: Natl. Speleo. Soc. Bull., v. 28, no. 3 .

Back, W., and Hanshaw, B. B., 1970, Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan: Jour. Hydrology, v. ID, p. 330-368.

Ball, M. M., 1967, Carbonate sand bodies of Florida and the Bahamas: Journ. Sedimentary Petrology, v. 37, p. 556-591.

Barnes, V. £., 1951, Paleozoic and Cretaceous of Eastern Llano Uplift; in South Texas Geological Society Annual Field Trip Guidebook.

Bathurst, R. G. C., 1958, Dlagenetlc fabrics 1n some British D1nant1an limestone: The Liverpool and Manchester Geol. Journal, v. 2, p. 11-36.

168 169

, 1964, Diagenesis and Paleoecology, p. 357-376 Tn Imbrle, J. and Newell, N. D., eds., Approaches to Paleoecology; John Wiley and Sons, New York, N. Y. , 432 pp.

______, 1966, Boring algae, mlcrlte envelopes, and 111hi- fication of mollusk blosparites: Geol. Jour., v. 5, p. 89-109.

Bernard, H. A., LeBlanc, R. J., and Major, C. H., 1962, Recent and Pleistocene geology of Southeast Texas; 1n Geology of Gulf Coast and Central Texas, Guide book of Excursions: Houston Geological Soc., p. 175-224.

Berner, R. A., 1966, Diagenesis of carbonate sediments: Interaction of magnesium 1n sea water with mineral grains: Science, v. 153, no. 3732, p. 188-191.

Biscaye, P. E., 1964, Distinction between kaolinlte and chlorite in Recent sediments by X-ray diffraction: Am. Mineralogist, v. 49, p. 1287-1289.

Bischoff, J. L., and Fyfe, W. S., 1968, Catalyst, Inhibi tion, and the calc1te-aragon1te problem: Am. Jour. Sd . , v . 266, p . 65-79 .

Blank, H. R., and Tynes, E. W., 1965, Formation of caliche 1n situ: Geol. Soc. America Bull., v. 76, p. TTST-l 392.

Boone, P. A., 1968, Stratigraphy of the basal Trinity (Lower Cretaceous) sands of central Texas: Baylor Geological Studies Bulletin 15, 64 pp.

Brown, P. R., 1966, Pyrtt1zat1on in some mulluscan shells: Jour. Sedimentary Petrology, v. 36, p. 1149-1152.

Buchblnder, B., and Friedman, G. M., 1970, Selective dolomitlzatlon of m1cr1t1c envelopes: A possible clue to original mineralogy: Jour. Sedimentary Petrology, v. 40, p. 417-418.

Butler, G. P., 1970, Holocene gypsum and anhydrite of the Abu Dhabi Sabkha, Truclal Coast: an alternative explanation of origin, p. 120-1 52 1_n Rau, J. L., and Dellwlg, L. F ., eds., Third Symposium on Salt: Northern Ohio Geological Society, Cleveland, Ohio.

Cahill, J. M., 1967, Hydraulic sand model study of the cyclic flow of salt water 1n a coastal aquifer: U.S.G.S. Prof. Paper 575-B, p. 240-244. I 70

Castle, R. A., 1969, Stratigraphy and sedimento1ogy of Basal Cretaceous sediments, North-Central, Texas: Master's Thesis, L.S.U., 133 p.

Chanda, S. K., 1967, Selective recrystal 11zation 1n lime stones: SedImentology, v. 8 ., p. 73-76.

Chave, K. E., Oeffeyes, K. S., Weyl, P. K., Garrels, R. M., and Thomson, M. E., 1962, Observations on the solu b ility of skeletal carbonates 1n aqueous solutions; Science, v. 137, p. 33-34.

Chillngar, G. V., Blssell, H. J., and Wolf, K. H., 1967, D1agenes1s of carbonate rocks; 1n Diagenesis of Sediments (Larsen, G., and ChilTngar, G. V., eds.}: Elsevier, London, p. 179-322.

Damon, H. G., 1940, Cretaceous conglomerates on the east side of the Llano Uplift: Ph.D. Dissertation, University of Iowa, 92 pp.

Deffeyes, K. S., Lucia, F. J., and Weyl, P. K. , 1965, Dolom11 1zat1on of Recent and P11o-Plelstocene Sediments by marine evaporlte waters on Bonaire, Netherlands Antilles; 1_n Dol om111 za ti on and Lime stone Diagenesis, A Symposium (Pray, L. C., and Murray, R. C., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 13, p. 71-88.

Devine, S. B., 1971, M1neralog 1ca1 aspects of the surficial sediments of the deep Gulf of Mexico: Ph.D. Disser tation, L.S.U., 160 pp.

Dunham, R. J ., 1962, Classification of carbonate rocks ac cording to depositlonal texture; 1n Classification of Carbonate Rocks (Ham, W. E., edTJ: Am. Assoc. Petroleum Geologists Mem. 1, p. 108-121.

______, 1 969, Vadose Pisolite 1n the Capltan Reef (Per mian), New Mexico and Texas; 1n Depositlonal En vironments 1n Carbonate Rocks~TFr1edman, G. M., ed .): Soc. Econ. Paleontologists and Mineralogists Spec. Pub. 14, p. 182-191 .

______, 1 970, Meniscus cement; 1_n Carbonate cements [Brlcker, 0. F., ed.), Johns Hopkins Press, Balti more, p. 297-300.

Dutcher, L. C., and Thomas, H. E., 1966, The occurrence, chemical quality and use of ground water 1n the Tabulbah Area, Tunisia: U.S.G.S. Water Supply Paper 1757-E. 171

Emery, K. 0., and Rlttenberg, S. C., 1952, Early diagenesis of California basin sediments 1n relation to origin of oil: Bull. Am. Assoc. Petroleum Geologists, v. 36, p. 735-806.

Evamy, B. D., 1963, The application of a chemical staining technique to a study of dedolomitization: Sed i men - tology, v. 2, p. 165-170.

, 1969, The prec1p1tatlonal environment and corre- Tation of some calcite cements deduced from a r t i f i cial staining: Jour. Sedimentary Petrology, v. 39, p. 787-792.

Fischer, I. S., 1968, Interrelation of mineralogy and tex ture within an Ordovician Lexington Limestone sec tion In Central Kentucky: Jour, of Sedimentary Petrology, v. 38, p. 775-784.

Folk, R. L., 1962, Spectral subdivision of limestone types; 1n Classification of Carbonate Rocks (Ham, W. E., ed\): Am. Assoc. Petroleum Geologists Mem. 1, p. 62-84.

______, 1 965, Some Aspects of recrystallization 1n an cient limestones; j_rt Doloml t1 zatlon and Limestone Diagenesis, A Symposium (Pray, L. C., and Murray, R. C., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 13, p. 14-48.

______, 1968, Petrology of Sedimentary Rovks: Hemphill, Austin, Texas, 170 pp.

______, 1970, Eocene (Jackson) Moody's Branch Marl, Montgomery, La. (ll.T. # TC-851-TLa. ); 1n Carbonate Cements (Brlcker, 0. F., ed.): Johns Hopkins Press, Baltimore, p. 113-166.

Friedman, G. M., 1964, Early diagenesis and 1ithification in carbonate sediments: Jour. Sedimentary Petrology, v. 34, p. 777-813.

______, 1968, The fabric of carbonate cement and matrix and Its dependence on the salinity of water; 1_n Recent developments 1n carbonate sedlmentology 1n Central Europe (Mueller, G. and Friedman, G. M., eds.): Spr1nger-Verlag, New York, N.Y., p. 11-21. 172

Forgotson, J. M. , Jr., 1 956 , A correlation and regional stratigraphlc analysis of the formations of the Trinity Group of the Comanchean Cretaceous of the Gulf Coast Plain; and the genesis of the Ferry Lake Anhydrite: Gulf Coast Assoc. Geological Soc., v. 6 , p. 91-108.

Galehouse, J. S., 1969, Counting grain mounts: number per centage vs. number frequency: Jour. Sedimentary Petrology, v. 39, p. 812-184.

Garrels, R. M., and Christ, C. L., 1965, Solutions, Mine rals, and Equilibria: Harper and Row, New York, N. Y, , 450 pp.

Gavish, E., and Friedman, G. M., 1969, Progressive dia genesis 1n Quaternary to Late Tertiary carbonate sediments: sequence and time scale: Jour. Sedi mentary Petrology, v. 39, p. 980-1006.

Gebelein, C. D., and Hoffman, P., 1970, Algal origin of dolomite 1n 1nterlaminated 1 1mestone-dolomite sedi mentary rocks: 1_n Carbonate Cements (Bricker, 0. F., ed.), Johns Hopkins Press, Baltimore, p. 319-326.

G1le, L. H., and Hawley, J. W., 1966, Periodic sedimenta tion and soil formation on an alluvial-fan piedmont in southern New Mexico: Soil Sc1. Soc. Am. Proc., v. 30, p. 261-268.

Glnsburg, R, N., 1957, Early diagenesis and 1ithlfIcation of shallow water carbonate sediments 1n South Florida; 1n Regional Aspects of Carbonate Deposi tion (LeBTanc, R. J. and Breeding, J. G., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 5, p. 80-100.

Goldlch, S. S., and Parmalee, E. B., 1947, Physical and chemical properties of Ellenberger rocks, Llano County, Texas: Bull. Am. Assoc. Petroleum Geolo gists, v. 31, p. 1982-2020.

Golubic, S., 1969, Distribution, taxonomy, and boring pat terns of marine endollthlc algae: Am. Zoologist, v. 9, p. 747-751.

Graf, D. L., and Goldsmith, J. R.( 1956, Some hydrothermal syntheses of dolomite and protodolomite: Jour. Geology, v. 64, p. 173-186. 173

Grim, R. E., 1968, Clay Mineralogy: McGraw-Hill, New York, N.Y., 596 p.

______, Dietz, R. S., Bradley, W. F., 1949, Clay mineral composition of some sediments from the Pacific Ocean off the California Coast and the Gulf of California: Bull. Geol, Soc. Am., v. 60, p. 1785-1803.

Gross, M. G., 1965, Carbonate deposits on Plantagenet Bank near Bermuda: Bull. Geol. Soc. Am., v. 76, p. 1283-1290.

Hanshaw, B. B., Back, W. , and Deike, R. G., 1971, A geo chemical hypothesis for dolom1t 1zation by ground water: Econ. Geology, v. 6 6 , p. 710-724.

Harms, J. C., and Fahnestock, R. K., 1965, Stratification, bed forms, and flow phenomena (with an example from the Rio Grande); j_n Primary Sedimentary Structures and their Hydrodynamic Interpretation (Middleton, G. V., ed.): Soc. Econ. Paleontologists and Miner alogists Spec. Publ. 12, p. 84-115.

Hennlngsen, E. R., 1962, Water diagenesis 1n Lower Creta ceous Trinity aquifers of Central Texas: Baylor Geological Studies Bull. 3, 37 pp.

Hsu, K. J., and S1egenthaler, C., 1969, Preliminary ex periments on hydrodynamic movement induced by evapo ration and their bearing on the dolomite problem: Sedimentology, v. 12, p. 11-25.

Illlng , L. V., 1954, Bahamian calcareous sands: Bull. Am. Assoc. Petroleum Geologists, v. 38, p. 1-95.

, Wells, A. J., and Taylor, J. C. M., 1965, Pene- contemporary dolomite In the Persian Gulf; j_n Dolo- mltlzation and Limestone diagenesis, A Symposium (Pray, L. C., and Murray, R. C., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 13, p. 89-111.

Imbrle, J. and Buchanan, H., 1965, Sedimentary structures In modern carbonate sands of the Bahamas; in Primary sedimentary structures and their hydrody namic interpretation (Middleton, G. V., ed.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 12, p. 149-172. 1 74

Irion, G., and Muller, G., 1968, Mineralogy, petrology, and chemical composition of some calcareous tufa from the Schwabische Alb, Germany; iji Recent developments in carbonate sedlmentology in central Europe (Mul ler, G.f and Friedman, G. M., eds.): Springer- Verlag, New York, N.Y., p. 157-171.

Kahle, C. F., 1965, Possible roles of clay minerals in the formation of dolomite: Jour, of Sedimentary Petrol ogy, v. 35, p. 448-453.

Kendall, C. G. C., and Skipwlth, P. A. d‘ E., 1968, Recent algal mats of a Persian Gulf lagoon: Jour, of Sedimentary Petrology, v. 38, p. 1040-1058.

______, 1969, Holocene shallow water carbonate and evaporite sediments of Khor al Bazam, Abu Dhabi, southwest Persian Gulf: Bull. Am. Assoc. Petroleum Geologists, v. 53, p. 841-869.

Kinsman, D. J, J., 1969, Modes of formation, sedimentary associations, and diagnostic features of shallow- water and supratldal evaporites: Bull. Am. Assoc. Petroleum Geologists, v. 53, p. 830-840.

Kitano, Y., Kanamorl, N., and Tokuama, A., 1969, Effects of organic matter on solubilities and crystal form of carbonates: Am. Zoologist, v. 9, p. 681-688.

Klein, G. de V., 1965, Dynamic significance of primary structures 1n the Middle Jurassic Oolite Series, Southern England; 1_n Primary sedimentary structures and their hydrodynamic Interpretation (Middleton, G. V., ed.): Soc. Econ. Paleontologists and Miner alogists Spec. Publ. 12, p. 173-191.

Kohlmeyer, J., 1969, The role of marine fungi 1n the pene tration of calcareous substances: Am. Zoologist, v. 9, p. 741-746.

Kohout, F. A., 1960, Cyclic flow of salt water In the B1s- cayne aquifer of Southeastern Florida: Jour, of Geophysical Res., v. 65, p. 2133-2141.

Kornicker, L. S., and Purdy, E. G., 1957, A Bahaman fecal pellet sediment: Jour, of Sedimentary Petrology, v. 27, p. 126-128.

Krauskopf, K. B., 1967, Introduction to Geochemistry, McGraw-Hill, New York, N.Y., 721 pp. 1 75

Krumbeln, W. E., 1968, Geomicrobiology and geochemistry of the "Nari-L1me-Crust" (Israel); 1n Recent develop ments 1n carbonate sedlmentol ogy~Tn Central Europe (Muller, G., and Friedman, G. M., eds.): Springer- Verlag, New York, N.Y., p. 138-147.

Land, L. S., 1967, Diagenesis of skeletal carbonates: Jour, of Sedimentary Petrology, v, 37, p. 914-930.

, 1 970, Phreatlc versus vadose meteoric diagenesis of limestones: evidence from a fossil water table: Sedlmentology, v. 14, p. 175-185.

______, and Epstein, S., 1 970, Late Pleistocene diagene sis and dolom1tizatlon, North Jamaica: Sedlmen- tology, v. 14, p. 187-200.

Lindholm, R. C.f 1969, Detrital dolomite in Onondaga Lime stone (Middle Devonian) of New York: its Implica tions to the "dolomite question": Bull. Am. Assoc. Petroleum Geologists, v. 53, p. 1035-1042.

