Triassic Rocks of Texas TED GAWLOSKI Creative Thinking Is More Important Than Elaborate

Triassic Rocks of Texas TED GAWLOSKI Creative thinking is more important than elaborate

FRANK CARNEY, PH.D. PROFESSOR OF GEOLOGY BAYLOR UNIVERSITY 1929-1934

Objectives of Training at Baylor

The training of a geologist in a university covers but a few years; his education continues throughout his active life. The purposes of train­ ing geologists at Baylor University are to provide a sound basis of understanding and to foster a truly geological point of view, both of which are essential for continued professional growth. The staff considers geology to be unique among sciences since it is primarily a field science. All geologic research in­ cluding that done in laboratories must be firmly supported by field observations. The student is encouraged to develop an inquiring ob­ jective attitude and to examine critically all geological concepts and principles. The development of a mature and professional attitude toward geology and geological research is a principal concern of the department.

THE BAYLOR UNIVERSITY PRESS WACO, TEXAS f

BAYLOR GEOLOGICAL STUDIES

BULLETIN NO. 41

Stratigraphy and Environmental Significance of the Continental Triassic Rocks of Texas

Ted Gawloski

BAYLOR UNIVERSITY Department of Geology Waco, Texas Spring 1983 Baylor Geological Studies

EDITORIAL STAFF

Jean M. Spencer Jenness, M.S., Editor environmental and medical geology

O. T. ward, Ph.D., Advisor, Cartographic Editor what have you

Peter M. Allen, Ph.D. urban and environmental geology, hydrology

Harold H. Beaver, Ph.D. stratigraphy, petroleum geology

Rena Bonem, Ph.D. paleontology, paleoecology

William Brown, M.S. structural tectonics

Robert C. Grayson, Ph.D. stratigraphy, conodont biostratigraphy and sedimentary petrology

Don M. Greene, Ph.D. physical geography, climatology, earth sciences

Don F. Parker, Ph.D. igneous geology, volcanology, mineralogy, petrology

Ker Thomson, D.Sc. W. M. Keck Foundation Chair in Geophysics

STUDENT EDITORIAL STAFF

C. Elaine Cartographer, Student Editor

The Baylor Geological Studies Bulletin is published by the Department of Geology at Baylor University. The Bulletin is specifically dedicated to the dissemination of geologic knowledge for the benefit of the people of Texas. The publication is designed to present the results of both pure and applied research which will ultimately be impor­ tant in the economic and cultural growth of the State.

ISSN 0005-7266

Additional copies of this bulletin can be obtained from the Department of Geology, Baylor University, Waco, Texas 76798. $1.05 postpaid. CONTENTS

Page Abstract 5 Introduction 5 Purpose 5 Location 7 Procedures 8 Previous works 8 Acknowledgments Descriptive Geology Regional geologic setting 12 Regional structural elements Distribution and Character of the Eagle Mills Formation Geologic setting Thickness .15 Lithology 16 Distribution and Character of the Sycamore Formation Introduction Regional geologic setting contacts 18 Lithology of the Sycamore conglomerate Outcrop regions of Sycamore conglomerate 20 Distribution and Character of the Dockum Group Introduction Geologic setting Stratigraphic contacts 22 Lithology of Dockum rocks 23 Distribution and Character of the Bissett Formation 27 Introduction 27 Interpretive geology 27 Sedimentary History of the Eagle Mills Formation 27 Depositional environment 27 Paleotectonic implications of origin 27 Ancient depositional models 32 Sedimentary History of Sycamore Rocks 33 Suggested depositional models for Sycamore sedimentation 34 Sedimentary History of Dockum Rocks 37 Tecovas Formation 37 Trujillo Formation 37 Depositional systems 38 Sedimentary history 38 Provenance area 40 Depositional models 40 Sedimentary History of the Bissett Formation 41 Age determination Depositional environment 41 Comparison of the Continental Triassic Rocks of Texas 41 Conclusions 43 Appendix 44 References 45 Index 48 ILLUSTRATIONS

FIGURE Page 1. Index map of wells penetrating Eagle Mills Fm., NE Texas 6 2. Outcrop and locality map, Sycamore rocks, central Texas 7 3. Outcrop and locality map, Dockum rocks, W Texas 8 4. Structural map of Texas during Triassic deposition 9 5. Outcrop and locality map, Bissett Fm., SW Texas 9 6. Physiographic map of Texas with outcrops of Triassic rocks 7. Map of Interior Mesozoic Basin and Ouachita Tectonic Belt 8. Late Triassic paleogeography of Texas 9. Thickness of Eagle Mills Fm 10. NW-SE cross-section of Eagle Mills Fm., S Arkansas 16 11. W-E cross-section of Eagle Mills Fm., central and E Texas 16 12. Slab of coarse-grained red sandstone, Titus Co 17 Slab of Eagle Mills conglomerate, Titus Co 17 14. Slab of Werner Fm., Titus Co 18 15. Slab of Werner Fm., Titus Co 18 16. Loc. 5. Sycamore Fm. overlying Pennsylvanian rocks 19 17. Loc. 14. Fluvial point-bar deposits of Travis Peak Fm., Lampasas Co 19 Loc. 19. Slab of Sycamore conglomerate 19 19. Loc. 3. Slab of Sycamore conglomerate 20 20. Loc. 18. Sycamore conglomerate at Elliot Creek 20 21. Loc. Exposed caliche zone between Sycamore and Cretaceous rocks ... .21 22. Loc. 5. Weathered conglomerate exposed at Pedernales Falls 21 23. Lithologic-stratigraphic relationships of High Plains sediments 22 24. Loc. 5. Channel sand of the Dockum Gp., W Texas 23 25. Loc. 1. Shales of the lower Dockum Gp., Co 23 26. Loc. 8. Slab of Dockum intrabasinal conglomerate 24 27. Loc. 6. Slab of Dockum extrabasinal conglomerate 24 28. Loc. Outcrop of Tecovas Fm., Garza Co 24 29. Loc. 4. Outcrop of Tecovas sandstone 24 30. Loc. 7. Tecovas exposures, Garza Co 25 Loc. 14. Tecovas shales, Palo Duro Canyon 25 32. Loc. 5. Massive sandstone channel 25 33. Loc. 5 Outcrop of Trujillo Fm 26 34. Loc. 12. Sigmoidal foreset cross strata 26 35. Loc. 14. North rim of Palo Duro Canyon 26 36. Orientation of cross-beds in Dockum sandstone 26 37. Pennsylvanian to Cretaceous lithofacies-environment maps 28 38. Postulated Precambrian basin margin of the Gulf of Mexico 29 39. Late Mesozoic sea-floor spreading of Pangaea 29 40. Map of the pre-Mesozoic to Middle Jurassic Gulf Coast region 41. Generalized W-E cross-section, E Texas basin 32 42. Map of Gulf Coast region during Louann deposition 32 43. Facies development of upper Rotliegendes in NW Europe 33 44. Map of Llano Uplift 34 45. Plan and cross-section of Van Horn alluvial fan system 35 46. Map of Gorak Shep alluvial Eureka Valley, Calif 36 47. Caliche zone in upper Sycamore conglomerate, Lampasas Co 37 48. 19. Topography of present day Sycamore Fm 37 49. Loc. 5. Distributary channel-fill 38 50. Loc. 5. structure, Dockum deposits 38 Facies developed at high and low stand depositional phases 39 52. Map of Triassic Intertropical Convergence Zone and climate regions 43 53. Map of Upper Triassic climate-sensitive sedimentary rocks 43 Stratigraphy and Environmental Significance of the Continental Triassic Rocks of Texas

Ted Gawloski

ABSTRACT

The continental Triassic rocks of Texas are repre­ fan deltas, and lakes. The coarse sandstone and conglom­ sented by four distinct but similar rock groups, which erate are products of high-energy, short duration depo- exist both in outcrop and in the subsurface and include: sitional events. the Eagle Mills Formation (south-central and northeast Sedimentation was greatly affected by alternating Texas), Sycamore Formation (central Texas), Dockum matic conditions that produced changes in base level, Group (west Texas), and Bissett Formation (southwest water depth, lake area and the type of streams that flowed Texas). They are clearly terrigenous in nature, derived into the depositional basins. Character of the rock groups principally from older Paleozoic sedimentary rocks. strongly suggests semi-arid to arid deposition typical of The rock groups are composed in part or entirely of the low latitude desert regions of today. Thus, the rocks mudstone, siltstone, medium- to coarse-grained sand­ forming the Eagle Mills, Sycamore, Dockum, and Bissett stone and pebble- to boulder-conglomerate (intrabasinal Formations appear to be continental clastic products and extrabasinal). The sediments were deposited in allu­ deposited during a major semi-arid to arid climatic epi­ vial fans, braided and meandering streams, lobate deltas, sode, such as that of Late Triassic time.

PURPOSE of the Interior Gulf Coast Basin and consists of as much The continental Triassic rocks of Texas are repre­ as 7000 ft of alternating sequences of coarse red sand­ sented by four distinct but similar rock groups: 1. the stone, siltstone, and conglomerate deposited as bed-load Eagle Mills Formation of northeast Texas, 2. the Syca­ to mixed-load streams on alluvial fans, which extended more Formation of central Texas, 3. the Dockum Group from the Ouachita Tectonic Belt southeastward into sub­ of west Texas, and 4. the Bissett Formation of southwest siding grabens. Texas (Table 1). The Sycamore Formation of central Texas is a highly The Upper Triassic Eagle Mills Formation accumu­ locahzed, poorly sorted, coarse conglomerate consisting lated in an arcuate pattern bordering the eastern edge of of pebbles and cobbles of hard limestone, dolomite, and the Ouachita Tectonic Belt from Falls and Robertson chert in a matrix of coarse quartz sand. It is a distinctive Counties in south-central Texas northeastward through unit in that it differs in composition and character from Bowie County and eastward into Arkansas and Missis­ the underlying Paleozoic rocks and the overlying basal sippi (Fig. 1). It is a subsurface unit, now buried beneath Cretaceous deposits. Rocks of the Sycamore Formation thousands of feet of Jurassic and Cretaceous sediments. were deposited upon an erosional surface characterized The Eagle Mills formation represents the initial deposits by a series of valleys and divides that extend north,

thesis submitted in partial fulfillment of the M.S. degree in Geology, Baylor University, 6 BAYLOR GEOLOGICAL STUDIES

TABLE 1. NOMENCLATURE CHART, UPPER TRIASSIC eastern New Mexico, southeastern Colorado, far ROCKS OF TEXAS. western portions of Kansas and Oklahoma. It com­ posed of up to 2000 ft of complexly interrelated terrigen­ ous clastic facies ranging from mudstone to conglomer­ CENTRAL TEXAS EAST TEXAS SOUTHWEST TEXAS WEST TEXAS ate (McGowen et 1979, p. 2) (Fig. 3). The sediments

TERTIARY accumulated in: 1. alluvial fans, 2. valleys of braided and [ PLIOCENE) CRETACEOUS CRETACEOUS meandering streams, 3. lake basins, and 4. highly con­ JURASSIC structive lobate deltas. Within the Dockum Group sev­ eral depositional cycles are recognized from the high and

CHINLE FM. low stand of lake level within the basin. Principal sedi­ ? o ment sources were the Ouachita-Marathon Tectonic

TRUJILLO FM. SYCAMORE EAGLE MILLS Belt, Wichita Mountain System, the Diablo Platform,

FORMATION FORMATION the Llano Uplift and the Sacramento Uplift (Fig. 4). DO C TECOVAS FM. The Bissett Formation (Lower-Middle Triassic) is exposed in the Glass Mountains of southwest Texas (Fig. BISSETT 5). The deposits consist of interbedded sandstones, lime­ stone and chert conglomerates, reddish-brown mud- PE PERMIAN PALEOZOIC PALEOZOIC stones and thin beds of limestone and dolomite (King, UNDIFFERENTIATED 1935, p. 1544). Physical and fossil evidence indicate a pre Dockum Triassic age for the Formation, the Llano Uplift. The Llano region served as which represents initial Triassic alluvial fan and fan-delta the primary source area for the of the Sycamore sedimentation immediately north of the Ouachita Tec- Formation. These were probably laid down as a tonic in southwest Texas. bed-load deposit in high gradient wadi stream channels The purpose of this study, therefore, is to describe the or alluvial fans (Fig. 2). character, distribution, and environmental significance The Dockum Group (Upper Triassic) was deposited in of the continental Triassic rocks of Texas. Each of these a fluvial-lacustrine basin, which formed in west Texas, rock groups was deposited under similar environmental

O 3 50 ' I j ftWelllocation "i

I ' HOPKINS ; R 1

ZA

ELLIS

HENDERSON-,

HILL

FALLS ,

\

Fig. 1. Index map of northeast Texas showing location of wells that penetrate the Eagle Mills Formation. CONTINENTAL ROCKS OF TEXAS 7

conditions characteristic of semi-arid regions. All of the CENTRAL TEXAS (SYCAMORE FORMATION) rock groups are largely terrigenous in origin, most . derived from the same tectonic elements. It is, therefore, Sycamore Formation crops out west-central probable that a correlation exists between these rock types, and that the paleogeography of the Triassic of the area located west-northwest of Austin, southwest of Dallas, and west Waco, Texas. The study area includes Texas can be determined. , all or parts Blanco, Brown, Burnet, Comanche, Hays, Lampasas, Llano, McCulloch, Mason, Mills, San Saba LOCATION ' Travis Counties (Fig. 2) and lies within the physio­ graphic regions of the Lampasas Cut Plain and the West- The continental Triassic deposits of the study area ern Cross Timbers (Fig. 6). occur, both in outcrop and in the subsurface, over large areas of northeast, central, and northwest Texas. For this NORTHWEST TEXAS GROUP) investigation the area of study is subdivided into four separate regions: northeast, central, northwest, and The Dockum Group was deposited in a broad shallow southwest Texas. basin stretching across what is now the Panhandle region of Texas. This investigation is concerned chiefly with NORTHEAST TEXAS (EAGLE MILLS FORMATION) rocks that crop out along the Caprock Escarpment Armstrong, Borden, Crosby, This study area extends from south-central Texas Dawson, Deaf Smith, Dickens, Fisher, Garza, northeastward to the border. It includes Howard, Kent, Lynn, Lubbock, Martin, Mitchell, Mot- all or parts of Bowie, Camp, Cass, Delta, Ellis, Falls, ley, Nolan, Oldham, Potter, Randall, Scurry, Sterling, Franklin, Freestone, Hill, Hunt, Kaufman, and Swisher Counties (Fig. 3). Most of the Triassic de- Limestone, McLennan, Navarro, Red River, Robertson, posits of west Texas occur beneath younger deposits of and Rockwall Counties (Fig. 1). The trend of the Eagle the Llano Estacado (Fig. 6). Structurally the area lies Mills closely follows the eastern edge of the buried Oua- within the boundaries of the Midland Basin, Matador chita Tectonic Belt. Arch, and the Palo Duro Basin (Fig. 4).

Fig. 2. Outcrop and locality map of Sycamore rocks, central Texas. 8 BAYLOR GEOLOGICAL STUDIES

SOUTHWEST TEXAS (BISSETT FORMATION) value. However, some reports describe the stratigraphy and depositional history of these Triassic rocks groups. The Bissett Conglomerate crops out the area the concerned chiefly with the Glass Mountains northern Brewster and southern stratigraphic relationships and age of the deposits. More Pecos Counties of Texas. Structurally the area within Triassic describe the Marathon Uplift 4, 5). relationships with the initial opening of the Gulf of Mexico. Currently, there is a renewed interest in the PROCEDURES Triassic deposits of Texas because of the discovery of The methods of this investigation included: a litera- uranium within the Dockum Group and the association ture review, 2. field reconnaissance of selected Sycamore of the Eagle Mills Formation petroleum exploration and Dockum outcrops, 3. microscopic examination of Ouachita Tectonic Belt. slabbed samples and cores, and 4. the analysis of north- the section that follows literature describing east Texas logs of wells that penetrate the Eagle Mills or rocks of Texas is divided groups, one for each of the underlying Paleozoic rocks. In addition, the rocks of the Triassic stratigraphic units and the final for works the Eagle Mills were described from cores of the Humble, dealing recent and ancient depositional models. Titus County, Texas. From this information, six structural cross sections were prepared to better illustrate EAGLE MILLS FORMATION the paleotectonic regime that existed during Eagle Mills , , Very little work has been done to date concerning the stratigraphy and environmental significance of the Eagle Formation in Texas. There is some confusion in PREVIOUS WORKS nomenclature of the various subsurface formations of the The continental Triassic rocks of Texas have, for the Gulf Coast region, most of which have no outcrop equi- most part, been ignored by many authors, mainly valents. This confusion is due chiefly to the use of such because of their lack of significant deposits of economic names as Permian or Trinity for formations of unknown

Outcrop Localities pattern

Fig. 3. Outcrop and locality map of Dockum rocks, west Texas. CONTINENTAL ROCKS OF TEXAS 9