Logan, B. W,, Read, J. F., and Davies, G. R., 1970, History of carbonate sedimentation, Quaternary Epoch, Shark Bay, Australia; in Carbonate Sedimentation and En vironments, Sharlc- Bay, Australia: Am. Assoc. Petroleum Geologists Mem. 13, p. 38-84.

Lozo, F. E., and Stricklin, F. L., 1956, Strat1graphlc notes on the outcrop Basal Cretaceous, Central Texas: Trans. Gulf Coast Assoc. Geol. Soc., v. VI, p. 67-78.

Lucia, F. J., 1962, D1agenes1s of a crinoidal sediment: Jour, of Sedimentary Petrology, v. 32, p. 848-865.

Malone, Ph. G., and Towe, K. M., 1970, Microbial carbonate and phosphate precipitates from sea water cultures: Marine Geology, v. 9, p. 301-309.

Matthews, R. K., 1967, D1agenet1c fabrics 1n biosparites from the Pleistoceneof Barbados, West Indies: Jour, of Sedimentary Petrology, v. 37, p. 1147-1153.

______, 1968, Carbonate diagenesis equilibration of sedi mentary mineralogy to the subaerial environment; Coral Cap of Barbados, West Indies: Jour, of Sedi mentary Petrology, v. 38, p. 1110-1120. 176

Meisler, H., and Becher, A. E., 1967, Hydrologic signifi cance of Ca/Mg ratios 1n ground water from carbonate rocks 1n the Lancaster Quadrangle, Southeast, Penn sylvania: U.S.G.S. Prof. Paper 575-C, p. 232-235.

Miln, C., 1968, The hydro logic setting in Los Angeles County, California; i_n Proceedings of the limited professional symposium on salt-water encroachment into aquifers (May 4-5, 1967): Louisiana Water Resources Research Institute, La. State Univ., Baton Rouge, p. 127-145.

Moore, C. H., 1970, Recent Intertidal cements--the1r miner alogy, texture, and significance, Grand Cayman, British West Indies (abst.): Am. Assoc. Petroleum Geologists Program Annual Meeting, 1970, p. 861.

______, and Allen, S. F., 1971, Preliminary dlagenetic model of carbonate beach sequences (abst.): Am. Assoc. Petroleum Geologists Program Annual Meeting, 1971, p. 354.

Muller, G., and Irion, G., 1969, Subaerial cementation and subsequent dolomltlzatlon of lacustrine carbonate muds and sands from Paleotuz Golu ("Salt Lake"), Turkey: Sedlmentology, v. 12, p. 193-204.

Multer, H. G., and Hoffmelster, J. E., 1968, Subaerial laminated crusts of the Florida Keys: Bull. Geol. Soc. Am., v. 79, p. 183-192.

Murray, R. C., 1960, Origin of porosity in carbonate rocks: Jour, of Sedimentary Petrology, v. 30, p. 59-84.

______, and Lucia, F. 0., 1967, Cause and control of dolomite distribution by rock selectivity: Bull. Geol, Soc. Am., v. 78, p. 21-36.

Nagtegaal , P. J. C., 1 969a, Microtextures 1n Recent and fossil caliche: Leldse Geol. Mededel 1ngen, v. 42, p. 131-142.

______, 1969b, Sedlmentology, paleocl1matology, and diagenesls of the post-Hercynlan continental deposits 1n the South-Central Pyrenees, Spain: Leldse. Geol. Mededel 1ngen, v. 42, p. 143-238.

Neumann, H. C., Gebeleln, C. D., and Scoffln, T. P., 1970, The composition, structure, and erodablllty of sub- tidal mats, Abaco, Bahamas: Jour, of Sedimentary Petrology, v. 40, p. 274-297. 1 77

Newell, N. 0., and Rigby, J. K. , 1 957 , Geological studies of the Bahama Bank; 1n Regional aspects of carbonate deposition (LeBl anc R". J. and Breeding, J. G., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 5, p. 15-72.

, Purdy, E. G., and Imbrie, J., 1960, Bahamian oolitic sand: Jour. Geol., v. 6 8 , p. 481-497.

Nicols, R. A. H., 1966, Petrology of an 1rregular-nodular bed, Lower Carboniferous, Anglesey, N. Wales: Geol. Magazlne, v. 103.

Oborn, E. T., 1960, A survey of pertinent biochemical literature: U.S.G.S. Water Supply Paper 1459-F, p. 111-190.

______, 1960, Iron content of selected water and land plants: U.S.G.S. Water Supply Paper 1459-G, p. 191-211.

Orme, G. R., and Brown, W. W. M., 1963, D1agenet1c fabrics in the Avonlan limestones of Derbyshire and North Wales: Proc. Yorkshire Geol. Soc., v. 34, p. 51-66.

Owen, L. W., 1968, The challenge of water management: Orange County water district; i_n Proceedings of the limited professional symposium on salt-water en croachment Into aquifers (May 4-5, 1967), Louisiana Water Resources Research Institute, La. State Univ., Baton Rouge, p. 105-122.

Parry, W. T., and Reeves, C. C., Jr., 1968, Clay mineralogy of pluvial lake sediments, southern High Plains, Texas: Jour, of Sedimentary Petrology, v. 38, p. 516-529.

Perkins, B. F., 1971, Traces of rock-boring organisms in the Comanche Cretaceous of Texas; i_n Trace Fossils, a field guide to selected localities; in Pennsyl vanian, Permian, Cretaceous, and Tertiary rocks of Texas and related papers (Perkins, B. F., ed.), p. 137-148, LSU School of Geosc. Misc. Publ. 71-1.

Perkins, R. D., and Halsey, S. D., 1971, Geological sig- nificane of microboring fungi and algae 1n Carolina shelf sediments: Jour, of Sedimentary Petrology, v . 41, p. 843-853. 1 78

Pingatore, N., Jr., 1970, Diagenesis and porosity modifica tion 1n Aeropora palmata, Pleistocene of Barbados, West Indies: Jour, of Sedimentary Petrology, v. 40, p. 712-722.

Plummer, F. B., 1950, the Carboniferous rocks of the Llano region of central Texas: Univ. Texas Publ. 4329, 170 pp. Potter, P. E., 1967, Sand bodies and sedimentary environ ments: a review: Bull. Am. Assoc. Petroleum Geologists, v. 51, p. 337-365.

Purdy, E. G., 1968, Carbonate diagenesis: an environmental survey: Estratto da Geologlca Romana, v. VII, p. 183-228. Puri, H. S., and Collier, A., 1967, Role of microorganisms in formation of limestones: Gulf Coast Assoc. Geological Soc. Trans., v. XVII, p. 355-367.

Reeves, C. C., Jr., 1970, Origin, classification, and geo logic history of caliche on the southern High Plains, Texas and eastern New Mexico: Jour. Geol., v. 78, p. 352-362.

Ruhe, R. V., 1967, Geomorphlc surfaces and surficial de posits 1n southern New Mexico: New Mexico Insti tute of Mining and Technology Mem. 18, p. 1-66.

Runnells, D. D., 1969, D1agenes1s, chemical sediments, and the mixing of natural waters: Jour, of Sedimentary Petrology, v. 39, p. 1188-1201.

Rutle, E., 1958, Kalkkrusten 1n Spanlen: Neues Jb. Geol. PalSont., Abh., 106, p. 52-138.

Schmaltz, R. R., 1970, Beachrock formation on Enlwetok Atoll: 1n Carbonate Cements (Bricker, 0. F., ed. ), Johns HoplTlns Press, Baltimore, p. 17-24.

Schroeder, J., Dwornlk, E., and Paplke, J. J., 1969, Pri mary protodolomite 1n echinoid skeletons: Bull. Geol. Soc. America, v. 80, p. 1613-1616.

Scoffin, T. P., 1970, The trapping and binding of subtldal carbonate sediments by marine vegetation in Bimini Lagoon, Bahamas: Jour, of Sedimentary Petrology, v. 40, p. 249-273. 179

Sel1ards, E. H., Adkins. W. S., and Plummer, F. B., 1966, The geology of Texas (vol. 1, Stratigraphy), Univ. of Texas Press, Austin. 107 pp.

Shinn, E. A., Ginsburg, R. N., and Lloyd, R. M., 1965, Recent supratldal dolomite from Andros Island; j_n Dolomlt1zat1on and Limestone Diagenesis, A Sympo sium (Pray, L. C., and Murray, R. C., eds.): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 13, p. 112-123.

Shinn, E. A., 1968, Practical significance of blrdseye structures In carbonate rocks: Jour, of Sedimentary Petrology, v. 38, p. 215-223.

______, 1968, Selective dolom1 1 1zat1on of Recent sedi mentary structures: Jour, of Sedimentary Petrology, v. 38, p. 612-617.

______, 1969, Submarine 11thlfIcatlon of Holocene carbo nate sediments 1n the Persian Gulf: Sedlmentology, v. 12, p. 109-144.

, Lloyd, R. M., and Ginsburg, R. N., 1969, Anatomy of a modern carbonate tidal flat, Andros Isl.: Jour, of Sedimentary Petrology, v. 39, p. 1202-1228.

Shinn, E. A., 1968, Burrowing 1n Recent 11me sediments of Florida and the Bahamas: Jour. Paleontology, v. 42, p. 879-894.

Simons, D. B., Richardson, E. V., and Nordln, C. F., Jr., 1965, Sedimentary structures generated by flow 1n alluvial channels; 1_n Primary Sedimentary Struc tures and their Hydrodynamic Interpretation (Middle ton, G. V., ed. ): Soc. Econ. Paleontologists and Mineralogists Spec. Publ. 12, p. 34-52.

Stanley, 0. J., Swift, D. J., and Richards, H. G., 1967, Fossl11ferous concretions on Georges Bank: Jour, of Sedimentary Petrology, v. 37, p. 1070-1083.

Stanton, R. J., Jr., and Uarme, J. E., 1971, Stop 1: Stone City Bluff; 1_n Trace Fossils, a guide to selected localities 1n Pennsylvanian, Permian, Cretaceous, and Tertiary rocks of Texas and related papers (Perkins, B. F., ed.), p. 2-10, LSU School of Geosc . M1sc. Publ. 71-1. 130

Stehll, F. G., and Hower, J., 1961, Mineralogy and early diagenesis of carbonate sediments: Jour, of Sedi mentary Petrology, v. 31, p. 358-371.

Stricklin, F. L., and Smith, C. 1., 1968, Limestone beach deposits in the Lower Cretaceous (Aptian) of Cen tral Texas--environmental reconstruction (abst.): Geol. Soc. Program 1968 Annual Meeting, p. 291-292, Mexico City.

______, Smith, C. I., and Lozo, F. E., 1971, Stratig raphy of Lower Cretaceous Trinity deposits of cen tral Texas: Univ. Texas, Bureau Econ. Geology Rept. Investigations 71, 63 pp.

Suess, E., 1970, Interaction of organic compounds with calcium carbonate--I. Association phenomena and geochemical Implications: Geochimlca et Cosmo- chlmica Acta, v. 34, p. 157-168.

Swinchatt, J. P., 1967, Formation of large-scale cross bedding in a carbonate unit: Sedlmentology, v. 8 , p. 92-120.

______, 1969, Algal boring: A possible depth indicator Tn carbonate rocks and sediments: Bull. Geol. Soc. Am. , v. 80, p. 1391 -1 396.

Taylor, B. J., 1971, Thallophyte borings 1n phosphatic fos sils from the Lower Cretaceous of southeast Alex ander Island, Antarctica: Paleontology, v. 14, p. 294-302.

Taylor, J. C. M., and Illlng, L. V., 1969, Holocene inter tidal calcium carbonate cementation, Qatar, Persian Gulf: Sedlmentology, v. 12, p. 69-107.

Tebutt, G. E. , 1967 , Diagenesis of Pleistocene limestone on Ambergris Cay, British Honduras: Ph.D. Disser tation, Rice Univ., 138 pp.

Tennant, C. B., and Berger, R. W., 1957, X-ray determina tion of dolomlte-calc 1te ratio of a carbonate rock: Am. Mineralogist, v. 42, p. 23-29.

T6 th, J., 1963, A theoretical analysis of ground-water flow 1n small drainage basins: Jour. Geophysical Research, v. 6 6 , p. 4795-4812.

Trichet, J., 1967, Essal d'exp!1cation du d£pot d'aragonite sur les substrats organlques: Acad. Sc1. Comptes Rendus, S6 r. D, v. 265, p. 1464-1467. 101

Vischer, G. S., 1965, Fluvial processes as Interpreted from ancient and Recent fluvial deposits; 1_n Primary Sedi mentary Structures and their Hydrodynamic Interpre tation (Middleton, G. V., ed.): Soc. Econ. Paleon tologists and Mineralogists Spec. Pub. 12, p. 116-132.

Ward, W. C., 1970, Diagenesis of calcareous eolianites of the Northeastern Yucatan Peninsula, Mexico: Ph.D. Dissertation, R1ce Univ.

______, Folk, R. L., and Wilson, J. L., 1970, Blackening of Eol1an1te and calclte adjacent to saline lakes, Isla Mujeres, Quintana Roo, Mexico: Jour, of Sedi mentary Petrology, v. 40, p. 548-555.

Weyl, P. K., 1960, Porosity through dolom1tlzat1on- conservat1on-of-mass-requ1 rements: Jour, of Sedi mentary Petrology, v. 30, p. 85-90.

Wilson, R. C. L., 1967, Diagenetlc carbonate fabric varia tions 1n Jurassic limestones of southern England: Proc. Geol. Assoc., v. 78, p. 535-554,

Winland, H. D., 1968, The role of Hi-Mg calclte in the preservation of mlcrlte envelopes and textural fea tures of aragonite sediments: Jour, of Sedimentary Petrology, v. 38, p. 1320-1325.

Winters, E., and Simonson, R. W., 1951, The subsoil: Advan. Agron., v. 3, p. 2-92.

Wolf, K. H., 1965, "Gra 1 n-d1m1nut1on" of algal colonies to mlcrlte: Jour, of Sedimentary Petrology, v. 35, p. 420-427.

Wozab, P., and Williams, S., 1967, Flow problems in lime stone plains, Jamaica, W.I.--A progress report: International Assoc. Sc1. Hydrology Publ., v. 73, p. 402-409.

Zenger, D. H., 1965, Calcite-dolomite ratios vs. Insoluble content 1n the Lockport Formation (Niagaran) 1n New York State: Jour, of Sedimentary Petrology, v. 35, p. 262-265. APPENDICES APPENDIX A

PROCEDURES

Field Work

During the field season 1969-1970 approximately 600 samples were collected from 18 sections of the Hensel Sand,

Cow Creek Limestone, Hammett Shale, and uppermost Sycamore

Conglomerate. L1thofac1es were differentiated on the bases of lithology and sedimentary structures and thicknesses were determined by hand level and measuring tape.