Fig. 4. Structural map of Texas showing major tectonic elements affecting deposition of Triassic rocks and suggested source area for Dockum sediments. Adapted from Kiatta, 1960, M.S. thesis, Texas Tech College, p. 52. age (Shearer, 1938, p. 723-724). The Eagle Mills Forma­ tion has been classed as Pennsylvanian, Permian (Weeks, 1938; 1945; and Crosby, 1967), or Jurassic 1943 and Swain, 1949) on the basis of strati- graphic succession. This information, however, the fossil evidence of a Late Triassic age for the Eagle Fig. 5. Index map of southwest Texas showing the outcrop region Mills determined by Scott et (1961). of the Bissett Formation. The Eagle Mills Formation was named by Shearer (1938) from wells drilled near the village of Eagle Mills, Calhoun County, Arkansas. He described the Eagle Mills Formation as consisting of red shale and sand overlying Woods and Addington (1973), Todd and Mitchum Pennsylvanian and Mississippian sediments and underly­ Burgess and Walper (1980) describe the ing the Smackover limestone. He suggested that the nature and character of the Eagle Mills Formation and Eagle Mills red beds are early Mesozoic, probably Trias­ relate this to the origin of the Gulf of Mexico. They sic or Jurassic in age (Shearer, 1938, p. 724). Hazzard et suggest that the Eagle Mills red beds were deposited in al. described the Eagle Mills Formation as a Per­ grabens and half grabens resulting from postorogenic mian red bed sequence overlying the Morehouse Forma­ normal faulting or from the initial rifting of the conti­ tion and underlying the Werner conglomerate and anhy­ nents during Late Triassic time. drite. Scott et al. (1961) described the nature, character, and thickness of the Eagle Mills Formation from wells in SYCAMORE FORMATION extreme northeast Texas and Arkansas. Furthermore, Scott recovered a fossil leaf impression from the Humble, While there is some question that Sycamore rocks are well in Hempstead County, Arkansas. He of Cretaceous age they were for many years included determined this leaf to be of Late Triassic age. At about within the Cretaceous section. During those years basal the same time, E. J. Crosby et al. (1961) described the Cretaceous strata in Travis and Burnet Counties were stratigraphy, thickness, and paleotectonic implications first described by Taff who referred to the conglom­ of the Eagle Mills Formation in the Gulf Coast region. erates and sandstone that rest unconformably upon Recent studies by Vail and Mitchum (1967), Rain­ Paleozoic rocks as the "Trinity Sands" and included water, (1968), Vernon (1971), Rowett (1972), within these the Sycamore conglomerates. R. T. Hill 10 BAYLOR GEOLOGICAL STUDIES

40

pattern

Fig. 6. Physiographic map of Texas showing the area of outcrop for the Triassic rocks. Adapted from Raisz.

(1901) laid the basic geologic groundwork of central Texas in his study of the "Geography and Geology of the Black and Grand Prairies of Texas." Hill subdivided the Travis Peak Formation into three distinct lithologic units, in ascending order the Sycamore sand, Cow Creek beds and the Hensell sands. This work includes a general description of the topography of the pre-Cretaceous surface. summarized existing knowledge of the structural The Travis Peak Formation Travis, Blanco, and geology of the Texas foreland. His summary, which was Hays Counties was described by Cuyler (1931). He pro- compiled from a vast variety of literature, furnishes the detailed descriptions of the "basar tectonic framework referred to in this report. Later, described lower conglomerate 1962, his summary was updated and (within study area), and described the regional uncon- and (1956) suggested elevation of the at the top ofthe Sycamore conglomerate. Sycamore sandstone to formation rank. They also be- 1932 described certain Pennsylvaman conglom- that the conglomerate was nonmarine in origin, crates of central Texas that crop out McCulloch possibly pre-Cretaceous in age. County and may be correlative with the Sycamore For- Getzendaner (1956) also believed that the coarse con- He determined that stream direction during late surrounding the Llano Uplift were older than Paleozoic was westward away from the Cretaceous, possibly Jurassic or Triassic in age. He was ,, , , the first to differentiate the coarse terrigenous gravels of In 1934, Sellards described the major structural Sycamore Formation from the coarse sand and con- ments of central Texas emphasizing the faulted rocks of glomerate of the Lower Trinity Group. In a guidebook the Llano Uplift. These structural elements described by San Angelo Geological Society Getzendaner Sellards are those referred to report. states' Cretaceous conglomerates on the east side of the Llano there are some reasons for believing that pre-Cretaceous rocks Uplift region were described by Damon (1940). He may outcrop locally, at present, in Central Texas. ... Around the divided the into two distinct units: an Central Mineral region and in some directions extending out for upper marine marginal unit and a lower fluvial unit considerable distance a conglomeratic material , margmal, but piedmont and valley-flat fill, in part, Damon described the lower conglomerate as continental to the Jurassic and Triassic (Getzandaner, p. character, deposited before the advance of the Cre- 77-78). taceous seas. He suggested that the lower conglomerate Boone (1968), in a study stratigraphy and deposi- may be as old as Triassic. tional history of the basal Trinity sands between the In 1950, Plummer described Lower Pennsylvanian Colorado and Brazos Rivers, included the Sycamore strata of the Llano area, which apparently were the chief conglomerate as a continental facies of Travis Peak contributors to the Sycamore conglomerate. Eardley deposition. CONTINENTAL TRIASSIC ROCKS OF TEXAS 11

Further studies on the stratigraphy and sedimentary chiefly from the ancestral Rocky Mountains or from the history of the Sycamore Formation and initial Cretace­ Wichita Mountains in Oklahoma. Eight years later, Pat- ous deposits were presented by Dreyer and Young ton described the Triassic rocks of Potter County, et Dreyer also suggested that portions of the Texas, following the divisions previously established by basal conglomerate, considered Cretaceous in age, may Gould. actually be older. Another division of the Triassic System was presented An extensive study of the Cretaceous-pre-Cretaceous by Hoots (1926) who studied the outcrop area in the contact and the effect of paleoplain (pre-Sycamore) southeastern part of the Llano Estacado as part of an topography was completed by Bain in 1973. Additional exploration for salt and potash deposits for the U.S. work on the basal Cretaceous of the central Texas Geological Survey. He defined a lower unit of red clay region was completed by Dobbins (1974). He related and gray cross-bedded sand overlain by an upper unit of Sycamore deposition to fault-controlled valleys in the red clay. He attempted to correlate his divisions with Colorado River region and also suggested that the Syca­ those of Drake and Gould, suggesting that Gould's Teco­ more was older than Cretaceous. vas and Trujillo formations graded into his lower unit, Recent studies on the Sycamore Formation in central and that his upper unit was the same as that of Drake Texas were presented by Ledbetter (1976) and Blalock (Hoots, 1926). (1976) . Blalock stated that the Sycamore conglomerate is Adams in his study of the Triassic of west Texas not a clearly defined stratigraphic unit, differing in com­ described the structure, geomorphology, and lithology of position from location to location. Ledbetter described the Dockum Group from subsurface and surface data. the stratigraphy and depositional environment of the He defined several properties used to distinguish Triassic Sycamore Formation and divided it into three informal deposits from the underlying Permian deposits, which at units: a lower conglomerate unit, a middle sand and first glance appear quite similar. In addition, Adams caliche unit, and an upper conglomerate unit. Hazelwood considered that the Triassic deposits of west Texas corre­ (1977) discussed the paleogeomorphic evolution of the late with continental Triassic deposits farther west and Wichita Paleoplain in the Colorado River region of cen­ suggested the name "Santa Rosa" for the basal sand and tral Texas and suggested a Jurassic age for the Sycamore "Chinle" for the upper shale, both of which are forma- conglomerate. tional names from the upper Triassic deposits of western Whigham, in 1978, described the basal Cretaceous New Mexico. marginal marine deposits within the central Texas area. Roth (1932) stated that the source of the siliceous In this, however, she excluded the Sycamore conglomer­ conglomerates of the Dockum Group was probably to ate from her discussion of Cretaceous units. the southwest in the area of the Glass Mountains. How­ GROUP ever, well suggested that the provenance lay to the northwest in the southern Sangre de Cristo Moun­ The first detailed studies on the Triassic deposits of the tains. Southern High Plains were completed by Cummins in The next study of the Triassic rocks of Texas was by 1889and 1890. these reports, he described the Triassic Green in 1954. This is a detailed comprehensive analysis exposures of northwest Texas and gave them the name of the stratigraphy, paleontology, petrology, and sedi- "Dockum beds" after outcrops near Dockum, in Dickens mentology of the Dockum Group in northwest Texas. He County. modified the two formation division as established by One year later, Drake in the first comprehensive Gould referring to the red beds that overlie the Trujillo report on the Triassic rocks of west Texas divided Cum­ sands in the southern part of the area as the Chinle mins' "Dockum beds" into three units: a lower sandy clay Formation. bed, a middle unit of sandstone and conglomerate, and Recent studies dealing with the stratigraphy, petrol­ an upper bed of sandy clay. Also in E. D. Cope ogy, and paleocurrent indicators for the Upper Triassic described the vertebrate remains found within the Dockum of west Texas were made by Kiatta in and Cazeau in Group and confirmed the opinions of Cummins and From lithologic and sedimentary analyses of the Drake as to the Triassic age of the deposits (Green, sandstones, both authors agree that the Llano Uplift p. 9). probably served as the immediate provenance area for In and 1907 Gould, two reports on the geology the Dockum and water resources of the Texas Panhandle, described Matthews (1969) in a geologic description of Palo the Dockum rocks of the Canadian River valley and Palo Duro Canyon described each of the formations there Duro Canyon regions. In these reports he them exposed and the processes of weathering and erosion that as ft of interbedded shales, sands, and conglom­ formed the canyon. He examined and described outcrops erates. In addition, Gould subdivided the Dockum along the eastern edge of the Caprock Escarpment in Group into two formations: the lower Tecovas Borden and Garza Counties. Formation and an upper Trujillo Formation composed A paleocurrent analysis of the Upper Triassic sand­ of massive ledges of cross-bedded sandstone and con­ stones of the Texas High Plains was completed by Cra­ glomerate interbedded with red shales. mer in 1973. He also suggested a probable southeastern Baker proposed that the coarse sandstone and source area for the Dockum conglomerates of the Dockum Group were derived Walker (1978) described the geomorphic evolution of 12 BAYLOR GEOLOGICAL STUDIES

the southern High Plains, paying particular attention to similar in composition to the Sycamore Formation are the factors that controlled the deposition of the Ogallala included in this work. Glennie (1970) described desert Group. In this study he mentioned a land­ sedimentary environments. addition, much of the cur­ scape developed on Triassic rocks. rent literature on sedimentary environments and prod­ The most comprehensive study of the distribution and ucts was summarized by Reineck and Singh (1970) and depositional framework of the Dockum Group in north­ Reading (1978). west Texas was completed by McGowen et in 1979. Since pole positions are important to any paleoclima- They described in detail the complex depositional facies, tological interpretation, a brief review of paleoclimato- cyclic sedimentation, sediment dispersal patterns and literature was undertaken. Robinson (1972) placed occurrence of uranium in the Dockum sediments. special consideration on the reconstruction of paleomag- netic data. FORMATION Finally, the literature on the formation, classification, Work on the Bissett Formation was done almost and morphology of caliche is an enormous one. Of the exclusively by P. B. and E. H. Sellards in the 1930's. works consulted in the progress of this investigation They described the Bissett Formation as dominantly those most useful were by Rightmire (1967) and Reeves limestone and chert conglomerate with interbeds of red (1970) and (1976). These works included theories of shale, sandstone, and some local limestone (caliche) and origin, formation, morphology, and age of certain west marl beds. Plant and vertebrate remains found within the Texas caliches. In 1976 Mikels presented an extensive sediments by King and Sellards seem to indicate an Early review of the origin and significance of caliche, with Triassic age for the Bissett Formation. emphasis on the caliches of central Texas. These caliche studies provided important information concerning the DEPOSITIONAL MODELS interpretation of Sycamore caliche deposits. In addition to those works dealing specifically with Triassic rocks in Texas there are a number of studies ACKNOWLEDGMENTS relating to depositional models applicable to the deposi­ tion of Triassic sediments. Blissenbach (1954) described Appreciation is extended to O. T. Hayward for sug­ the geology of alluvial fans in semi-arid regions with gesting the topic and for providing valuable assistance special emphasis on agents of alluvial fan deposition. and guidance throughout the course of this study. Special Other studies dealing with the lithology and depositional thanks go to Walter Youngquist, Harold Beaver, and processes of ancient and recent alluvial fans are those by Robert Packard, members of my thesis committee, who Hooke (1967), McGowen and Groat (1971), and Mc­ critically reviewed this report. The field assistance and Gowen et al. (1978). guidance of Joan Hancharik and Curtis Koger are grate­ Butzer and Hansen (1968) in a study of the desert and fully acknowledged. Appreciation is also extended to my rivers of Nubia (Egypt) provided models of wadi stream parents for financial assistance in typing and preparation deposition. Descriptions of specific arid stream deposits of the thesis manuscript.

DESCRIPTIVE GEOLOGY

REGIONAL GEOLOGIC SETTING tural provinces. These include, from east to west the Interior Mesozoic Gulf Basin, the Ouachita Tectonic The continental Triassic rocks of Texas consist of a Belt, the Llano Uplift, the Bend Arch, the Concho Arch, complex clastic sequence composed of conglomerates, the Wichita Mountain System and the Dockum Basin sandstones, siltstones and shales deposited by a variety of (Fig. 4). terrigenous depositional systems during a period of semi- aridity. They exist both in outcrop and in the subsurface INTERIOR MESOZOIC GULF BASIN across portions of northeast, central, southwest, and The Interior Mesozoic Gulf Basin formed during Late northwest Texas. Figure 4 illustrates the area of deposi­ Triassic time probably as a result of the separation of the tion for the Eagle Mills, Sycamore, Bissett and Dockum Afro-South American Plate from the North American rock groups and associated tectonic features that affected Plate. The basin consisted of a discontinuous series of their sedimentation. grabens and half grabens not unlike those of the Triassic basins of the northeastern United States. The faults closely follow the trend of the Ouachita Tectonic Belt REGIONAL STRUCTURAL ELEMENTS from south-central Texas northeastward through north­ The pre-Triassic surface was a continental erosional ern Louisiana and southern Arkansas to the Florida surface of moderate relief transecting a number of struc­ panhandle (Fig. 7). These graben systems are probably CONTINENTAL ROCKS OF TEXAS 13

sas, and Oklahoma (Fig. 8). The location and geometry of the Dockum Basin probably reflect Paleozoic struc­ tural elements lying within an area roughly defined by the Midland Basin. It is bounded on the north by the Bravo Dome and Amarillo Uplift, which are structural elements probably originating during Late Mississippian time. To the south lie the Matador Arch and Central Basin Plat­ form, which apparently exerted little influence on sedi­ mentation (McGowen, et 1979, p. 2).

ARCH The Concho Arch is a broad pronounced asymmetrical arch, which formed as a result of the Late Mississippian orogeny of the lands adjacent to the Ouachita Foldbelt Fig. 7. Map of the Gulf Coast region showing the location of the (Eardley, 1951, p. 246) (Fig. 4). The Sycamore deposits Interior Mesozoic Basin and the Ouachita Tectonic Belt. appear to have been restricted on the west by topographic elements related to the Concho Arch. The uplift of the arch began during Pennsylvanian time; it was struc­ associated with the initial rifting of the continents during turally and topographically high by Sycamore time, and Late Triassic time. may well have been a source for some Sycamore (Blalock, 1976, p. 62). DocKUM BASIN BEND ARCH The Dockum Basin is a broad shallow depression that underlies parts of Texas, New Mexico, Colorado, Kan­ The Bend Arch is an elongate asymmetrical structural

Fig. 8. Paleogeographic reconstruction of Texas during Late Triassic time showing major depositional systems and source areas for the Eagle Mills, Sycamore, Dockum, and Bissett rock groups. Adapted from McGowan et al., Texas Bureau of Economic Geology Rept. of Invest. 97, p. 6. 14 BAYLOR GEOLOGICAL STUDIES