Thin Section Analysis

Four hundred and fifty thin sections were prepared and stained with potassium ferrocyanlde according to the procedure outlined by Evamy (1963); of these, 180 were point counted; the remaining thin sections, mainly from the Hammett Shale interval, were classified according to texture and fabric (Dunham, 1967); the dlagenetlc modifi cations of each thin section were also noted.

Point-counted thin sections were first scanned at low magnification (x 1 0 ) to locate a line of traverse through a homogeneous part of the slide (I.e ., not bur rowed) thought to be representative of the lithology sampled. One hundred and seventy-five tallies were made

183 104

along this line in the following manner: grain types, plus their diagenetlc alterations, 1 f any, were noted once, re gardless of sl2 e, every time they formed an intersect with the cross-hairs; distinctly different patches of calcite or dolomite matrix were counted as units because of the fineness of the component crystallites; dissimilar cement fabrics, whether existing as one crystal or as crystal groups, were also counted as separate entitles. Thus, each grain and separable 1 ntergranular texture and fabric was noted once every time it occurred under the cross hairs.

This type of po'nt counting procedure is similar to the line method used on loose grain mounts, but differs slightly because some of the recorded units are groups of similar crystals rather than Individual crystals. The method yields a number frequency for the categories en countered; it underestimates the area of the larger units

{mainly grains) while also underestimating the number per centage for the smaller units (mainly cement and matrix types) (Galehouse, 1969). Nevertheless, this approach was more expedient than counting on a 1 arger-than-the- 1argest- graln-1nterval basis because It allowed for the collection of data on diverse cement types without necessitating the tailing of a prohibit1vely large number of points. 18b

Insoluble Residue Analysis

One hundred and fifteen specimens from the Hensel ,

Cow Creek, and Hammett Intervals were ground to a suitable size (^10 mm) for acid digestion. Ten grams of every crushed sample were weighed out, placed in 100 ml beakers, and treated periodically with aliquots of 15% glacial acetic acid until no noticeable effervescence could be perceived. Reslduums were then washed twice with distilled water, oven dried, weighed, and the percent insoluble resi due calculated for each sample.

Preparation of Standards for Bulk X-ray Analyses

Bulk sample calclte, dolomite, and quartz percen tages were determined by X-ray diffraction according to the method of Tennant and Berger (1957). In this method, the

Intensity (ruler heights) of the first order calcite

(3.03 A), dolomite ( 2 . 8 8 A), and quartz (3.35 A) peaks from individual samples are measured, peak he 1 ght-1 ntensity ratios calculated, and the ratio values compared to c a li bration curves. Three of these curves (calclte/dolomite; calcite/quartz; dolomite/quartz) were constructed from the

X-ray analyses of a rtific ia lly prepared mixtures. 136

Laboratory Procedure for Curve Construction

1. Individual portions of Iceland spar (low Mg calcite; CaC03), pure dolomite, and quartz crystals were ground for 20 minutes in an electric mortar and pestle.

Each was sieved and the <200 mesh fraction X-rayed to check for purity.

2. From the <200 mesh fractions, 10, 20, 40, 60,

80, and 90 weight percent mixtures of ca1cite-dolomite, calcite/quartz , and dolomite/quartz were prepared by vary ing the respective mineral proportions 1n a two-gram sample. Weighing was done on a Mettler electronic balance with a <.0001 gram accuracy. The mixtures were placed in containers and homogenized 1 n an electric shaker for 20 mi nu tes .

4. The samples were packed in aluminum holders,

X-rayed (at 2000 full scale count and time constant of five), and ruler helghts- 1 ntenslties of the major peaks measured. Values for the ratios dolomite/dolomite + cal clte, dolomite/dolomite + quartz, and calclte/calclte + quartz were calculated and plotted against percent dolo mite and calclte respectively. The line of best f it was determined for each by linear regression analysis.

These three curves are presented in Figures A1 and

A2. DoJ,/Cokita + Dokxnit* D o l./Q ti ♦ Dol. gure A2. andard curve fo - e t i c l a c from e v r u c d r a d n ta S . 2 A e r u ig F 0 e Al. St r and e t i c l a c m fro s e v r u c d r a d n ta S . l A re K) 30 uarz mixtures. s e r u t x i m rtz a ju c - e t i m o l o d y = 533 + 01 (* -3 3 33) 3 -3 01 (* + 533 = y . e r u t x i m e t i m o l o d 30 40 50 60 y = .526 + 0 I(* * 54.29) * I(* 0 + .526 = y 090 70 ONX) SO 90 too 10

Col./Qti. + Cal 187 188

Bulk Sample and <2u Analysis

Bulk samples were prepared in the same manner as above. Because a typically three-component system was being analyzed, two of the three calibration curves had to be used to arrive at the absolute value of each phase within the sample. Which two curves were used depended mostly on which two of the three components were most abundant in a sample. After the relative percents

(dolomite-calcite, do 1omite-quartz, and ca 1cite-quartz) were determined for each specimen they were set equal to one another {i.e ., ca 1c1 te :dolom 1 te * dolomite:quartz ; calc1te:quartz = quartz:dolomite ; dolomite:calcite = calcite:quartz) and the absolute values arrived at. Be cause the two component curves are very Inaccurate below

1% values, components may be recorded as < 2 % or <<5% after calculation on a three-component basis.

The less than 2y fraction was analyzed according to a method developed and used at the Louisiana State Univer sity Geology Department (Devine, 1971). After a dispersed

<2g fraction was separated, approximately .05 grams clay from the removed aliquot was centrifuged onto a glass slide. The sample was then glycolated for 24 hrs. and

X-rayed from 2°-31o20 at 2°/min at a suitable full scale and time constant. They are then X-rayed again from 24.3-

(24.8°2g) from the 004 chlorite peak (25.1°20) (Biscaye, 189

1964). The ruler helght-peak intensities are measured for these two peaks and the expanded 001 smectite, 001 illite , and 101 quartz peaks. These measurements and sample weights form the Input for a computer program developed to calculate a semi-quantitative weight percent analysis of the sample components. APPENDIX B

DISCUSSION OF SIGNIFICANT VARIATIONS IN THE OUTCROP AND OUTCROP BY FACIES INTERACTION ANALYSES

The significant variations in the outcrop analysis are given 1n the analysis of variance tables (Table Bl).

Significant variations among outcrops occur in most vari ables analyzed but no trends could be discerned, which, if encountered, may have indicated a lateral (i.e ., down depositional dip) change 1 n the deposltlonal or post- depositlonal histories of the Cow Creek. Instead, the means for a particular variable are randomly distributed.

Either more samples are needed to delineate trends, or no trends exist and the means represent random variations in the abundance of a variable.

The significant outcrop by facies interaction varia tions are presented 1n Table B2. Most of these variations could be eliminated with more samples, or if the facies would have been defined on petrographic criteria rather than on field descriptions. A brief explanation of each variation is given below.

190 TABLE B1. Significant Outcrop by Facies Interactions

iables Var Ratios Matrix-Matrix Diagenesis Grains-Grain Diagenesis ' : F Oysters 2.61** Calci te Moldic Cavity Filled Grs(P_) 2.65** 1.99** ' Mollusks NEl -2

Pel 1ets 1 .59* ne 3 2.16** (Mold. Cav. F. (Ps) + Indeter 2 .02** minate Grs)/Mol 1u sks

Rextl.-Inverted 1 ,52* Micritized Grs. 1,72* NE4-5 1.51* Grs. Oysters + Unidentified

RextlInv. + Dolomi te Indetermi nate 1 .65* — r'e 2 1,83** Matrix (NE)/Cement (HPV) 5.50** Grs. Micritized Grs. Bladed Cement PBO + PBC + PFC 2 .01** 1 .84** 1 .54* (Nd) 00 + PB4C PEO^g/Rextl. Grs. * Micrite Rims 1 .96** 4.27** Equant Cement Quartz 1.59* 6.57** PE3O

Iron-Content 2.14** PE4 O 3.11**

F pe 5. 6 0 7.37** PE 5-e^m 1.83** i PE2 2.50**

pe 3 2.18** ! pe 4 2.08** PE5-6 2.60**

- 161 TABLE B2. Analysis of Variance Tables

^sVari abl e Mollusks (other Oysters Echi noi ds Intraclasts than oysters) Source d.f. M.S. F M.S. F M.S. F M.S. F 14 outcrop 240.51 2.77** 617.27 9.77** 19.04 3.87** 3.86 1.61

4 faci es 854.38 9 .8 6 ** 236.71 3.75** 8.90 1 .81 2.76 1.15 outcrop x 43 113.71 1 .31 164.87 2.61** 7.15 1 .45 2.64 1 .1 0 faci es 105 Error 86.69 63.19 4.92 2.39

Total det. dolo. N\V aria b le Detrltal Pel lets Quartz Total clastic grs.] Dolomite d. f. Source M.S. F M.S. 1 F M.S. F M.S. F 1

14 outcrop 2 1 .6 8 4.00** 594.87 3.44* 41.07 1 .05 0.01 2.06*

4 fac i es 58.69 10.82** 2225.18 12.85** 143.16 3.67** 0.03 4.86** outcrop x 43 8.60 1 .29 274.36 1 .59* 36.57 0.94 0.007 1 .25 faci es 105 Error 5.42 173.13 0. 005 TABLE B2 (continued)

able Total elastics Micrite rims Micrite Rims Dolomicrite Rims Total, allochems Total AllocFems Source d.f. M.S. F M.S. F M.S. F M.S. F

14 outcrop 50.26 0.75 189.63 4.66** 182.95 1 .93* 0.09 1 .1 0

4 faci es 85.02 1 .27 151.85 3.73** 313.58 3.31* 0.03 0.34 outcrop x 43 45.63 0 .6 8 79.80 1.96** 139.71 1 .47 0.06 0.73 faci es 105 Error 66.81 40.69 94.86 0.08

\ V a r i a b 1 e Dolomicrite rims M1c.Rims+Dolo.Rms.Micritized grains Micritized grains Total allochems Total allochems

14 outcrop 0.06 1 .84* 0.05 1 .28 242.53 5.99** 0.1 6 4.01**

4 facies 0.08 2.42 0.23 5.68** 229.05 5.66** 0.1 5 3.86** outcrop x 43 0.04 1 0.06 1 .38 81 .18 2 .01** 0.06 1 .51* faci es .26

105 Error 0.03 0.04 40.48 0. 04 TABLE B2 {continued)

^xVarl able Mol die Cav.Filled (Mold.Cav.F.Grs.+ Rextl.-Invtd.Grs. Rextl.- Inverted + (P9) Grains Indeterm. Grs.) Indeterm. Grs. ( * i) Source d.f. M.S. F M.S. F M.S. F M.S. F 14 outcrop 182.70 3.99** 227.90 3.97** 191.25 2.92** 156.60 2.16*

4 faci es 246.71 5.38** 449.82 7.84** 191.90 ' 2.93** 327.59 4.49** outcrop x 43 64.65 1.41 80.02 1 .40 112.44 1 .72* 120.71 1.65* faci es 105 Error 45.90 57.37 65.57 73.03

^sVariabl e Mold.Cav . Fid.Grs. {fold. Cav.Fill (P$) + M1cr1 te(NE-|) and Cse. Microspar- MollusksIndeterm.) / Mo 1 lusks Fine Micro spar (NEJ Fine Pseudospar (NE^) d.f. Source M.S. F M.S. F M.S. F M.S. F

14 outcrop 0.18 4.37** 0.30 4.40** 253.86 1 .66 901,43 6.72**

4 facies 0.35 8.85** 0.50 7.39** 4291.56 28.08** 1783.80 13.31** outcrop x 43 0.11 2.64** 0.14 2 .02** 303.46 1.99** 290.12 2.16** facies 105 Error 0.04 0.07 152.83 134.06 TABLE B2 (continued)

^sV a r i a b1 e Cse. Pseudospar Dolomicri te Fine Dolonicrospar Cse.Oolomicrospar- Cse. Dol ospar (RE3.5 ) (.NE4-5> (RE'I) m 2) Source d.f. M.S. F M.S. F M.S. F M.S. F

14 outcrop 124.05 2.30** 108.39 1 .12 207.64 3.10** 130.48 0.83

4 faci es 280.36 5.20** 431 .67 4.46** 227.78 3.40** 138.51 0 .8 8 outcrop x 43 facies 81.82 1.52* 54.01 0.56 122.53 1.83** 70.31 0.45 105 Error 53.95 96.88 67.08 157.59

x^Variable Calc.Matrix(LNE) Matrix (ZNE + RE) Fibrous Cement Bladed Cement Dolo.Matrix(ERE) Cement (PF2_4 0; PF2 4 C) (pb2.4°) d.f. Source M.S. F M.S. \ F M.S. F M.S. F

14 outcrop 37.36 0 .6 6 46.37 4.92** 0.14 0.89 6.72 0.82

4 faci es 96.70 1 .70 94.69 10,04** 0.30 1 .85 12.71 1 .56 outcrop x 43 42.66 0.75 51 .87 5.50** 0.23 1 .43 6.67 0.82 facies 105 Error 56.83 9.43 0.16 8.16 TABLE B2 (continued)

l(pb2q+pb2c+pf2_4) 'N'SssJ(a r i a 1b e Bladed Cement pb3c pb4c (pb2 c) pb3_4c d.f.Source M.S. F M.S. F M.S. F M.S. F

14 outcrop 145.28 4.36** 1 1 0 .2 2 2.29** 1 .18 7.00** 5.43 2 . 10 *

4 faci es 828.11 24.83** 660.51 13.73** 0.62 3.66** 10.94 4.24** outcrop x 43 61.51 1.84** 69.83 1 .45 0.72 4.27** 3.99 1 .54* facies 105 Error 33.35 48.11 0.17 2.58

Monocrystal 1i ne Optically Continuous ^SVariable Equant Cement PE5-6°mm Equant Cement pe4o (PE4 O111) (PE3O) d.f. Source M.S. F M.S. F M.S. F M.S. F

14 outcrop 8.27 2.74** 6 . *2 2 . 21 ** 65.13 19.36** 26.06 4.55**

4 facies 9.13 3.03** 39.23 13.52** 40.75 1 2 . 11** 72.23 12.62** outcrop x 43 3.56 1 .18 5.31 1 .83** 22.11 6.57** 17 .80 3.11** faci es 105 Error 3.01 2.90 3.36 5.73 TABLE B2 (continued)

^sVari able Random Equant 0 PE5-6 Cement (PEg) PE3 PE4 Source d.f. M.S. F M.S. F M.S. F M.S. F

14 outcrop 19.01 11 .82** 242.74 4.57** 702.83 5.41** 452.51 6.67**

4 faci es 23.07 14.34** 78.28 1.47 2617.51 20.15** 897.01 13.21** outcrop x 43 141.17 2.08** faci es 11 .85 7.37** 132.75 2.50** 282.99 2.18** 105 Error 1.61 53.17 129.87 67.89

'NsvNVari abl e I {PE + PEO+PE 0m)2 _ 6 PEr ^ PE2-3° + PE2-3 A11ochems 5-6 e(pe*peo+peoj4_6 £(PF+PBQ+PBC) 2_4 Z(PF+PB0+PBC) 2_4 d.f. Source M.S. F M.S. F M.S. F M.S. F

14 outcrop 8.21 4.75** 59. 4b 1 .62 323.48 0 .8 8 188.16 1 .93*

4 facies 10,96 6.35** 53.66 1 .46 1132.25 3.07* 235.15 2.48* outcrop x 43 1 .21 facies 4.50 2.61** 44.34 323.77 0 .8 8 143.92 1 .48 105 Error 1 .73 36.66 369.08 97 .31 TABLE B2 (continued)

Iron content of Partially Dolomi- a r i a b 1 e < «0 )j_# Z elastics w/PF or PB Allochems-Cements tized Grains Rextl .-Inv . Grs.tN^) z elastics Source d.f. M.S. F M.S. F M.S. F M.S. F

14 outcrop 7.08 11.06** 5.55 1 .68 8.97 4.81** 124.86 3.02**

4 faci es 15.06 23.53** 22.40 6.78** 16.02 8.58** 21.62 0.52 outcrop x 43 faci es 7.60 11.87** 3.39 1 .0 2 4.00 2.14** 43.62 1.05

105 Error 0.64 3.31 1 .8 6 41 .42

Vari a b1e Part. Pol. Grs. Oysters + Unident. ______Grs. Source

outcrop 0.26 4.00**

facies 0.02 0.34 outcrop x facies 0.08 1.28 105 Error 0.07 1 99

Sedimentoloqical Variables

Oyster fragments--an apparently random distribution

throughout the Cow Creek lithofacles.