feature extending northward from the Llano to the of the Llano Uplift are clearly the primary source of the Red River Uplift in north-central Texas (Fig. 4). The for the Sycamore conglomerate. The diverse rock Bend Arch is believed to have undergone slight uplift and types and the distribution of the different forma­ tilting to the west during Late Permian and Early Triassic tions of the Llano Uplift show a close lithologic correla­ 1960, p. 50). tion with the cobbles, pebbles, and sand of the Sycamore conglomerate. The region along the eastern border of the . WICHITA MOUNTAIN SYSTEM Llano Uplift may be divided into four distributive pro­ vinces (Damon, 1940, p. 59). These include the Round The Wichita Mountain System of southwest Okla­ Mountain Province, Marble Falls Province, Lampasas homa and the Texas Panhandle is composed of three Province, and San Saba Province. folded mountain chains. They are, from east to west, the Arbuckle, Wichita, and Amarillo Mountains (Fig. 4). Round Mountain Province The Arbuckle Mountains consist of a large complex anti­ This province is in the southern part of the Llano cline composed of a series of faulted subparallel folds region and was the provenance area for most of trending northwest-southeast across southwestern Okla­ Sycamore conglomerate in the Pedernales area. In this homa. The Wichita Mountains consist of three en province the Ellenburger Group is most widely exposed echelon anticlines with granitic cores flanked by Cam­ though it was formerly covered by the Smithwick brian, Ordovician, Mississippian and Pennsylvanian sed­ Formation. iments overlapped unconformably by Permian (Eardley, 1951, p. 237). The Amarillo Mountains are a Marble Falls Province series of buried hills with Precambrian crystalline cores, The Marble Falls Province lies west of the Colorado which extend west-northwest across the Texas Panhan­ River area in the northern Llano region. This area dle. The crystalline core of the Amarillo Mountains is includes outcrops of Ellenburger Marble Falls overlain by Pennsylvanian and Permian strata (Eardley, Limestone, and Smithwick Shale, as well as igneous and 1951, p. 238). metamorphic rocks of Cambrian and Precambrian age. These appear to have contributed chiefly to the region TECTONIC BELT immediately east of the Llano Uplift in the vicinity of The Ouachita Tectonic Belt is a series of sinuous folds Austin. flanked on the north by thrust fault blocks. This tectonic belt extends from southwest Texas northeastward Lampasas Province through Oklahoma and Arkansas almost reaching the The Lampasas Province includes the area of the Llano buried extension of the Southern Appalachian Moun­ region west and southwest of Lampasas. The Sycamore tains 1973, p. 4). The Ouachita Foldbelt conglomerates of the Lampasas region were derived is exposed only in the Marathon Uplift of southwest principally from the Ellenburger Group with minor con­ Texas and the Ouachita Mountains of southeast Okla­ tributions from an igneous metamorphic area to the homa and southwest Arkansas; between these two points southwest of the Ellenburger outcrop belt. The Lampasas it is buried beneath thick sections of Mesozoic and Province appears to have contributed chiefly to the rocks Cenozoic rocks. northeast of the Llano area in the region of Lampasas. San Saba Province LLANO UPLIFT The San Saba Province is in the northern and north­ Local relationships in the Llano Uplift area suggest western margin of the Llano Uplift and includes outcrop that late Paleozoic rocks were deposited over the Llano belts of Ellenburger and Upper Pennsylvanian forma­ region and were uplifted, tilted, and eroded (Cheney and tions. This province apparently furnished the Sycamore Goss, 1952, p. 2263). Bedrock of the Llano Uplift is deposits of the Elliot Creek region (locality 20), which composed of intensely folded and metamorphosed gran­ extend northward toward the Brownwood area (locality ites; gneisses and schists of Precambrian age and more 37). However, within the area north of the Llano region gently folded and faulted limestone, shales and sands of the lithologic makeup of the Sycamore conglomerate early and middle Paleozoic age. This regional uplift differs from outcrop to outcrop suggesting that highly caused the erosion of Paleozoic rocks from the Llano localized streams supplied sediments to areas separated dome at the time of Sycamore deposition, because rocks from each other by drainage divides (Dreyer, p. 36). CONTINENTAL ROCKS OF TEXAS 15

DISTRIBUTION AND CHARACTER OF THE EAGLE MILLS

GEOLOGIC SETTING Stratigraphically, the Eagle Mills Formation uncon- The Eagle Mills Formation exists in discontinuous overlies weathered Paleozoic sediments and is occurrence in the subsurface from south-central Texas overlain by the Werner Formation, Lou- northeastward across southern Arkansas and northern formation, Smackover Formation, Louisiana into western Mississippi. Figure 1 shows the location of wells that penetrate the Eagle Mills rocks in (Table 2). Texas. The eroded north edge of the Eagle Mills Formation THICKNESS follows the trend of the Ouachita Mountains across west- Mills Formation is rarely penetrated to its ern Arkansas and eastern Oklahoma and the buned trend and its thickness, therefore, is difficult to northeastern and south- figure 9 shows the total thickness of the Eagle Mills central Texas. represents the northernmost Formation in southern Arkansas, northern Louisiana, extent of the Interior Basin 4). Mississippi, and northeast Texas. In Hempstead County, Arkansas the Humble, well pene- TABLE 2. COLUMN OF THE MESOZOIC almost 7000 ft of Eagle Mills rocks, presently the ROCKS OF NORTHEAST TEXAS. maximum known thickness formation (Fig. 10). Of all the well logs examined in Texas during this investigation only 43 penetrate the Eagle Mills, and of z these only were drilled through to underlying Paleozo­ FORMATION GROUP ic rocks. Thicknesses vary substantially from as little as 60-70 ft (Texas, Red River County) up to Z more than 1,360 + ft (Humble, Titus

In Texas the Eagle Mills shows no clear thickness trends COTTON VALLEY

except where it pinches out against Paleozoic rocks at the

northern edge Interior Gulf Basin. Because of the erratic thickness pattern, many geologists believe that z HAYNESVILLE deposition Eagle Mills was controlled by normal

faulting associated with the initial rifting of the Gulf Coastal Basin (Figs. 10, 5 S

DC z SMACKOVER ARKANSAS o N o OKLAHOMA o

M E 0 NORPHLET

UJ 25

o

WERNER

CO EAGLE MILLS a a. TR I MISS.

5 N PALEOZOIC

Appendix 3, Core description of the Humble, well, Titus Fig. 9. Thickness, in feet, of the Eagle Mills Formation in north­ County, Texas (3 p.), is available for costs from the Department of eastern Texas, southern Arkansas, northern Louisiana, and western Geology, Baylor University Mississippi. 16 BAYLOR GEOLOGICAL STUDIES

LITHOLOGY The shale is usually red to red brown in color but may The Eagle Mills Formation of northeast Texas (Hum­ be purplish red or gray green. It is silty, and at some ble, consists of alternating layers of sandstone, horizons is mottled green by small clay clasts. siltstone, shale, and conglomerate (Figs. 12, 13). The conglomerate consists mostly of pebbles of round The sandstone is commonly brick red, but in places is milky quartz and brick red, fine-grained orthoquartzite white or gray green in color. The texture ranges from fine (up to 3 in diameter). The matrix is commonly fine to to coarse grained (conglomeratic). The sand is commonly coarse grained argillaceous, slightly porous, red sand­ poorly sorted and argillaceous, becoming more calcare­ stone. Conglomeratic layers become progressively coarser ous and slightly chloritic toward the base of the forma­ downward (Fig. 13). tion (Fig. The principal lithology of the Eagle Mills in Arkansas in the Eagle Mills Formation is principally is red silty shale, red clay shale, or mudstone with gray- red to purplish red, argillaceous, and occasionally be­ green and red siltstone and sandstone. The sandstones, comes finely micaceous in character. It is frequently for the most part, are not porous and contain mottled with areas of blue-green clay or shale probably calcareous, or siliceous cement. Where siliceous cement indicating reducing conditions. predominates, the sands are represented by sedimentary

Fig. Generalized northwest-southeast cross-section across southern Arkansas showing location of wells penetrating the Eagle Mills Formation. The Humble # 1 Royston (2) penetrates over 7000 ft of Eagle Mills red beds. Eagle Mills rocks were probably restricted to grabens along the front of the Ouachita Mountains. From Vernon, 1971, AAPG Mem. 15, v. 2, p. 975, used with permission.

Fig. Generalized west-east cross-section across central Texas into the East Texas basin. The Eagle Mills red beds were deposited in graben and systems probably associated with Late Triassic rifting of Pangea. From Vernon, 1971, AAPG Mem. 15, p. 968, used with permission. CONTINENTAL TRIASSIC ROCKS OF TEXAS 17

Limestone and dolomite, in the form of The overlying Werner Formation represents the base caliche-like blebs and inclusions or pebbles, are also pres­ of the Jurassic sequence in the Gulf Coast. It is composed ent though not common. In some places thin conglome­ of up to 50 ft of clear to gray crystalline anhydrite with ratic beds a few inches thick occur in the shales, siltstones, occasional solution cavities (Fig. 14). In some wells a thin and sandstones. Furthermore, there are minor traces of dense limestone layer and a conglomerate consisting of siderite, anhydrite, vein calcite, and dolomite in the Eagle pebbles of white quartz, gray chert, and red and green Mills Formation of Arkansas (Scott, et 1961, p. 12). igneous pebbles underlie the anhydrite (Fig.

Fig. 12. Coarse-grained red sandstone from the Humble #1 Calfee Fig. 13. Section of Eagle Mills conglomerate from the Humble #1 core, Titus County, Texas. Calfee well showing milky quartz and orthoquartzite pebbles in a red coarse-grained sandstone matrix.

DISTRIBUTION AND CHARACTER OF THE SYCAMORE FORMATION

character from locality to locality. Most reports com­ INTRODUCTION pleted to date place the Sycamore Formation stratigraph- The Sycamore Formation of central Texas is a poorly ically as part of the basal Cretaceous deposits in the sorted, coarse terrigenous conglomerate. It is a distinc­ central Texas region. However, as early as 1940, some tive unit, differing greatly in composition and structure doubt was expressed concerning a Cretaceous age for from underlying Paleozoic rocks and the overlying basal Sycamore rocks. Damon (1940, p. 86) stated that the Cretaceous deposits. The Sycamore Formation is highly lower conglomerate (Sycamore) was deposited prior to localized, cropping out radially along the northern, east­ the advance of the Cretaceous sea, accumulation proba­ ern and southern flanks of the Llano Uplift. It appears to bly occurring soon after the Llano region was sufficiently have been derived primarily from the erosion of Paleo­ elevated to undergo extensive erosion. Recent studies by zoic rocks of the Llano region. The character and distri­ Dobbins (1974) and others also suggest a possible pre- bution of Sycamore rocks was significantly affected by Cretaceous age for the Sycamore conglomerate. For the the tectonic history of the Llano region during the time of purpose of this investigation, therefore, the Sycamore Sycamore deposition. conglomerate will be defined as the lowermost coarse A major problem in interpreting Sycamore rocks is terrigenous conglomerate deposit immediately beneath that the Sycamore conglomerate is not clearly defined as Cretaceous rocks unconformably overlying Paleozoic a stratigraphic unit, for it differs in composition and rocks and distributed in discontinuous outcrop from near 18 BAYLOR GEOLOGICAL STUDIES

Fig. 14. Section of the Werner Formation showing clear-to-gray crys­ Fig. 15. Section of the Werner Formation directly above the contact talline anhydrite with small solution cavities, Humble #1 Calfee core. with the Eagle Mills Formation. The conglomerate contains poorly sorted gravels of white quartz, green and gray chert, and some igneous fragments in a matrix of green-to-gray fine-grained sandstone. Directly above the conglomerate is a thin layer of dense fossiliferous limestone in Brown County southeast to the southern with stylolites. side of the Llano in Kimble County (Fig. 2). is present overlying Paleozoic just west of Lampasas REGIONAL GEOLOGIC SETTING (localities 14 and 15); from this point northwestward toward Sycamore rocks crop out along the The Sycamore Formation is exposed in an outcrop belt contact. In addition outliers of that extends northwest to southeast around the northeast Sycamore rocks occur at localities 25, 26, and 27 in the margin of the Llano Uplift (Blalock, 1976, p. 50) (Fig. 2). vicinity of Rochelle, McCulloch County. However, Sycamore outcrops south and southeast of the The Sycamore conglomerate occurs at an elevation of Llano in Gillespie and northern Blanco and Travis 960 ft in the area of Pedernales Falls, locality 5, at ft Counties suggest that Sycamore deposition was a prod­ at Elliot Creek, locality 18, and about 1600 ft near uct of radial drainage away from the Llano Uplift. The Brownwood, Brown County. Since the cobbles of Syca­ Sycamore conglomerate thins away from the Llano more rocks were clearly derived from Llano source areas, region in all directions strongly indicating that this region these elevations suggest that the region has been tilted was the primary source area. south or southeastward since deposition of Sycamore At locality 5 the Pedernales River and its small tribu­ conglomerate (Damon, 1940, p. 85). tary streams have dissected the Sycamore conglomerate and underlying Paleozoic rocks revealing excellent sec­

tions. Here the conglomerate wedges out to the northwest STRATIGRAPHIC CONTACTS between the rising Paleozoic floor and the nearly hori­ zontal beds of the overlying Cretaceous sediments. The The Sycamore conglomerate unconformably overlies black limestone and shale of the basal Marble Falls Form­ rocks of Paleozoic age and is unconformably overlain by ation dip S E at an angle of (Fig. 16). In the the more normal clastic deposits of the Travis Peak For­ Colorado region the old Paleozoic surface is almost a mation, here considered the initial Cretaceous deposits of plane striking N E and dipping gently eastward. The the central Texas region. These Travis Peak con­ basal Sycamore conglomerate here continuously overlies sist of fluvial, deltaic, and marginal marine deposits and the Paleozoic surface (Damon, 1940, p. 40). are probably rocks of the oldest Trinity Group in central The Sycamore conglomerate is absent at locality 12 Texas. where the Walnut Formation of the Fredericksburg The Travis Peak Formation appears to represent a Group overlies Paleozoic rocks of the Ellenburger Group. product of marine transgression across a gently subsiding This was an area of positive relief upon which the Syca­ land mass. As the waters deepened, the deposits changed more conglomerate was never deposited or on which it from coarse to fine materials becoming more calcareous was deposited and from which it was later eroded prior to in the upper portion (Damon, 1940, p. 23). Fredericksburg deposition. The Sycamore conglomerate The Travis Peak Formation of the Trinity Group can CONTINENTAL TRIASSIC ROCKS OF TEXAS 19

Fig. Locality 5. mi northeast of Johnson City at Falls State Park, Blanco County. Here the Sycamore Formation rests unconformably on steeply dipping Pennsylvanian limestones and shales of the Marble Falls and Smithwick Formations. The conglomerate ranges from ft in thickness where it wedges out between the overlying Cretaceous deposits and the underlying Pennsylvanian strata. The Sycamore here consists of numerous cobbles of hard EUenburger dolomite and limestone with some pebbles of chert. The matrix, which is composed of coarse quartz sand and is badly weathered at the surface.

be divided into three major fades: a lower sand and conglomerate facies, a middle sand fades, and an upper silt and clay facies. The basal Travis Peak Formation is typically fine to coarse sands with interbedded silts and clays. The sands are commonly quartzose, well sorted and exhibit cross-bedding. Most of the sand is friable; however, ledges are occasionally formed as calcium car­ bonate cementation increases upward through the se­ quence (Whigham, 1978, p. 1). Localities 14 and 16 are examples of fluvial deposits of the Travis Peak-Twin Mountains Formation. The fluvial deposits are repre­ sented by an upward fining sequence grading from fine­ grained quartz sand at the base to silt at the top. The lower facies consist of quartz sand and sandy conglomer­ ate. The basal conglomerate is composed of well-rounded pebbles of quartz and chert increasing upward in abun­ Fig. Locality 14. Fluvial point-bar deposits of the Travis Peak dance and size in a matrix of clean coarse-grained quartz Formation exposed at Lynch Creek, Lampasas County. These deposits sand (Fig. 17). consist of large-scale cross-bedded coarse-grained sandstone with peb­ bles of limestone and chert.

LITHOLOGY OF THE SYCAMORE CONGLOMERATE The Sycamore conglomerate crops out radially around the Llano Uplift, but it is most widely exposed on the northern and eastern margins of the uphft where it occurs in a belt extending northwest from the northern Llano region (Blalock, 1976, p. 24). The Sycamore Formation is typically a poorly sorted coarse terrigenous unit consist­ ing of a sequence of interbedded sandy, pebble-to-cobble conglomerate with thin sand drapes (Fig. 18). The cob­ bles consist largely of chert, dolomite, limestone and sandstone derived from exposures in the Llano region. Most of the conglomerate, ranging in size from sand to boulders, is derived from rocks of the Ordovican Ellen- burger Group. Cobbles and pebbles of Sycamore con­ glomerate are well rounded, but sphericity decreases with increasing size (Dobbins, 1974, p. 29) (Fig. 19). In addi­ Fig. 18. Locality 19. Slabbed section of Sycamore conglomerate composed of pebbles and cobbles of dolomite and chert. Many of the tion certain exposures of Sycamore conglomerate are chert pieces are highly polished suggesting that they may have been composed of sand, pebbles, cobbles, and boulders regolith cherts from a pre-Sycamore soil here incorporated into the derived from Marble Falls limestone of Pennsylvanian Sycamore conglomerate. 20 BAYLOR GEOLOGICAL STUDIES