Pellets--the variation in pellet concentration is

between the lagoonal nodules and other units and would

probably be eliminated if these two facies would have been more properly defined and more samples of the nodules would have been collected.

Quartz--most quartz occurs in the lagoonal and shoal units and the least quartz 1 s present 1 n the upper offshore- foreshore intervals. Host variation exists between the shoal-1agoona1 beds because these are arbitrarily defined on a difference in bedding thickness. They are gradational petrographically and the shoal unit is often composed of quartz-free lime grainstones, which are Invariably absent in the lagoonal sequence; thus, the shoal may contain lit tle or no quartz, if composed of grainstone, or be quartz rich, if composed of lime packstones (see p. 24).

Diagenetic Variables

Cement Types and Cement Ratios.- - It is felt that most of these variations occur because of the variation in the pore size of the sediment, which was not recorded 1 n the analysis (for example, a .25 mm equant cement [PE^] can only be precipitated 1n a pore .25 mm or larger). It should be noted that most of the variations for cement size 200

groups disappear when one group is proportioned to another

(see p. Ill for discussion). Two other factors also in

fluence the abundance and size of the cement types:

a) whether packstones or packstones a nd grainstones make

up the shoal unit; b) experimental error in identifying

recrystallized matrix as cement and recrystallized cement

as recrystal 1ized matrix (Appendix D4).

Matrix-Matrlx Ratios.--The various sizes of re crystallized matrix vary from facies to facies because of:

a) experimental error (see cements above); b) presence of

grainstones in the shoal beds, and packstones in the upper

offshore unit; c) randomness 1 n resultant size of recrys

tallized matrix ( 1 t was more Important 1n this study to determine where most recrystal 1 ization took place rather

than what the resultant crystal sizes were).

Recrystal 1ized-Inverted Grains, Micritized Grains.--

A possible reason for the variations In these two dlage-

netic modifications originate because the absolute number of mollusks and oysters counted 1n each facies is not con

sistent. Goth of these variations are in part also a re

sult of calichlfIcatlon or the presence of supratidal se quence at the beach top (as Is micritized grains/(oysters +

unidentified grains). 201

Micrite Rims.--The ira 1 n reason for variation here is that the recorded number of micrite rims does not accurately describe the actual number of micrite rims that developed.

This is because many rims were dolomitized; micrite rims and dolomicrite rims do not show outcrop by facies varia tion.

Moldlc cavity filled grains/Mol1usks.--Variations arise here because of: a) the presence of a caliche or supratldal sequence at the beach top, b) extensive leach ing and collapse of the beach sequence at outcrops 13 through 16, and c) dolom1t 1 zat1 on, or absence of dolomite, in the shoal unit. APPENDIX C

CLAY MINERALOGY OF THE HAMMETT SHALES AND CARBONATES

Genera 1

The clay suite 1n the Hammett limestone and dolomite units (Facies IV) and underlying claystone-mudstone Inter val (Facies I II ) 1s predominantly composed of ill 1 1 e; kao- lin lte and chlorite occur In lesser amounts but 1 n constant proportions to one another (F1g. Cl). The quartz content ranges from 8 % to 2 2 % in the carbonates and claystone ex cept 1 n the uppermost and lowermost beds (near the contacts with the Sycamore Conglomerate and Cow Creek Limestone, respectively) where 1t 1s somewhat higher. Smectite, a l though abundant 1n the time equivalent Hensel alluvial red mudstone, 1s Invariably absent from these two offshore marine deposits.

Figures C2 and C3 Indicate that Hammett limestones contain Intermediate percentages of 1 1 1 1 te and chlorite; dolomite samples, on the other hand, contain either high or low percentages of chlorite and llllte , but this rela tionship is not as clear as for the limestones. If these variations were due to experimental error 1t does not seem likely that all the limestones would cluster in the centers of the graphs. 202 Chlorit* (Hit* 203 Figure Cl. Plot of chiorite-dolomite-i11ite in Hammett Shale dolomite and limestone. 204

9 Cloytlona ^udilo

90 • Dolomite

a *0 • 0

*7,

Figure C2. Plot of 1111te vs. chlorite In Hammett Shale carbonates.

O Cloyiton« Muditorn

■ Dolom ite

•S

• •

* I

7 0 •

i *> * 3 2 * 413* *64™"* 80 90 % Dolontt* in Solu ble Fraction Figure C3. Plot of 1ll1te vs. percent dolomite of Hammett carbonates. Di scuss ion

The i 111te-chlor 1te-kaol1nite suite of the Hammett shales and carbonates also characterizes both Recent and ancient marine sediments {as reported by innumerable work ers in Grim, 1968). Smectite, forming on the arid Hensel alluvial plain (Alexander and others, 1939) during the de position of these sediments, 1 s probably absent as a result of conversion to 1 1 1 1 te and chlorite upon its transporta tion into the marine environment (Grim, 1968). Kaolinite, although present 1 n small amounts, 1s still on the average more abundant in the marine Hammett shales and carbonates than it 1s in the alluvial plain mudstone facies (XIII)

(compare Fig. Cl to Table 1); thus, kaolinite was altered or destroyed during subaerial weathering, or formed diagenetically In the marine sediments, rather than being con verted to 1 1 1 1 te and chlorite 1 n the marine environment as suggested by Grim and others (1949). The fact that the

Hammett shales, limestones, and dolomites contain the same clays, in approximately the same amounts, denotes that clay diagenesls was essentially uniform throughout these three llthologies. However, an explanation for the llllte - chlor 1te-carbonate relationships plotted 1n Figures C2 and

C3 completely eludes the author.

The higher percentage of quartz 1n the Hammett units

Immediately superjacent and subjacent to the Sycamore con glomerate and Cow Creek Limestone is presumably the result 206

of the more selective sorting processes which operated in the shallower, near strandline environments in which these beds were deposited. APPENDIX D

OTHER DIAGENETIC ASPECTS OF THE COW CREEK LIMESTONE

D1 . Lith 1f 1 cat 1 on of the Nodules and Nodu 1 a r B e cTs

Nodules and nodular beds contrast markedly with their enclosing rocks in grain and matrix content and d 1a- genetic modification (see text Tables 3, 4, 5, 6 ). By far, their most distinct petrographic characteristic is the presence of abundant, large quartz free microspar pellets

(Fig. D1 — A); they also contain fewer quartz and allochemi- cal grains than do the matrix rocks (F1g. Dl-B). The de tailed field characteristics and Interpretations of these two units are given on p. 28.

Petrographic examination of the nodules representing burrow fills (see p. 30) reveals that detritus rich micro spar separates the pellets 1 n the diffuse, peripheral zone of a nodule (F1 g. Dl-C). Inward toward the center of a burrow f i l l , the Interpellet area contains only a few quartz grains which float 1 n a microspar-pseudospar matrix

(Fig. Dl-D); pellets are more tightly packed inward from the margins. In nodules other than burrow f ills , Included shell material is angular and poorly sorted; both of these textures are the result of in situ shell accumulation, Figure D1. Cow Creek potpourri

A. Large (2 mm) quartz free fecal pellets recrystal lized to microspar 1n foss1 1 1 ferous quartz rich ( v .f .-f.g r.) microspar matrix. Photo of contact of nodule with matrix.

B. Fossi 1 1ferous quartz-rich microspar matrix which encases pelleted nodules of burrowing origin.

C. Contact of pelleted nodule (left) with arenaceous matrix (right); note mixing of matr 1x-pel 1 ets 1 n middle and complete absence of matrix and quartz on left.

0. Large microspar pellets 1n arenaceous microspar matrix.

E-F. Central line of crystal juncture 1n grains which display relict ultrastructure (E) or are composed of clear calcite which, for all appearances, is a moldlc cavity f i ll (F).

G. Large equant cement (PEi.gO) In optical continuity with recrysta 1 1 fzed mollusk fragments; cement is

H. Recrystallized optically continuous cement (PEgO) enclosing a remnant patch of bladed firs t genera tion cement (center). This Indicates recrystal lization of g ra1 ns and cement took place after Initial cementation. 208

F IG U R E D l 209

followed by breakage and redistribution by burrowing orga

nisms (Stanton and Warme, 1971).

Nodular units have the lowest least square means of any of the point counted units for the grain diagenetic modifications listed below: Micritized gra1ns/(oysters + unknowns) Moldlc cavity filled gralns/Mollusks (also highest inverted grains and inverted gra1ns/Mol1usks Moldlc cavity filled grains + 1ndetermlnates/Mol1 usks (also highest [Inverted grains + Indetermlnates]/ Mollusks) Micrite Envelopes-Mlcrlte Envelopes/Al1ochems Dolomicrite Envelopes/Al1ochems Micrite Envelopes + Dolomicrite Envelopes/Al1ochems Dolom1t1zed A11ochems-Dolomlt1zed A11ochems/(Oysters + Unknowns)

The matrix 1_n the nodules 1s also uniquely dissimi lar to the matrix of other units 1 n that 1 t 1 s composed almost solely of coarse microspar and pseudospar (NE3;

.016-.164 mm) with lesser amounts of very coarse pseudo spar (NE^ g*. .064-1.0 mm); much of this 1s 1n optical con tinuity with recrysta111zed mollusk fragments. Even though the surrounding matrix may be completely dolomitlzed, this alteration never affected the nodules to any great degree; dolomite content In other restricted lagoon beds and 1n the shoal unit Is, at the least, three times greater than

1n the nodules. Furthermore, lit t le 1f any recognizable cement 1s present 1 n the nodules and nodular beds.

The pelleted nodules and nodular beds are hypothe sized to have originated through the processes of burrowing and current redistribution of burrowed sediments, 210

respectively (p. 33), because they (nodules) are branched and both contain abundant pellets and less detritus than do the surrounding beds. Burrowing organisms populated

the sediment during slow rates of deposition in an environ ment where currents were weak enough to allow carbonate muds to accumulate, but were periodically strong enough to

bring 1n fine-grained terrigenous sands. This explanation accounts for distribution, composition, and textural as

pects of the nodules and nodular beds, but does not answer

the question of why they are so diagenetlcally distinctive.

Fecal pellets found 1n recent carbonate environments consist of ingested material bound together by an organic mucous (Kendall and Sk1pw1th, 1969; Trlchet, 1 967). 11 1 ing

(1954) was one of the firs t workers to note their profusion

1n shallow, low energy carbonate environments. Under ex

tremely low energy conditions, and where scavengers are

scarce, pellets become semi- 1 1 th1 f 1ed on or slightly below

the sea bottom (Kornicker and Purdy, 1957). The exact mechanism by which this 1 1 th1 f 1catlon takes place is not

known, but 1t 1s thought to Involve a bacterial precipita

tion of aragonite (1111ng, 1954; Kornicker and Purdy,

1957) or be related 1n some other way to the organic con

tent of the pellets. Trlchet (1967) concluded that pre cipitation or aragonite In pellets results from super

saturated conditions caused by: 1 ) phys1ochem1ca1 reac

tions with Included monosaccharides and uronlc acids, or 21 1

2 ) bacteriological destruction of organtc-caldum complexes, which causes the release of excess calcium and brings about supersaturated conditions. In any event, diminutive recrystallization {micr1 tizatlon) accompanies this cementa tion, resulting in the loss of any Internal pellet struc ture {Newell and Rigby, 1957).

It has already been noted (p. 30) that differential compaction took place over the nodules. Moreover, at one outcrop, the tubes of serpulld worms (which encrust solid substrates) were found on the surface of a nodule, suggest ing that the nodules may have hardened while exposed on the sea floor; however, the few worm tubes on this one nodule may also be Interpreted as pure coincidence.

On the foregoing bases, 1t is reasonable to suggest here that the nodular units became semi- 1 1 th lf 1ed while s till in close proximity to the marine sediment-water inter face. Pellets, along with the bloturbated shelly, sandy muds enclosing them, contained abundant organic matter.

These organlcs, or their bacteriological destruction,

Initiated the precipitation of aragonite within the nodules or beds (Berner, 1968). Sometimes these original nucleation (precipitation) centers continued to grow outward by further cementation of the surrounding sediments which con tained somewhat less abundant, disseminated organlcs.

This type and time of 11th1f 1ca 1 1 on not only ex plains the sometimes diffuse and Irregular nature of the 212

contacts with the surrounding matrix, but also the d iffe r ential compaction features, and the origin of the large

"cannonball" concretions, which are thought to represent diagenetically enlarged burrows or sediment volcanoes

(Shinn, 1968). Stanton and Warme (1971) and Stanley and others (1967) suggest a similar origin for nodules and concretions 1n the Cretaceous and Miocene, respectively.

Early cementation undoubtedly resulted In the loss of some porosity and permeability and must have sealed off the nodules from dolom 1t 1z1 ng fluids which permeated through the surrounding beds (Shinn, 1968, notes that early 11th1f 1cat1on Inhibits dolom1tizat1on In Recent supratldal sediments). Dolom1t1zat1on appears to have taken place after the matrix 1n the nodules recrystal 11 zed to microspar and pseudospar as witnessed by the minor re placement of some microspar and pseudospar crystals.

Preferential dolomi11zat1 on of certain lagoonal sequences was brought about through the growth of dlagenetlc dolomite on detrltal dolomite seeds (p .101; also Undholm, 1969).