Fig. 19. Locality 3. Slabbed sample of Sycamore conglomerate show­ Fig. 20. Locality Sycamore conglomerate exposed at Creek ing large rounded pebbles of dolomite and smaller pebbles of chert in a consists of boulders, cobbles, and pebbles of dolomite, limestone, and matrix of coarse quartz sand with caliche infilling. The sample is well chert derived primarily from the EUenburger and Marble Falls rock cemented and many of the clasts contain calcite groups of the Llano region. Arrow indicates a graded sequence of conglomerate representing waning stages of a single sediment influx. age. Other outcrops of Sycamore rocks consist largely of thick. In Brown County, at the northern margin of the boulders derived from the Pennsylvanian sand­ outcrop Sycamore rocks are 4-5 ft thick and they stone. In areas of good exposure Sycamore rocks typi­ continue to thin northward. cally consist of thick structureless beds of caliche- The matrix of Sycamore conglomerates consists of cemented conglomerate capped by thin sand drapes, quartz sand grains and chert fragments often cemented in which appear to represent cycles of sedimentation. The white to pink caliche (Fig. 19). The ratio of matrix to Sycamore rocks of the Elliot Creek and Pedernales Falls cobbles increases with transport distance away from the sections exhibit such sand drapes (Fig. 20). In areas of Llano source area, and at similar distances the dimen­ good exposures Sycamore conglomerate can be subdi­ sions of the maximum cobble size decrease. At locality vided into an upper and lower unit. 38, for example, the matrix consists almost exclusively of The upper unit is characteristically a caliche zone and coarse quartz sand and small chert fragments; the cobbles is present only where there is a complete section of Syca­ are sparse and small, consisting almost entirely of chert. more rocks. This zone varies in thickness from 2-10 ft The Sycamore conglomerate is commonly cemented by (Fig. 21). calcite possibly derived both from the cobbles and from The basal Sycamore section is a poorly sorted, massive caliche, for it is most common where caliche cementation boulder to pebble conglomerate (Fig. 22). also exists. Sycamore outcrops vary widely in particle size, matrix composition, and degree and character of cementation. OUTCROP REGIONS OF SYCAMORE CONGOLMERATES Since grain size of a clastic deposit is the measure of the energy of the depositing agent, the maximum cobble size The Sycamore conglomerate varies in composition of the Sycamore conglomerate is significant to an inter­ and character from place to place. It is possible, there­ pretation of the depositional history. Total stream energy fore, to define for the Sycamore conglomerate areas of increases with an increase in discharge. Transportation of exposure in which composition and character are inter­ boulders, cobbles, and pebbles consumes energy causing nally consistent but which differ from those of adjacent a variation in grain size and thickness of the deposit. areas of Sycamore exposure. During this investigation Generally there is a downcurrent decrease in grain size seven areas characterized by internal consistency were formed as a result of abrasion and progressive sorting identified. These include the Pedernales Falls, Sycamore (Mackin, 1948, p. 465-466). The Sycamore conglomer­ Creek, Lampasas, Elliot Creek and Brownwood areas ates, therefore, are almost classic indicators of sediment and possible areas of outcrop of Dockum rocks. distribution from a localized source. The thickness of the formation varies from outcrop to PEDERNALES FALLS AREA outcrop, generally decreasing away from the Llano In this area the conglomerate rests upon the steeply region. Nearest the Llano region (locality 5) at Peder­ dipping limestones and shales of the Smithwick and nales Falls, Sycamore conglomerates exceed 100 ft in Marble Falls Formations (Pennsylvanian). Here the thickness where they pinch out against the Paleozoic Sycamore conglomerate consists of boulders and cobbles Smithwick and Marble Falls Formations below and the of hmestone, dolomite, chert, and sandstone. Dolomite overlying Cretaceous deposits of the Travis Peak Forma­ cobbles and pebbles, from Marble Falls and Ellen- tion above. In Lampasas County near Nix (localities 14 & burger sources, form most of the conglomerate in 15), the Sycamore Formation is approximately 15 ft this area. The matrix is primarily coarse brown quartz CONTINENTAL TRIASSIC ROCKS OF TEXAS 21

Fig. 21. Locality The upper caliche zone is exposed and forms a Fig. 22. Locality 5. Weathered basal conglomerate exposed here at clear boundary between Sycamore rocks and more characteristic basal Pedernales Falls is composed almost entirely of dolomite cobbles and Cretaceous sediments. The boundary between the upper and lower pebbles derived from the Ellenburger Group and Marble Falls Forma­ Sycamore is found at the lower part of the picture. tion. The conglomerate at this locality is similar in lithology, composi­ tion, and structure, including sand drapes, to that found at Creek, Lampasas County. sand with some indurated caliche, which contributes a light pink color. Sand drapes are also present within the and by shallow marine deposits of the upper Travis Peak conglomerate of this area (Fig. 22). Formation.

SYCAMORE CREEK ELLIOT CREEK The type locality of the Sycamore conglomerate is on Sycamore Creek, and this exposure is particularly signif­ At Elliot Creek the Sycamore conglomerate consists of icant to this investigation. Here a stream exposes the 25 ft of red-brown to brown pebble-to-cobble con­ Smithwick shale, Sycamore conglomerate, and overlying glomerate alternating with thin layers of sand, which basal Cretaceous The Sycamore Formation con­ apparently represent waning stages of separate stream sists of a coarse basal conglomerate of several pre- flows. The pebbles and cobbles consist of hard limestone, Cretaceous rock types including the Ellenburger Group dolomite, and chert derived from the Ellenburger Group and Marble Falls Formation of the Llano area and and Marble Falls Formation. The conglomerate at grades upward to a mixture of sand pebbles, silt and clay Creek is similar to the Sycamore conglomerates at Ped­ (Young et 1972, p. 65). ernales Falls (Fig. 20).

LAMPASAS AREA BROWNWOOD AREA The Lampasas area lies north and northeast of the The Sycamore conglomerate in the Brownwood area Llano Uplift. The Sycamore conglomerate in this region consists primarily of small yellow, black and red chert in is represented by the Nix locality (14, 15) 12 mi west of a coarse brown quartz sand matrix. Gravels are much Lampasas. Here cobbles and pebbles of chert and dolo­ finer and pebbles less numerous in the northern localities mite are evident within an indurated caliche and sand in Brown County, and limestone pebbles are noticeably matrix. The Sycamore Formation is overlain by more absent northward from the EUiot Creek region to the typical Travis Peak deposits of mixed-load fluvial streams Brownwood area.

DISTRIBUTION AND CHARACTER OF THE DOCKUM GROUP

INTRODUCTION GEOLOGIC SETTING The Dockum Group of west Texas and New Mexico is Rocks of the Dockum Group were deposited in a basin a complex continental clastic sequence consisting of that underlies parts of Texas, New Mexico, Colorado, mudstone, siltstone, sandstone, and conglomerates. These Kansas, and Oklahoma (Fig. 8). This study is concerned terrigenous deposits accumulated in alluvial fans, braided chiefly with those sediments that were deposited in the and meandering streams, lobate deltas, and lakes. west Texas area. 22 BAYLOR GEOLOGICAL STUDIES

The southern boundary of the Dockum Group approx- rocks is generally smooth and regular, with evidence imately follows the east bank of the Pecos River down- of significant erosion prior to the deposition of the Trias- stream from the New Mexico-Texas border to eastern sic rocks (Gould, 1907, p. 21). In addition, though the Reeves and northern Pecos Counties. From this point it retreat of the Permian sea was followed by erosion and extends northeast to Sterling County, includes much of local folding, the erosion was not extensive, because most Mitchell, Scurry, and Borden Counties, and continues northward along the Caprock Escarpment to the Cana­ dian River valley (Adkins et 1932, p. (Fig. 3). The Caprock Escarpment is an armored surface of mas­ OGALLALA GROUP sive caliche-cemented rock that underlies the Texas High BRIDWELL FORMATION Plains surface and serves as a protective covering for the consists of well-sorted, reddish-brown, quartzose sand, siltand clay underlying Dockum sediments (Fig. 6). occasionally interlaminated with thin discontinuous channel gravels COUCH FORMATION composed primarily of unconsolidated, pinkish-gray, coarse­ STRATIGRAPHIC CONTACTS grained calcareous sand and gravel (from Walker, 1978 and & Reeves, The Upper Triassic Dockum Group of west Texas unconformably overlies the red shales of the Upper Per­ GROUP mian Quartermaster Group. Except for the sparse out­ PURGATOIRE FORMATION CREEK FORMATION crops of the Bissett Formation in far southwest Texas fine grained moderately yellow shale and low to brown, fucoidal sand- thin light gray to yellowish there are no Lower or Middle Triassic deposits in the stone with bentonitic shale in- brown limestone beds state. This contact between the Quartermaster and terbeds Dockum Groups represents a time gap of about 35 mil­ FORMATION lion years. However, the upper surface of the Permian to brown, cross thinly laminated dark-gray to bedded, ripple marked sandstone moderately yellowish-brown with some fine-grained coarse shale and scattered light gray sand and pebble layers (the limestone and yellowish sand- Purgatoire is analogous stone lenses to the Duck Creek and chi sections)

FREDERICKSBURG GROUP EDWARDS FORMATION hard light-gray to grayish yellow thick bedded, fine to coarse­ grained limestone with abundant rudist remains PEAK LIMESTONE light-gray to light-yellowish-gray argillaceous, fossiliferous lime­ stone and thin light-gray interbeds. WALNUT FORMATION light-gray to brown argillaceous sandstone, calcareous shale, and argillaceous limestone.