This alteration process and Its locus of operation are considered 1n the construction of the Cow Creek dla genetlc model . APPENDIX D

OTHER DIAGENETIC ASPECTS OF THE COW CREEK LIMESTONE

D2. Grain Polom1t1zat1on (R^E)

General

The two predominant allochem types of the Cow Creek

Limestone and Hammett Shale are oysters and other unidenti fied mollusk fragments. Oysters were In itia lly composed of primarily low Mg calclte, as are Recent oysters, and other mollusks of aragonite, as 1s evident by their selec tive recrysta 111zation or representation by spar filled dissolution molds. The fecal pellets of the restricted lagoonal facies are presumed to have been composed of ara gonite; echlnold skeletons consisted of high Mg calclte.

Of these three carbonate types, only low Mg calcite is stable under surface, fresh water conditions (Chave, 1962;

Friedman, 1964; Land, 1967).

Partial dolomite replacement of allochems in these rocks is common but 1s apparently quite preferential.

Oysters and echlnolds are selectively replaced; only rarely are original aragonltlc grains replaced, except where the dolomi1 1za1 1 on of a bed or part of the sequence has gone to completion. Dolomite may replace oyster fragments 1n

213 214

three different ways: 1 ) peripherally; 2 ) as very thin, distinct replacement zones paralleling the foliated shell structure, and 3) randomly. Dolomite replacement crystals are approximately the same sizes or slightly smaller than those contained within the matrix, if any is present.

The selective dolom111za 1 1 on of only certain grains is influenced by many factors, among these being the in i tial grain mineralogy. Schroeder and others (1969) report protodolomite forming 1n Recent echlnolds, and Land and

Epstein (1970) report selective dolom111za11 on of original high Mg ca ld te red algae from the Pleistocene of Jamaica.

Graf and Goldsmith (1956) suggest that dolomite shoudl form more easily on high or low Mg c a ld te (versus aragonite) because the crystal structure of dolomite 1s already pres ent. However, because of the known Instability of arago nite and high Mg ca ld te outside the marine environment,

Stehl1 and Hower (1961) submit that either of these miner als could be preferentially dolomitlzed over the more stable low Mg c a ld te .

The crystal sizes and fabrics of a grain Influence the 1ntragranular porosity and permeability, and these in turn Influence the flow rates and pathways of fluid move ments, and Ion diffusion Into and out of a grain. The type and distribution of organic matter, along with Its varying rates of decomposition may also play a role in the selective dolom1t1zat1on of grains. Numerous researchers 215

of the Recent supratldal areas have noted that pellets and bioclasts withstand dolomit1zat1on (Deffeyes and others,

1965; Illing and others, 1965) as opposed to aragonite muds; shells are apparently the most resistant of the three.

Distribution of Dolomitlzed Grains

Grain replacement by dolomite 1s fairly homogeneous throughout all facies, but slightly more grains are dolomitized 1 n the upper offshore-foreshore sequence than in the subjacent units. Furthermore, slightly less oysters and unidentified grains were replaced in the shoal facies than in any other, whereas more were replaced in the nodular beds and nodules.

Interpretation of Dolomitlzed tiraln Distribution

Grain dolom1t1zation displays the least variation of any dlagenetic modification throughout the facies of the Cow Creek Limestone; it 1s therefore considered to be the least Important for discerning facies specific diagenetlc environments, or differences in the early paleohydrologic framework. Even though this modification is restricted mostly to the originally low Mg caldte grains.

Pellets in the Cow Creek Limestone perhaps resisted dolo- mitlzation because of high organic content and early 1 1 1 h1 - fication (Trichet, 1967). Aragonite bioclasts are 216

presumed to have gone unaffected because they had very dense, homogeneous or nacreous shell structures; oysters with their sometimes less compact foliated shell structure

and calcite composition were more susceptible.

Whether these data indicate a preferential replace ment of low Mg calcite before the aragonite grains stabi

lized, or whether it reflects a preferential replacement of original low Mg c a ld te over calcite formed through neo morphism, 1s uncertain. If the former was true, grain dolomitization would necessarily be early; aj such, varia

tions between facies would be expected because the Cow

Creek facies do show differences 1n grain envelope and matrix dolomitization. Thus, the homogeneous distribution of dolomitlzed oysters (and unknowns) most likely is a re

sult of preferential replacement for original low Mg cal cite in a hydrologic setting that developed after the ma jority of other dlagenetlc modifications were imposed on

the Cow Creek Limestone. APPENDIX D

OTHER DIAGENETIC ASPECTS OF THE COW CREEK LIMESTONE

D3. The Irregular, Medial Crystal Juncture L1ne-- A Method for IdentlfyTng Recrystallized Mollusk Fragments

Excepting relict structure and broken micrite enve lopes, Folk's (1965) and Bathurst's (1958, 1964) criteria for distinguishing recrystallized from cement fabrics are of limited use when dealing with altered mollusk fragments, particularly those which are composed of large, equant or prismatic crystals. Within an individual shell, straight sided crystals may be as common as ones with serrate edges and the enfacial junctions (Bathurst, 1964) are sometimes as common as triple crystal junctures In shells displaying relict structure. Grains with an Inner bladed fringe, followed by a coarsening Inward macosalc of equant crystals

(druse of Bathurst, 1958), are often encountered. Moreover, calcite precipitated locally from solutions carrying the extra carbonate produced by aragonite Inversion may contain

Impurities exsolved from the Inverted shells; crystals pre cipitated from these solutions would mimic calcite formed through recrystal 112a1 1 on-1nversion processes because they would contain the aforementioned Impurities (Purdy, 1968).

217 218

I am 1n full accord with Tebbutt (1968), who concludes that the original shell structure controls to a large degree the crystal shapes and distributions, types of crystal boun daries, and occurrence of enfacial junctions in recrystal-

II zed shel 1 fabrics.

A feature useful 1n identifying a bioclast recrys tallized 1s an Irregular line of crystal junctures which extends down the longitudinal center of bioclast. This central junction appears 1n shells with and without relict structure (F1g. Dl, E, F) and 1n some Instances, staining reveals an Iron zonatlon towards the center of a mollusk fragment. Friedman (1964) suggests a mlcrosolutlon- m1crodepos1t 1on process for effecting aragonite Inversion to c a ld te . Purdy (1968, p. 199) states, when referring to three photographs, that "with respect to corals where

1t is evident that the volume for volume replacement of aragonite by low Mg. ca ld te 1s effected along a very I r regular line that presumably constitutes a microvoid on one side of which aragonite Is being dissolved and on the other side of which calcite 1s being precipitated." I see no reason why this statement cannot be equally applied to

Inversion of aragonltlc mollusks. The Irregular line seen

1n many Inverted bloclasts 1s thought to be a fossil culmi nating microvoid that formed when Inverted crystals, grow ing at approximately equal rates from either side of the shell, met at a central stabilization front (Fig. Dl, E). 219

This commissure would not be seen: a) when differing shell structures occurred on either side of the bioclast; b) in transverse sections of elongate mollusk fragments; c) in spherical or nearly spherical grains; d) when inversion proceeded from only one side of a fragment* and e) in tan gential cuts. Crystal size Increases toward the shell center as a result of the gradual elimination of many o ri ginal calcite nucleatlon sites* and continued selective growth on others (Fig. Dl , F).

* APPENDIX D

OTHER DIAGENETIC ASPECTS OF THE COW CREEK LIMESTONE

D4. Recrystallized Cements

The only real dilemma coped with 1n the accumulation of cement data and Its subsequent analysis, was the task of differentiating cement fabrics from matrix recrystalliza tion fabrics. This problem has perplexed carbonate petrographers for ages, but criteria developed by Bathurst (1958,

1964) and Folk (1965) have lent to Its resolution. The enigma 1n the Cow Creek, however, Is not only one of de ciphering recrystal 11 zed matrix from cement, per se, but one of delineating recrystall 1zed matrix from recrysta 1 -

11 zed cement. Ofttlmes this 1s Impossible, particularly

1n samples from the lagoonal sequence. Here, the recrys tallized 1ntergranu 1ar materials are recorded under cement headings but are coded ?NPE, ?NPE0, ?NPEOm, and ?NB0, to

Indicate the uncertainty Involved, rather than being lumped into puristic "N" categories.

Yet, there do exist equant cements that have defi nitely undergone recrystal 11zatlon. These are most abun dant 1n the upper offshore-foreshore sequences and are recognized on the basis of their being 1n optical

220 221

continuity with recrysta11 1 zed mollusk fragments (Tig. Dl ,

G, H); even though Individual crystals can have Irregular boundaries {F1g. D2, A-C) they do exist side by side with

"ideal," non-recrysta11ized cements (Fig. Dl , D). The ratios EPE0/N1 and EPEO/Ni + Indeterminate and their com ponents (Table 7, cement distributions) show that most re crys ta1 1 1 zation of cement and grains took place at the top of the beach.

Shinn (1969) reports that In samples from the Per sian Gulf, previously recrysta111 zed grains become cemented with optically continuous cements; 1n fact, the cement type

1s controlled by Its substrate. In the Cow Creek there 1s no evidence of this. Contrarlly, rare occurrences of relict druse, which 1s surrounded by a cement which 1s 1n optical continuity with a recrystallized mollusk fragment

(PEO; F1g. Dl-H), suggest that grain recrystal 11zatlon took place not before, but after, initial cementation. Unfortu nately, insufficient data precludes this from being the rule rather than the exception. Figure 02. Cow Creek potpourri.

A-C. Recrystallized grains and equant cements, some which are optically continuous. In A-B, cements have loafy form and serrate edges, whereas in C, cements have straight edges and has more of a "good" cement appearance. FIGURE APPENDIX D

OTHER DIAGENETIC ASPECTS OF THE COW CREEK LIMESTONE

D5. Equilibrium Calculations

+ 2 -2 CaCO 3 calcite + C03

_ [Ca'2] [C03 2] _ 1 0 -8.34 calcite " ------—— “ 10 [CaC03] because [CaCO^] = 1 for pure calcite

However, in (Ca gg^e 0 5 )^0 3 * assuming ideal solution

(CaC03] .95 NCaC0 3 therefore

lAP(Ca, Fe)C0 3 = [Ca+2] [C03' 2] = [CaC03] x 10 ’ 8 ' 34

223 224

+ 2 +2 To calculate the ratio of concentrations of Fe and Ca

in aqueous solutions for (Ca ggfe 05^C83

[Ca+2] [CO ' 2] ,Ca+2 mCa+2 yCO. " 2 mCO. " 2 ------J ------— = K [CaCOj] XCaC03 NCaC03 spca1cite

« TO' 8 *34

[Fe+2] [C O /2] yFe +2 mFe +2 yCO/ 2 mCO/ 2

[FeC03] siderite AFeCOjN FeCOj

= K = 10"10*5 spsideri te and combining these two

yCa + 2 mCa + 2 XCaC03 N CaC03 Kcalc1te

yFe +2 mFe +2 ' XFeCOj N F.C0 3 Ksider1te

+ 2 +2 assuming yCa or yFe In dilute aqueous solutions, the + 2 +2 concentration ratio of Ca and Fe for (Ca g 5 pe Q5 ^ 8 3

is obtained from:

.05 x 10 ‘ 1 0 * 5 x XFeC0 3 x mCa +2 mF +2 = ------a -34------he .95 x 10 x XCaC0 3

10~U ,8 mCa* 2 XFeCQa

10’ 8 - 36 ” XCaC03

- 10 ‘ 3 ' 44 mCamCa+Z x xmo 3

mFe-4 = 10~3 *4 because ACaCO, * .95 and AFeCO, is mCa J assumed to approximate unity. APPENDIX D

D6 . Diagenetic Code from Folk (1 965)

Summary of Code far Autkigenu CaUttt

I. Mode of Formation P: Passive Precipitation P: Normal pore filling Ps: Solution-lit) I): Displacive Pieripitation N : Neomorphism N : a* a general term , or where exac t process unknown. \,: Inversion from known aragonite. N,: Kecrystallizafion from known ralnte. N.»: 1 iegrading (also NY, NYl. N»: Origtnal fabric strained significantly . N,; Coalesrive (as opposed to porphyroid i. (the above may be combined, as Nnt,). R: Replacement

I I . Shape E: Fquant, axial ratio < I i ‘. f B: Bladed, axial ratio I }: I to 6 :1 F: Fibrous, axial ratio > 6:!

III. Crystal Size Class I, 2, .1, 1. S. ft, or 7

IV. Foundation O; Overgrowth, in optical continuity with nuc leus. O: Ordinary 0„: Monorrystal 0„: Widens outward front nucleus C: Crust, physically oriented by nucleant sur face. C: Ordinary C»: Widens outward from nuc leus S: Spherulitic with no obvious nucleus (fibrous or bladed calcite only) No Symbol: randomly oriented, no obvious con trol by foundation

CryitaJ Site State

Si«, mm Name Symbol

extremely coarsely crystalline KCxn 7 4 0 ------very coarsely crystalline VC xn 6 1 0 ------roars ?ly crystalline Cxn 5 0 25 — medium c ryst.tlline M xn 4 I)% 1 ------—------finely c rystalline Fxrt J 0 . 0 1 6 ------very finely c rystalline V F xn 0 004 ------aphanocrystallirte Axn 0 0 0 1 —

225 APPENDIX E

MEASURED SECTIONS BURNET COUNTY

BLANCO COUNTY

TRAVIS COUNTY

HAYS COUNTY 227

Figure El. Location of measured sections Section 1 Loc: Turkey Bend Road .8 ml $. of Hwy 1431

Poorly »tcl. angular pebbly cae qti-feld. us; festooned; fines up ward. Lower sharp scour contact.

'10" Grew purple, uroor, green doloeltlc or calcareous mudstones with 1' bed of cee pebbly feId. as et base. Above thle mudstones ere grey with reddish lonei throughout.

t.13' Covered; q t * pebbles ecettered on slopes

3 ’ 6" Wh lte-plnklsh nodular lleonltlc arenaceous, chalky dolomitlc line audit.; stylolltlc near base.

'‘S' Hollusk grelnstonee containing pockets and sonee of whit c-plnltlih arenaceous veined dol. line mudstone. Lime gralnstonea decrease upwards at expense of mudstones. Very Irregular bdlng and Irregu lar lower contact.

11'6" Cse arenaceous mol lush line giwlnetonee. Accret Ionary.

10 k Crayleh-whIts fine.-cae. well rded. well std. aren. molluak grainstone Somewhat finer m middle than top and bottom. Accretlon- ary thlck-maes 1 ve bedding. Lower contact sharp. Ho festoons apparent .

10" As below in 3'4" unit, except no laminations are present. Burrowed In Upper part. 229

10’ Huff t« 1 c . dol.t vf.-flns qtl-det. dul mid v«rja r e n . u n ' u n mulluek pickitoitu; laminated. Nodular bde near base, nodules (burruwa?) In middle and Lap. Nodulti transect l»»lnjllant. slurp lower contact.

J‘9’ Buff, finely innaciouii m o 11 u s k packstoncs; 4 " - 6 " tk bils. weather- in; into l.S ’ -2r ik bad pickagci ; undulatory bdlng, sume luama- (lam | Gradational lovn contact

’9" Grayish white can-fine, finely ircniccoui, dolomitlc molluak packatonai can at bottom and middle becoming finer and more arenaceous at top, Hoetly fragmented valvea (gaetropoda, oyatara, Trlgonla, Cardlum {T >, TutritaX1Ida). Maaalvn bedding, Lower contact aharp a nd a ve n .

Grayiah brown alley oyater dolomite packatooe, valvea aeparated but unbroken. Undulatory lower contact.

Yellowish buff ailty, molluak dolomite wackaatonaa and packstore*. Both whole and frag, valvaa preaenr. In middle are cant molluak packetone lenaaa which contain whole oyater valvea. Lower contact undula t o r y.

|l 1 '1 Buff alley molluak dolomite packatonea. Lower contact hidden.

14 ' Covered

1 7 1 Slope of yellowleh buff a llt with email abundant oyater fragments. Lower contact apparently gradational. Upper contact picked where yellowish buff color enda on elope.

Cong., well rded, poorly ltd, dol. and qtl pebbles and boulders up to 10" in dla. with matrix of, and interbedded with, mad.-cae., angular, poorly atd. dol. qtz-feld. n . Cae to fine gradations with sharp contacts at the base of each unit. Upper contact hid den. Section 2 Loc: Travis Peak Section .3 ml S-SE of Cemetery on road going S of Hensel Ranch

COVER.LD

Uhltt'irijrlah whit* ca*. grained aranacaoua ln-gtat; Laaa irim - caoue than 7* unit; gradational lowar contact.

FInk-ya 11 owl ah rad vary aranacaoua aolluak la grat. and calc a*-; nodular baddad with rallct accratlon baddlng; nodulaa d ic rin i In ■ lit upwarda. Probably a raaldua la(t by Kacant laachlng.

Mb 1ta-graylab whlta caa gralnad aranacaoua aolluak la grat; aad.-tk accratlon bada ; lowar contact aharp.

Craylah whlta cae.-flna aranacaoua aolluak la. grat.-pkat.; caaat at tp; faatoon ibad--aa«Il at top and bottoa, largo In alddlo; thin plaLy locllnad bada at baae; nodular at baae; gradational 1owar c on tact.

Buff-white fine gr. vary aranacaoua la pkata and nad.-cae arena- caoua la pkata-grata; flaggy baddlng; nodular; gradational lowar contact.

Whlta, yallowlah buff fin* gr. very calc, aolluak bearing qti *e. and f . gr. detrltal doloalt* qt» adac., Laalnated with bad* 4"-6' tk. On* lnterbed of cae, aolluak bearing qti aa. Lowar contact aharp.

San* aa eubjacent unit but contain* abundant concave down aolluak fragaenta; platy baddlng near top.

Buff, fin* gr., vary calc, aolluak bearing qti aa and aranaceou* la. pkat; nodular bada 6 "-l’ tk; carbonaceoua debrla In lowar 1’ . Lowar contact gradational.

Cray aranacaoua aollua la. pkat.; aany whole valve* In lower 2* and at top. Becoaaa lor* aranacaoua upwarda. Bada are 1' tk at baa* but thin upwarda. Outcrop la laachad. Sharp lower contact. 231

Buff-gray oyater doloaitr pkat; valvea generated and cave aide down. separated and aoit are cun-

Graylsh buff allty oyater dolomite pkata-wksts; some rones of whole, separated valves, Alternating zones of cse-flne current oriented shell fragments, Thin l"-2" beds at base, b" beds In middle, and one 3' tk bed at top. Case zones grade to fine zones and truncate tops of tine rones. Even to slightly undulatory bedding planes.

Ll-dk gray silty mollusk. dolomite mollusk In.pkat.; bioplasts are fragmented; nodular bedding; lower contact hidden.

Covered to below cop of Sycavore Section 3

Loc: Twin Creek ^ .8 ml E of Stubblefield House

COVERED

Lt gray bracclatad dolonltlc 1*. adat. (palao<7> callcha)' aoaa brace la claaca llnonita atalnad; tk lrragular bada. Lowar con tact hlddaa. con

Crn, alley doloalta adat; ona bad; lowar contact aharp.

Wh 11 a-p Inklah whlta aranacaoua dolonltlc la adat; vary danaa- oodular. Thla unit and abova lying unit occur at a lowar laval than tha top of accratlon bada. Sharp lowar contact.

wnlta-ya 11 owl ah gray aad.-caa pabbly, aranacaoua, nolluak In. grat.; lanlnatad aad-tk avan bada. Uppar pt altarad by palao- callch1flcat Ion; aharp lowar contact.

Cray, tan, buff v. flna-flne v. calc aolluak baarlng q ti. aa. lntarbdad with aranacaoua (aolluak la. pkat. In uppar pt. Lowar pt. la laalnatad and contatna nodulaa which dacraaaa In friquucy upwarda; nodulaa 6M-1' tk and lanaold. Uppar pt. haa lrragular lantlcular (faatoonad T) nodular bada with aona In itia l dip. Sactlon coaraana upward. Lowar contact aharp.

Lt. gray v.f. v. calc. foaa. qtt aa. with lntarbada of araaacaoua aolluak la pkat. Thin, wavy lrragular bda l"-2" tk. Saaalngly aharp lowar contact plckad on changn In bdlng. 233

Lt. gray daloaltic aolluak la. pkat; bo»« larnt unbroken valves and tonaa of oriented lur r 11 e 1 1 Ida ; tk-maasivc bdlni(. Lower contact aharp.

Grnlah gray oyater doloalta pketone; whole and broken valves (none in growth position). Sharp lower contact.

Lt. gry. allty doloalta packetone; soma whole valvea; angular, poorly sorted bloclaeta; thin, Irregular beds 3"-*" tk.; aharp 1 owe r contact.

Lt. gt'ay doloalta Ilea pka t.-d o 1 oel t e wkat; caest bloclasta and whole oyater valvea in alddle; one eaaalve bed; weathers buff; aharp lower contact.

Gray Intercalated clayatone and molluak lime pkat.; wavy beds l"-3" tk; Gradational lower contact.

Cray to grayish brn. clayatone w/ a few molluaka; one aaaalvi homogeneous bed; lower contact gradational

Grayish brn. foaa 111feroua ailtatone.

Covered down paat T/Sycemore. 234

Section 4 1oc: 1.5 ml W of Turkey Bend Section on Hwy 1431

Maroon, rad, and green si Its to m - mudstone; poorly atd. angular, and. - cae. gr. faldapathic aa.; soma pabblaa; faatooa eroaa - bada; poorly axpoaad.

12 CCVSKXDi pabblaa on alopa.

Pebbly, v. aranacaoua aolluak liaa grainafcona; f. -tad. 6 *-7 gr. at baaa, aad. - caa. gr. at top; S'-I1*' tk. accratlon bada; aharp lowar contact.

Buff - whlta, v, aranacaoua caa liaa packatona - grain- 1*1' atona; aad. - caa. gr. qtc. ; s< pabblaa and granulaa; faatoon croaa - bada; acourad 1 r contact. Whlta, calc, foaalllfaroua v.f. - f, gr. aa. containing abundant large. thick aolluak valves naar baaa; nodulaa at baaa up to 2' in dia.i no nodulaa in uppar 3'; can't dia- tinguiah individual bada. Buff, calc., foaailifaroua (oyatar - aolluak) v.f. - f. gr. qts. aa.; nodular; aoaa nodulaa axa branched and alongata; 3*1' 1* tk. bada at baaa, platy - t*- bada at top; burrowed; aoaa depoeitional dip; aharp Imex contact. Grayish wh., aranacaoua (v.f. - f. gr.) aolluak line pack- atom; whole molluak valvaa. Buff to yellow calc., foaailifaroua v.f. - f. gr. qts as; thin (3- - 6"), lrragular bada.

Buff, very arenaceous, poorly atd., molluak liaa packstone; 5* f. - v.f. gx. qts. ad; whole oyatar valvea; beds 1* - I V tk.; aharp lower contact.

Crn. , silty, clayey ooloniita packatona; whole oyater valvea; 10' sharp lower contact.

Mint green to tan silty, clayey dolomite wackestone and lie* packstone; noma molluak molds; massive bding.; homo geneous . 235

2 3 ' COVERED

Buff - whit* f. - Md. gr. n . at baaa grading upward into 6* ailtatona; no other aad. atructuraa notadi abundant oyatar valvaa at top.

Borad doloaiita pabblaa

Sycanora Congloaaxata » Qts (1W> and doloalta (80%) pub- bla - cobbla congloaarata with rad, clayoy, aad. - caa. angular md. matrix; paoblaa poorly atd. but wall rdad.; thick to .uaaalva gxadaa bada with aharp lowar contact*. 236

Section 5 Loc: Burnet Co.; Hwy 1431, 1.8 ml W of Section 4 In creek on S side of road.

10* Maroon - rad auditont raottlad groan and yallow; larga, ii- ragular liaa audctona nodulaa; no obvious baddlng.

Whlta - gray, clayay, uanaoaoua, doloaitlc liaa aidstoM; 4*-4 * nodular to baddad > nay ba atalnad with oolora of ovarlying audatona.

COVthCT HIDDBH Whlta, pabbly, aranaaaoua caa. gr. aolluak liaa gralnatona; 6 * accratlon bada V - 1* tk.» vary badly waatharod; aharp lowar ooataet.

Tallow - pink calc, foaailifaroua, (. aad. gr. aa.i nod 3 ' 3 " ular (any bo a wuatharad rona> > aharp lowar contact.

Whlta - yullowlah buff, doloaitlc and calc., foaailifaroua, v.f. - f. gr. qta. aa.; nodular, aranaaaoua, liaa packatona throughout; anail aporadic nodulaa at baaa, thin, lrragular, 11' lanticular raaiatant bada in alddla, and larga (2* in dia.) apbaroldal concrotiona at top; aaaaiva bad at baaa, but baddlng can't bo diaoarnad throughout raat of aaquancai aharp lowar contact.

Whlta, wad.-caa. gr. liaa gralnatona and aranacaoua(v.f. - f. 4'9" 1 1 0 packatona; robuot nolluak valvaa in l ' . w n r 1 * and on up par aurfacar ona naaoiva bod (4* tk.) and ona tk. bad (1 *)j no aad. atructuraa oboarvadr aharp lowr contact. 1* Orylah grn., clayay, ailty doloalta wackaatona. sryian grn.. cl&yay ailty, oyatar doloalta packatona; 1 * 4 - both whola and fragaantad tk. oyatar valvaa, aharp lowar contact. Oray and buff oyatar - aolluak liaa packatona and doloalta

4 ' wackaatona; currant orlantad bioclaata; vary lrragular thin baddlng; baddlng diaruptad naar baaa; aharp lowar contact.

Lt. gray, clayay, ailty, doloalta wackaatona - audatona; 2 * 6 “ lrragular baddlngi burrowed?

Or nan and brown clayotooa; nonfoaailifaroua; can't dia- 5*6“ oarn any baddlng; foraa alopa; v. thin foaailifaroua ailtatona in alddla.

2 * Tan, oalc., ailtatonat no baddlng or aad. atructuraa can ba aaan. 1*1' COVBRXD 6 " S' faring v.f. -f. 9*. rrainantad; aharp lowar contact. 237

5* COVKREU

Lt. gray, tightly packed oyatar lime packatona; "oyatar 3“ haah "

Buff - gray uolomitic ailtatona; scattered oyatar frag, in uppar J'; t,.inly baadad (2s tk.); graoational lowar contact.

Sycamore Conglomerate» drain supported dolomite - gt*. pebble - cobble conglomerate; gradea upward into pebbly aa. - ailtatona; pabblaa throughout are poorly atd, wall o raed.; massive, homogeneoue bega. 238

Section 6 Loc: Burnet Co., Hwy 1431, R«d muds ton* Hickory Creek

hrkoalc - aubarKoaic aa.; angular, poorly P * :it' l v (ma.-w. idad.l ca*. s». »t ba.* -TJI..U., to 1.11 Ly 15 1 u p w a r d : maaaiv*. homoqanaous at ..as* over’atnoy laminat*d - faatoon cro..-b*ds; thinly : * ^ « u 1:0 P - lowar .com contact.

Maroon mudaton* mottlad and mtarbaddad with qraan mudaton.; 11* *OW* lima mudaton* nodulaa: .lop* format; lowar contact covarad.

COVSRXD

2 > Calc., poorly atd., mad. - ca*. gr. aubarkoaic **.t angular- aubrdad. gra. ; nodular. Rad, mad. - caa. gr. qtzoaa aa.; wall atd.; angular - rdad. 3 V gra.; isaiaiva and hamoganeoua.

1 ' C O V B K 1D

Whlta, papply, aranacaoua, wall atd., v. ca*. gx. molluak lima grainatona; aoma whola molluak valvaa; accratlon bad- d«d; aoma bada contain up to 25% wall rdad. qtc. pabblaa (2" - 3• In dla.) aharp lowar contact; badly woatharad.

" " * Whlta - lt. gray pebbly, aranacaoua (qtz.-feld.) aad. gr. 4 i molluak lima grainatona and calc, aa.; larga acala faatoon croaa-bada at top, but b*low thia ar* lrragular S' - 1* tk. bada; lamlnatad and bur row*d naar baa*; aharp lowar contact. , * - * ' T a n to whita, calc., toaaxi oaaring, v.i, -r. gr. qtz. aa. ; larga, xobuat, molluak valvaa and lrragular. aranacaoua 11m* packatona nodulaa In l o . i part; lamlnatad above thia; aharp Iowar contact. ^2 ’ Tan, calc, v.f. —f. gr. qtx. aa.; lamlnatad; no foaalla; ✓ aharp loai contact. / 239

-■ Buff to tan calc., foaailifaroua, v.f. - f. gr. aa. to ailt- ,' atona, ana inteibedded aranacaoua lima packatona; abundant 4*2" currant orrantad, email high apirad gaetropoue; thin bada (1* tk.) at top and baaa; flaggy (3“ -4"), lrragular, lantlcular bada in middla, and lamlnatad and v. amall faa toon croaa-bada in uppar part; ahtrc lowar contact, "Vhita, wall road,, wall atd., caa. lima grainatona,- larga 3* molluak valvaa throughout; tk. lrragular bada (3/4* - l'tk.) at baaa; bada 3" - 4 “ tk. at top; aranacaoua at top; grada tional lonmr contact. Buff - whita, v. aranacaoua(v.f. - f. gr.) dol. molluak oyatar lima packatona; burrowau; larga, broken oyatar valve near baaa; aharp lowar contact; thick (IV) bada at top ^^and 6" -9" tk. bade at baaa; aharp lowar contact. Tan - grniah. urn. clayey, ailty, oyatar dolomite packatona; - »articulatad and broken valvea; aharp lowar contact. Grniah brn. oyatar dolomita wackaatona - lima packatona; f. - mad. gr.: aoma currant oriantaa valvea. COVERED

Buff -tan aa.; f. -iaad. gr. in lower 4' grading to v.f. -f. gr. in uppar part; aparaa, fragmentad oyatar valvaa through out (ona thin oyatar haah bad in middle),- laminated through out; gradational lowar contact.

Sycana>re conglomerate i dolomite pebble - cobble conglomer ate with wad. - cae. gr. wall atd., wall rdad., qt* ad. matrix; pabblaa are wall rdad.. fairly wall atd., accretion bada; aharp lowar contact with faatoon croaa-bada compoaed of foaailifaroua, mad. gr., wall atd., rdad. qtr aa. Loc: Burnet Co., Hwy 1431; .65 ml E of Camp Creek

Pebbly eubarkosic aa.; mad. - v. cae. gr.; poorly atd., angular; pabblaa of qtz., chart, and dolomite; l o M t contact covered; vary poorly a\poaed.

Hiroon - red and green mudstone; soma lima mudstone nodulaa; Mitharea and forms slope.

COVERED 241

Whit*, mad. - v. c*«. i)i., v. arenaceous lime jrainatone; a few pebslea end molluak valves; accretion Lieu a up to l1 thick; irregular uede at jJ ii with n p p l c i ; lower scour con tact.

. ^ ' duff, calc. - dol., v.f. yt. qti aa.; thin oeds with inter nal laminations near top; foaailifaroua (oyater frag.) in lower 1 1; gradational lower contact. - “Wli Fte to duff arenaceous oyater - molluak lime packatona; v. abunuant whole oyater valvea in lower 2 ‘;scatterad whole valves in upper part; maasive imb (21 tk.)at top and tk. (11) bed at oase. _ Oyater uolomite packstone; large disarticulated oyster valvea.

Buff, c m . - v. cae. slightly arenaceous and paooly, well atd., molluak line grainatona; accretion beds; laminated in upper 2 1; aoma faatoon crosa-beds.

White, v. arenaceous, well atd. molluak line grainatona; -4 * cae., well rded, well atd., qts. and feld. gra.; large faatoon croes-bed seta up to 2• thick; lower scour contact.

Buff, calc. - dol. v.f. gr. as; laminated.

"brain supported dolomite and chert pebble - granule cong- j, lomerate with matrix of cae. -v. caa, gr. angular and rded. qta. ad.; pebbles are well rded. - eubangular; lenticular bade; lower contact hidden.

Sycamore Conglomerate) pebbly matrix-supported eaely. crystalline dolomite; aeettered qts. chart, and dolomite pebbles; grades dwnward over 10* into massive, grain sup ported dolomite - qts. pebble conglomerate. 242

Section 8 Loc: Burnet Co.. Hwy 1431 .3 ml E of Camp Creek

Whits, ciolomitic gHtiopod-bauing llis wacktftOMi thin ly bedded; iowar contact hidden.

Whit* - lt. 9 ity nod. - cm. gr., v. arenaceous aolluak 1 1 m grainatona; accratlon badai intarnally laminated; sharp lowar contact.

Buff, dolomitic - calc., foaailifaroua v.f. gr. aa.; abun dant molluak valvaa and gastropods; burrowad and nodular in middla and uppar part} aharp lowar contact.

Aranacaoua (v.f. - f. ;r.) molluak 11m packatona; whola > and fragnmntad molluak and oyatar valvaa; thin wavy bada; sharp lowar contact. Ct. gray v. aranacaoua, v. c m . oyatar lima grainatona; accratlon bada 1' - 2* thick; sharp lowar contact.

Mad.-c m . gr., calc., faldspathic - quartaoaa aa. ; aoma oyatar fragments; faatoon croaa-bada.

vary aranaoaoms, vary cm ., oyatar lima grainatona at top grading downward into v. cm ., granular, calc., foaailifar oua faldspathic - quartsoM as.; accretion bada; v, thinly baddad (l* tk.) at top; aharp lowar contact.

1*9* Vary coarse to granular faldspathic quartsoM as.; ona thick bad; aharp scour lowar contact.

Yellowish buff foaailifaroua, f. -mad. gr. as.; thick bads (1‘) with laminations; nodulaa at top; sharp ltnmr contact.

Sycamore Cong loam rata t pebbly (dol. -qta.) cMly. crystal line dolomite and grain «upported dolomite pebble - cobble conglomerate; upper aurrace is highly irregular and bored. 243

Section 9 Loc: Burnet Co., Hwy 142 6 ml E of INT wi th Hwy 281. Camp Creek

aidatoiu with 11m wdttona nodulaa aad lenticular iidMikoalc aa, bodiaa.

nodulaa aranacaoua 11m paokstoMa. vary aranacaoua aolluak 11m grainatonai f. - aad. gx. at baaa, aad. * can. gt. in alddla, and aad. g r . at topr highly laachad nodulaa aonaa in alddla; tk. to thin ac cratlon bada; aharp lowar contact.

pabbly. a. araaaoaoua. aad. - can. gr. oyatar liaa grainatona; aoaa larga oyacar valvaa at topr foatooa croaa-bada. Araaaoaoua, doloaltlc oyatar liaa packatona at baaa; araa aoaoua (v.f. gr.) oyatar doloalta paekatoaa at topr whola oy atar aand Trigonla ? at baaa; thin wavy bada (3“ - 1* tk.)r aoaa initial dip/r aharp lowar contact.

Vary araaaoaoua caa. oyatar lina grainatona - packatona awl calc., f. - and. gr. wall rdad., pabbly faldapathic - quartaoaa aa.r bocad pabblaa and larga coral haada at baaar aoaa grainatona cObblaa (boaehiodi?) in uppar 3 *; pabblaa (qta., cht., dol., faId.,) arc uoat abunda/it in lowar 4*; faatoon croaa-bada but any bo accratlon bada at top» aharp lowar contact1 ■bad - yollaw clayay ailtatonar lowar contact hiddan.

Syoaaora Congloaarata■ pabbly, tidy., caaly. eryatallina aucroaic do load t* and grain-aupportad doloadta pabbla - oobbla oonglOMaratar contact batwaan thaaa two can ba vor tical to horiaoatal, but no wall defined bada arc pcaoanti nay rapraaant palaooalicha > uppar aurfaoa (contact with ■nMHttt) ia vary lrragular, pittad, and atainad rad, and on it aru bocad pabblaa, coral haada, oyataxa, and worn tubaa. 244

Section 10 Loc: Burnet Co., Hwy 1431 on creek Just E of Smithwlck Cemetery on south side of road

laaatura pabbly, arkoiic - subarkoaic as.; only wear.harad uppar lurfto* axpoaad,

Rad and gxeen audatonas with acactarad vertical lima mudatona nodulaa (ca1icha).

nV COVKRHO

Aranacaoua, caa. - v . caa. g r . lima grainatona; lrragular 2S* bada S • thick.

Yallowiah'brn. calc., sparaely foaailifaroua. f. gr. aa.; larga (2* dla.) apharoidal concrationa (lina packatona and 7»T calc, aa.) in lowar part and aaall concrationa at top; thick baddad at baaa; vary thin laainatad bada at top.

Buff calc., foaailifaroua v.f. - f. gr. aa.; b u r r n a d in lowar 3'; v. aranacaoua liaa packatona nodulaa and lanaaa 5' in upper part; lrragular bada V - 1* tk. in loar part and v. thin platy bada (2" - 3" tk.) in uppar part; aoaa initial dipt aharp lowar contact. Buff - whit* wall atd., rdad., aolluak liaa grainatona and

4 ' v. aranacaoua poorly atd. liaa packatona; thick (1**‘) bada; abundant aalluak valvaa on uppar aurfaca; aharp lower contact.

Silty, clayay oyatar doloalta packatona; larga disarticu 2 * 1 0 * lated oyatar valvaa; aharp lwsi contact.

16’ COVERED

C>lc«t«ou(, i p i i M l y foaailifaroua, f. qtc. (•.: homo- 9 *imwi and maaaiva; thin ahala naar top.

COVERED; not ntora than 5' - fl* to Sycam»a Conqlocaarata. 246

Section 11 Loc: Travis Co. At Haynie Flat School about 5 ml E of Splcewood.

Silty, clayvy oyitar doloaiite picXiton* with l*rg* tk. ■hallad oyater valvai.

Dolcwait* M c l n t t o M lin* pack*ton*; pack*d with taolluak frag ment* and valve*r lower 4* ia vary thinly b*dd#d (S* -l^'tk.); upper 3* bedding ii obscured.

COVERED - Sycamore Cong, below lake level. 247

Fabbly, W d . - caa. aubarkoaic aa.r angular, poorly atd. gri faatoon croaa-bada; lowar acour contactaj caa. to fina grad ation*.

CCVRRRD

2 ' Modular liM nudaton* - wackaatona. - CONTACT C WE RAD

ahit* - gryiah.whita alightly atanacaoua - pabbly MOlluak 10*4* liaa grainatona; thin accratlon bade {6' - 8" tk. >j intar- nal lamination*; gradational lowar contact.

Aranaoaoua, mad. - caa. gr. molluak lima grainatona; aoma caa, ad. - granula tiia,Mil rdad. qta. gr*. t lanaa* of tk. aha Ha d molluak valva* throughout aoquancai fa*toon croaa-bada; individual bad* in croaa-bad sat* ara 1" - 2' tk.; croaa-bada aran't apparent naar baa*; gradational lowar contact. h*>a,3ir.a$

Yallowish-buff calc., f. gr. aa.; v, aranaoaoua liaa pack- ■tonaa intarbads in uppar 4 1; lima pack*ton* nodular son** and bada in lomt, isiddla, and top; bad* in uppar 4* ar* ixragular and S" - 7* thick; rast of aaction appaara masaiv* and homoganaoua; gradational lmtt contact.

vary aranaoaoua dolomitic oystar lima pack*ton*; poorly •td. t aoam intarbada of calc., foaailifaroua as.; tk. (1* - 2') irragulax badding at top and bottom; thinly baddad (6“) in middla; abundant molluak valvaa at top; sharp lowar contact. 248

Section 12 Loc: Travis Co.; Burrow p it at Intersection of Hwy 71 and 2322; on S side of road.

covmu>

- v* c m . gr. aolluak l i w grainatonaj finaat at base* M i l defined 1* - 2 ‘ tk. accretion bada, i m of which ara in t a r nally la n in a ta d i larga coral fragMats, oystara, and doIo n ite pabblaa in upper ara t p a r t ; sharp lowar contact r

White, slightly aranaoaoua, f.-aad. gr. lire grainatona - packstonar thin faatoon croaa-bada r gradational l*n*ar contact.

vary aranaoaoua (v.f. - f. gr.) nolluak liaa packstona and calc, foaallifaroua, v.f. gr. as.; v. thin - thin 10*4* bada ( V - 1*) in lowar 2S * r above this arc aranaoaoua liaa packstona nodulas and nodular bada which ara intarbaddad with calc. aa.j wavy, irregular bada at top. 249

xallowiah tan calc. - dol. foaallifaroua. i/.f. -f. gr, 3*3" mm. i nodular in aiddla; burroadj vary thin bada at baaa t tkly. baddad at topi gradational lowar contact. Lt. gray, v. aranaoaoua aolluak-oyatar liaa packaton*; ♦ • l' poorly »td>i aoatly angular gxainaj hnanganaoua, aaaaiva bad (3' tk.) with thin bada (3" - 5"tk.) at topi aharp '■ M , lowar contact. :*■ Silty, Clayay, oyatar dolomlta packatona. ,«r

Buff - gray ailty, clayay, mad. - caa., poorly atd. oyatar doloaita wackaatona and doloaitic lima packatana; avanly baddad intarbada V - 1' tk. i aoiaa bada ara diaruptad? lowar contact cowarad. z

rOVSRXD 250

Section 13 Loc: Blanco Co.; Hwy 2766, 8.3 ml E of INT. Hwy 281; about 3 ml down entrance road to Falls on the Pedernales State Park; Arrowhead Creek .

Cray - whit* - tan, ■11ty to aranaoaoua dolonutic liaa aud- 5'-7* a too* i nodular; abundant black or iron atainad braccia claata of dol. 11m audatona; ixrag. frteturaa; lowar contact covarad.

Lt. gray. v.f. - f. dolonitic, oatxacod* -oyatar liaa packaton*i thin irregular bada; lowar contact covarad.

Hhita, aad. - v. caa. gr. aolluak liaa grainatona; gradad froa a d . - caa. upward; accration bada; aharp lowar contact.

White, f. - caa. gr. aolluak-oyatar 11m grainatona - pack- f• atoaa; gradad fina to coaraa upward; faatoon croaa-bada; lowar acour contact.

Orayiah whita calc., foaallifaroua, f. - aad. gr. aa. and aranaoaoua liaa packstona; lowar 5 V ha a two thin intar- of v. aranaoaoua liaa packstona; uppar 4%'ia aainly 11 * «*upo* ad (SDK) of f. - aad. gr. aranaoaoua liaa pack* tona- aadalaa r aaaaiva, hoaoganaoua bad at baa*; 1* -2' tk. bada in uppar 4'; aharp, undul'*** lowar contact. 251

Greyish-whita, calc., fossrlifaroua. f. med, q r . qti. •harp lowar contact.

W r y aranaoaoua, doloaltic, poorly atd., nolluak liaa pacxatona and wall atd. nolluak-oyatar liaa grainatona which ooatalna littla or no ad.t baddiag appaara to fed aaaaiva aad hoaogeanoua tbioturbatnd7)j thin lanaaa of laxga bivalves> lower contact covarad.

Silty oyatar dolonita packatona?» nonraalatanti covarad < !■ with travertine to aoat of aaction la obscured.

1 8 ' COVSMD

Vary a 11ty, v. c a a .-gr. oyatar liaa packatona. a

Lt.gray. ailty, v. caa. gr. oyatar Ilia packatona.

Borad oyatar aneruatad, dolonita pabblaa at baaa of oyatar packed aad.-gr. aa. Sycaaora Congloanrate t drain aupportad dolonita pabbla ccnglOMrata wrth caaly. crystalline dolonita aatxix; also • o w pebbly, caaly crystalline dolonita. 252

Section 14 Loc: Blanco Co.; Hwy 2766, 6.8 ml E of INT. Hwy 281; Hiller Creek.

■ m m 1 lulatMii poorly txpoMd pabbly aubarkooic u .

M od. - V.CM. M l look 11M g » l M t o n i •eciatioi b*4at aharp lowar owUct.

gr. aolluak 11m gralutoM-paokatoMi faitooa f - 7 t lawor C M U c t ia aharp (loan) or gradatloMl.

Calcaraoua, foaaillfaroua, v.f. - f. gr. g u . aa. aad araaaoaoua 11m pa oka to aa (?) apharoldal coacrntiona; aparaa oyatar valvaa throughout! larga curroot oclaatod convax-up valvaa aad gaatxopoda la lowar 3*|'f vary nodular throughout, aad aa. boddlag (lialioa) ara bant (oonpaetloaal) around nodulaa ; lowar contact piekod on appaaraaoa of aodulas.

Calcaraoua, foaaillfaroua, v.f. - f. gr. qta. aa.i a currant orlontad blvalvaai thinly baddad (I*- I'tk.Ji aharp lowar contact.

Aranaoaoua (v.f. - f. gr.) aolluak-oyatar 11m packatonat two caa. to flna cyelaa with cm. gr. packatona and whola or oonvax-up orlontad bivalvaa at b n M aad flnar packatona f**' with fragMntad oyatar a at topi Maolva-tk. (2'-4' tk.) bada In lowar part aaparatad ay aeour contact frou tk (2') uppar bad; aharp lomr contact.

COVBMO 253

Section 15 Loc: Blanco Co.; Hwy 2766, 12 mi E of INT Hwy 281; Flat Creek.

M d , h i m , aad v x m i mdatsaaa with Um «s u d M d u l a a af eiayvy liaa adataa.

CCHKT CCMMD

•lightly areas — — aad. - o n . gr. aollusk liaa jt~1 n n » i j 12* aaarstloa bada r * l V tk. with iatarMl laalaatioae t aharp laaar oeatact.

hreaaaeses, (. gr. aallaak liaa gralaatsoar oaa. gr. laaaaa at baaat aadalar weatherlag of irragalar bada la laaar parti festoae eraaa bada la eppar pertr thia platy | I M * ) bada thrsegboat.

hraaaoaaas, f. aad. gr. aallaak liaa peekataaa graiartaaai 4* saaaa af paohad ayatar ralvaai faataaa eraaa bade aad wavy, laatiealax, thia bada I* tk.t aharp laaar oaatact. ~ Calaaraaaa. faaailifaraas, v.f. - f. gr. as. with thia (la- 2* tk.) laaaaa af f. gr. liaa packsteasi laaaaa bava irrag- 3*9” alar basaa aad flat tapsi as. bada ara tk. (1*) or platy (3“-d“)i laaar oaatact is eatreasly flat aad bariaaatalt - - aatira aactiaa la paths lad by atraaa araalaa.

Calaaraaaa, feesill faroaa, v.f. - f. gs. as.r oyatar- palacypad valvaa thrawghaut, bat aaaa of dlaartioulatad (m s artlaalatsd), aahbradad valvaa at base, aiddla, aad tap.

hraaaaa aw (v.f. - f. gr.) dolaaitlc ayatar liaa paokatonai v. aia— ataM la laaar parti who la aad frapaai tad oyatar valvaai tk. (l%*-2‘) badai gradatxaaal lover eoataet. 254

I»Wl»M»il yulloalah-bulf calc., aol. , toaailifaroua v.f. - f. gr. sa. aad doloa tic v. aranaoaoua liaa pacX- atoaa aaparatad by allty, oyatar dolonita packatoaa; bloclaata ra aa. aad liaa packatoaa ara caa. - v. caa. aad oyatar aol- aait* pactatoaa bada naataia largo uafragaaatad - fragaaatad ayatarai aa. - packatoaa bada ara avan, I ' - J V tk., wharaaa ilalaalto packatoaa bada ara Irregular aad V - l* tk.

§••13* COVBMD

graaa - graaaiah-bra. dol., allty, aad. gr. ae.t uppar bada ara r * " ^ with oyatar fragaaatar aporadle thia la tar bada of 10* clayay 1 1 m packatoaa la laaar partt doles! ta pabblaa la loaar I V i aa. la avaaly baddad (*■ tk.}.

W f ra coagloaaratai Matrlx-aupportad, pobbly (qta. -dol.) ~ 7 & eryatalllaa doloalta. 255

Section 16 Loc: Hays Co.; 5.2 ml E of Flat Creek on Hwy 2766. then 4.2 ml N. Deadmans Hole.

M d BidatM* with 11a adatoa nodules.

4 ._5 , nodular 11m aidstone - wackestone containing cobbles of underlying grainatona; highly irregular development.

Slightly arenaceous, nod. - v . css. lies greinstonsi se quence is finest at beset sons individual beds are graded 20* cee. to fins or have internal laminations; accretion beds 6*-2*i' tX. i base of accretion bada contains festoon cross beds i gradational lower contact.

Arenaceous (aed. gr.) cee. -v.cea. molluak grainatone- packstona; abundant broken, robust bivalves; Irregular 1' tk. beds at base; festoon cross-beds in upper 4'i lower oootact gradational into nodular sa. balow.

Calcareous, foeeiliferous, f. - ead. gr. aa. grading upward into aranaoaoua end. gr. packstone-grainstone; 4 ,j, nodular and irregular nodular bedding; aoaa thin, platy bada (festoon cross-beds7); soma nsdules say be lima packatona; tk. broken palacypod valves throughout; lower contact covered.

3 ' COVKMD vary aranaoaoua. oyatar linn packatona at baae and top i 20* doloadtic t bedding obacurad by travertine; forma vertical cliff faoa.

Silty, ayatar doloadte packatona bada with large, unbroka 4 ’3" oyatara aaparatad by dolonita uackeatone raid*tone; aharp lowar contact.

irregularly intarbaddad solluak-oyetar liaa packatona atf allty. clayay oyatar doloadte audatone-wackeatona; 7*2- V. thin (2“-3" tk.) to tk. <2* tk.) irregular bada» currant orlantad oyatarai laar contact covarad.

£, COVSMD, but allty, oyatar liaa packatona and calc., foe- allifnroua (oyatara) aad. gr. aa. on alopa.

Borad pabblaa and claata of dolonita pabbla conglomerate.

Syoaaora conglonarata i Qrain-eupported dolonita pabbla conglonarata with dolonita aatrlx intarbaddad with lenticular caa. gr. poorly atd. aa.i poorly axpoaad. 257

Section 17 Loc: Travis Co.; along S side of Hwy 1431 at Travis Peak post office; Hensel Ranch; Cow Creek

, .SlltitOM gradiag into clayay ailtatona and finally into 1 0 ’ thinly tnddad, luinatad ailty-clayay doloait*.

Rod and Mioon audatonaa nottlad graanr lanaaa and vartical nodulaa of lina imidatonas lanaoid bodiaa of faldapathic 1 8 * aubgraywackai caa. to fina gradationa within aa.t cut and fill contactaj faatoon cxoaa-badat aharp lowar contact. (UM>, iraMOtoui, oaaiy. oiyiUllliw telonlu with highly iingulu, whit*, finely oryttalliM intattadit U n g w U i , thick <2*) aad thia («“-«- tk.) beds; eons clay residue la aad batwaaa bedsi unit haada aad bows to follow tepo- oraphy developed oh underlying nodular unit (outarop desox.) white, arenaoeous, doloaltie liaa packatoaa waekaatoaa ; discrete nodulesj oalclte filled fractures; also soee sty- lolltas, ellckeaalldes; very Irregular - uidulatory lower contacti uppar baach surface la pock-earfeed aad potholed. (outcrop deecxiptloa)

Caa.-v.oaa. gr. liaa grainatona; secretion beds € “ -2* tk. ; sane bads are graded or laternally laminated; sequence is flaaat at its baas ; aharp lower contact.

Wed. css. gr. line gralaatona psckstnaat festoon cross- bads (outcrop).

Calcareous, foeslliferoue, f. - aad. gr. a*, with abundeat nodulest as. is aaaaiva (4* tk.) at baee, laminated in middle amd irregularly laminated <“wispy") in upper pert; nodulaa are arena oeous liaa packetone containing pellets and aagu- larmolluek fxsgMnts( nodules have diffuse contacts with ma trix; sharp lowar oontact.

Arenaoeous (v.f. - f. gr.; dol. oyster lima packatona; s o m small disarticulated, unbroken molluak and bored oy ster fragments; burrowed; leas dolomitic upardt aharp Ic m i oontact.

Silty, clayey oyster dolomite packstona; large, unbroken, robust oyatar valves.

Buff - grayish tan - gray 11m weeksstone and line peckstone and silty, clayey, dolomite weekestone mudstone; v. thin - thin bedding > angular oyster free, mollsuks, and eehiaoida; dolOsd.be always contains snailer and lass bioclasts; beds ara highly irregular; disturbed, and disrupted with dolomite apparently aquassad into more rigid, fractured limestone f bedding is thicker and 11asatone baconss more abundant upward at expense of dolonita; gradational inat oontact. Oray, clayay, iingultrly Italnatad, bunowad liltitoM and aa. with claystana in barbada in lowar 4*i aaetion gradaa Into raddiah hrn. clayatona; top oC clayatona ia gray and Intarbaddad with clayay dolonita waciastona.

Calcaraoua, foaaillfaroua (oyatar frag.) clayay, f. - and. gr. aa.t irragular lauinaa; burrowed? lanaa of llna packatona in nlddla» gradational lowar contact. dray * graanlah-gray clayay ailtatona at baaa grading up ward to Clayay, v.f. gr. aa.i lamina* ara irragular axoapt at top * A a » \hay ara avani buiaal. C O M L O U Oray aad dk. gray audatona with angular oyatar fragnanta? burrowadr aharp lowar contact. Taxturally inatur* aubgraywaoka fining upward into rvodula* (lino audatona) rad and groan nudatonaar raworkad pabblaa of lino nodulaa eonoantxatad at baaa. but acattaxad throughout t aharp lowar oontact.

Maroon and graaalah-whita audatona with dol. liaa audatona nodulaa.

C O M LOSS

Doloodtlc liaa wdatona nodulaa in rad to graan audatona and aad. caa. gr aubgraywacko.

Mad. - caa. gr. aubgraywackoi aaroon with graan mottling? thin allty, clayay aonaa? flnaa upwardi dol. liaa wdat o n a nodulaa at top t aharp lowar contact with rad widatona con taining liaa audatona nodulaa. 260

Section 18 Loc: Travis Co.; .3 mi N of Hwy 962. .5 mi E of Pedernales River. Hami1 ton Pool .

szniah.-qray doloaitlc clayay, alltatoM aad sixty, clayay lO' doloaita aidit w w j thia lenticular, irragular 1* tar bada (36* tk.)i irragular laminatioaa, burrowed; gradatiomal from ailtatone at baaa to dolomita at top.

3' ■ad aad graaa wdatona with buff-white nodular bxacciatad clayay dol. liaa mad a tone at baaa.

1 0 ' 3* Oradad caa. to fine feldspathic quartaoaa aa 7 C O M L O W

10* Bod aad groan aidatoaw with acattarad irragular, clayay liaa ■idatoma nodulaa.

10** M h ita^uff dol. liaa ■udatctna waelcaatona [ nodular.

1 9 ■ Mad.-v.caa. gr., wall rdad, wall atd., Molluak liaa grainatonar gr. qts. grs.

I 1 Nad.-caa. gr. molluak liaa grainatona; faatoon croaa-bada. 261

C O M UM»

F I m - ooaraa «r*n«aaou« (v. f. - f, gr ad. j 1 1 m pack- ■ t M M ' ^ r a l M t o a i with ainot in tar bada of dolonitic, calc., foaaillfaroua, qtr. aa.j biOclaata neatly rdad., poorly atd. - wall atd.t aoattaradlarga aolluak valvaa, but coaoaa* tratad at baaa aad aiddla of aactioar tk, (l*i' -2Vth.) Irragular badat c y c lic oaa. to fiaa gradatloaa with caa. M a t * haviag aharp baaaat aoan burrowing j laainatioaar faatooa croaa-bada in aiddla1 aharp lowar contactt ba- ooana vary aranaoaoua upward.

Clayay, allty, oyatar d91oalta packatoaar larga dlaartl- culatad, unbrokan valvaa.

Oyatar-nolluak liaa packatoaa aad alaor thia intarbada of clayay, allty oyatar doloadta wackaatooa audatona t thia, foaaillfaroua audatona in aiddla t aoaa bloclaata la liaa- atOM ara rdad. and 11matoaa aouatiMa coataina larga oyatar valvaa 1 achiooida, aarp. worn tubaa, alao praaant; irragular, v. thin-thin {1“-1' tk.) dlaruptad badat liua- atona pradoadnataa throughout axoapt w a r top j gradational lowar contact.

Clayay, ailty, dolonita wackaatcma-nudatona and oyatar- nolluak liaa packatona1 vary irragular dlaruptad bada S '- 1* tk.t liaa packatoaa appaara to havu boon dlaruptad by injactloo of dolonitat packatona Mkaa up SON of l

Rad - r«Miih ben. - gray el ays ton* and ar*n*c*ou«, clayay alltatonai slltstona contain* oyatar fragnanta but clay- aton* doaa notj gradational within and alao gradational lowar contact.

Gray, clayay, caa. gr. allt*ton* wiht abundant oyatar 5*7* fragaaatar lanaaa of aranaoaoua oyatar liaa packatona with angular, unatd. bioclaata? gradational lowar contact.

kaddiah brn. clayatona; hoaoganaoua; bacon*a allty at topi 7* lowar gradational oontact.

Brown to gray, clayay, can. gr. alltatonai acattarad anall 5, oyatar fragnantar lowar 3* contain* Irragular nodulaa of pyxltic, aranaoaoua, oyatar liaa packatonai lowar gradation al contact.

Gradational aaquanoa t Rad - aaroon aottlad graanlah whita - whit* throughout; caa. gr. taxturally ianatur* aubgraywacko 23* with doloadta pabblaa at baaa grading auccaaaivaly into fina •*.. ady ailtatona, and audatona; mottling at top M y b* duo to burrowing; borad dolonita pabbla* at top.

Rad nidaton* nottlad white-graanlah whita; alao thin aa. lanaaa. VITA

Richard Francis Inden was born January 13, 1943* in

Milwaukee, Wisconsin. He attended Messmer High School ana was graduated in 1960. In September, 1960, he enrolled 1n the University of Wisconsin-M11waukee and majored in geology. From September, 1964, through January, 1965, he held a teaching assistantshlp 1n the Geology Department.

He was graduated from the same Institution in January,

1965, with honors In the major field and senior honors.

In February, 1965, he enrolled in the University of

Illinois, receiving his Master of Science in Geology in

January, 1968. During the summers of 1965 through 1967 he worked as a geologist, consecutively, for Standard 011 o?

Texas, Pan American, and Chevron. His master's thesis wai entitled "Petrographlc Analysis and Environmental Interpre tation of the Breezy Hill Limestone in Illinois, Missouri,

Kansas, and Oklahoma." From February, 1968, through 1971 he was a Ph.D. candidate at Louisiana State University. He held a teach ing ass1 stantsh1 p and summer research assistantsh1 ps in

1968-69 and 1971; during 1969-70 he held a NASA Tralnee- ship. He was graduated 1n May, 1972.

Mr. Inden 1s a member of Sigma X1 and the Society of

Economic Paleontologists and Mineralogists.

263 264

In January, 1972, he began a post-doctoral appoint ment at the University of South Carolina Geology Depart ment. EXAMINATION AND THESIS REPORT

Candidate: Richard Francis inden

Major Field: Geology

Title of Thesis: Paleogeography, Diagenesis, and Paleohydrology of a Trinity Cretaceous Carbonate Beach Sequence, Central Texas,

Major Professor and Chairman

Dean of the Graduate School

EXAMINING COMMITTEE:

Date of Examination:

December 9, 1971