TRINITY GROUP ANTLERS FORMATION gray to purple loosely consolidated fine to coarse grained unfossilif- erous quartz sandstone with scattered lenses of quartz gravel. Basal part contains clay and quartz pebbles derived from underlying Triassic beds

DOCKUM GROUP CHINLE FORMATION restricted the southeastern portion the High Plains and consists of thin shales and sandstone which are lithologically similar to the Tecovas and Trujillo Formations TRUJILLO FORMATION consists of several ledges of fine to medium grained, reddish-brown to gray, cross-bedded and massive sandstone and conglomerate separated by red, maroon, and gray shales TECOVAS FORMATION consists of a lower sequence of varigated sandy shales and a upper sequence of purplish-red and orange shale with thin layers of soft sandstone

QUARTERMASTER GROUP (FORMATION) composed of primarily brick-red to vermillion shales interbedded Fig. 23. Lithologic and stratigraphic relationships of the Upper Perm- with lenses of soft red sandstone, clays, and gray shales. The lower ian, Upper Triassic, Cretaceous, and Tertiary sediments of the Southern red shales contain a considerable amount of gypsum in the form of High Plains. white to pink bands of satin spar CONTINENTAL ROCKS OF TEXAS 23

Fig. 24. Locality 5. Quartzitic, fine- to coarse-grained channel sand Fig. 25. Locality 1. Red, shales of the lower Dockum Group characteristic of the Dockum Group of west Texas. exposed at City cemetery. of the Permian structures that are reflected in Permian unconformity representing 167 million years exists be­ topography are also reflected in basal beds of the Triassic tween the Triassic and Pliocene sections typical of the deposits. Caprock Escarpment. However, in some of synclines Although there is good evidence for an unconformity beneath the northern Llano Estacado the Upper Jurassic between the Dockum and Quartermaster Groups, in Morrison Formation may be present, associated with many instances the boundary between them is difficult to Cretaceous formations, above the Dockum deposits. determine and appears to be gradational in nature. There Figure 23 shows the lithologic and stratigraphic charac­ are, however, certain lithologic features characteristic of ter of the various stratigraphic units that are associated each of the groups. Table 3 illustrates some of the com­ with rocks of the Dockum Group. mon lithologic differences between the Dockum and Quartermaster Groups in the west Texas area. LITHOLOGY OF DOCKUM ROCKS The Dockum Group is unconformably overlain by the The Upper Triassic Dockum Group of west Texas Ogallala Group of Pliocene (Tertiary) age and to a lesser comprises a unique and complex red-bed sequence con­ extent by Lower Cretaceous marine deposits chiefly sisting of colorful sand, shale, and conglomerate. south of In fact, in most areas of the Texas The sands are commonly quartzitic, fine to coarse Panhandle there is no stratigraphic record of the Juras­ grained, poorly sorted, and range in color from white to sic, Cretaceous, or lower Tertiary Periods, thus an red. Most of the sand is friable and is cemented by gray TABLE 3. LITHOLOGIC DIFFERENCES BETWEEN THE UPPER and red shale, silica, gypsum, and iron oxide (Adams, TRIASSIC DOCKUM GROUP AND THE UPPER PERMIAN 1929, p. 1048) (Fig. 24). QUARTERMASTER GROUP. The shales are commonly varigated, unlithified, mica­ ceous, and silty (Fig. 25). Although most commonly red DOCKUM GROUP QUARTERMASTER and maroon, the shales can also be gray, green, yellow or (UPPER TRIASSIC) (UPPER PERMIAN) brown in color. Two types of conglomerate are present within the Dockum Basin. The first type of conglomerate 1. Poor lithification of shales. 1. Good lithification of shales, consists commonly composed of mudstone, silt- compact and hard. stone, sandstone and caliche derived from within the 2. brick red. 2. yellow, light 1979, p. 10) (Fig. 26). The second brown, magenta, and gray. type of conglomerate is composed of constituents derived 3. Gypsum and anhydrite 3. Bedded anhydrite, gypsum occur only in minor amounts and salt. from sources outside the depositional basin. These con­ and are generally not bedded. glomerates are composed of clasts of chert, quartzite, 4. Abundance of mica and 4. Little or no mica or vein quartz with some silicified wood fragments (Fig. 27). phosphate. phosphate. Terminology applied to the Dockum Group of west 5. heter­ 5. fine Texas has undergone considerable evolution. Presently, ogeneous in nature with ex­ grained. however, the Dockum Group is divided into three forma­ tensive cross-bedding. tions, which can be traced for considerable distances. 6. Conglomeratic beds, both 6. No substantial conglomer­ intrabasinal and extrabasinal. atic beds. These are, from oldest to youngest the Tecovas, Trujillo, 7. Reptilian bones and species of 7. No evidence of fossil bones and Chinle Formations. Unio. or pelecypods. TECOVAS FORMATION 8. Weathers easily forming 8. Where exposed, weathers to The Tecovas Formation, named by Gould (1907, p. 21) dome-shaped masses. form a typical badland topography. for exposures on Tecovas Creek, Potter County, Texas, is present north of the Matador Arch where it can be 24 BAYLOR GEOLOGICAL STUDIES

Fig. 26. Locality 8. Slabbed sample of a intrabasinal con­ Fig. 28. Locality Outcrop of the Tecovas Formation, mi south glomerate composed of clasts of mudstone, siltstone, sandstone, and of Post, Garza County. At this locality the Tecovas consists of red caliche (scale in cm). siltstone interbedded with thin, fissile, light-gray to green sandstone and some coarse red sandstone.

Fig. 29. Locality 4. Outcrop of Tecovas sandstone showing some of Fig. 27. Locality 6. Slabbed sample of a Dockum extrabasinal con­ the dark-colored ironstone concretions that weather out of the sand at glomerate composed of chert, quartzite, vein quartz and silicified wood the surface. fragments (scale in mm).

divided into two members, a lower varigated sandy shale the Trujillo Formation is conformable. sequence and an upper dark-red or magenta shale Good outcrops of the Tecovas Formation occur in sequence. However, Patton (1923, p. 51) and Green Mitchell County (localities 2, 3) where the Tecovas con­ (1954, p. 17) have noted that because of the great varia­ sists of red fine sandy silts and laminated sandstones. The tions in color the divisions of Gould can not be carried muds and silts contain montmorillonite seams and small further south than the Palo Duro Canyon area. The thin plates of sparry gypsum. The sand beds are lenticular varigated shales of the Tecovas lie directly upon the and discontinuous, probably representing winnowing upper eroded surface of the Quartermaster Group, form­ stages of stream deposition in a margin. Other good ing a vivid color contrast. The Tecovas Formation usu­ exposures of the Tecovas occur just north of Fluvanna ally consists of sandy shales, typically calcareous, which (locality 6) and a few miles south and west of Post in are more or less cross-bedded and lenticular (Gould, Garza County (locality 8, 9) where the deposits consist 1907, p. 22)(Fig. 28). The color of the shale is due largely chiefly of red mudstone and siltstone with gray-to-red, to the presence of iron, and many dark-colored clay- finely cross-bedded, graded sandstones (Fig. 30). ironstone concretions weather out at the surface (Fig. In the Palo Duro Canyon area the Tecovas Formation 29). Interbedded with varigated shales are thin lenses of is about 200 ft thick and is composed of a lower lavender, white, yellow, or light-brown friable soft sandstone. The gray, and white shale sequence; a middle bed of white sand is poorly cemented and weathers easily. Many of the friable sandstone; and an upper unit of orange shale, outcrops exhibit sand-filled channels and intraforma- which underlies the more massive Trujillo sandstone tional conglomerates. The Tecovas attains its maximum (Matthews, 1969, p. 21). Figure illustrates the geologic thickness in an area roughly corresponding to the Palo formations present within the canyon. The Tecovas Duro Basin and thins rapidly southward toward the Mat­ shales form a relatively smooth slope readily distin­ ador Arch (Kiatta, p. The contact between the guished from the gullied, steeper slopes of the Permian Tecovas Formation and the overlying massive sands of shales beneath them. CONTINENTAL TRIASSIC ROCKS OF TEXAS 25

Fig. 30. Locality 7. Tecovas exposures near Post, Garza County, Fig. 31. Locality 14. Tecovas shales are well exposed in Palo Duro showing a finely cross-bedded graded sequence of loosely cemented Canyon where they form a relatively smooth slope (arrow) below the pebble conglomerate and coarse quartz sand. The outcrop is friable and more massive sandstone ledges of the Trujillo Formation. weathers easily.

FORMATION The Trujillo Formation, named by Gould (1907) for exposures along Trujillo Creek, Oldham County, Texas, consists principally of fine- to medium-grained cross- bedded sandstone, massive light-gray to reddish-brown sandstone, and thin lenticular quartzose conglomerate beds. The Trujillo consists of three to five well-defined ledges of massive, cross-bedded sandstone and conglom­ erate interbedded with red and gray shales and poorly consolidated cross-bedded sandstones (Gould, 1907, p. 26). He divided the Trujillo Formation into lower, mid­ dle, and upper units based upon the number of sandstone Fig. 32. Locality 5. A massive sandstone channel some 50 yd wide and ledges. However, sand ledges often converge or die out ft deep is exposed at locality 5. The channel is composed almost making correlation difficult. The sandstones are highly entirely of medium- to fine-grained, massive sand and is in sharp cross-bedded and typically light gray to buff colored but contact with the underlying shale and sand beds. may be light tan to yellowish brown where a large amount of iron oxide is present (Green, 1954, p. 29). The sands range from fine to medium grained and are locally con­ of prominent ledges of sandstone interbedded with red, glomeratic. They are cemented by calcium carbonate, maroon gray shales (Matthews, 1969, p. 23) (Fig. 35). clay, iron oxide, and silica (Kiatta, 1960, p. Triassic Peak in the northern part of Palo Duro Canyon Good outcrops of the Trujillo Formation may be seen State Park offers a unique exposure of the contacts along State Highway in Mitchell County where they between the Permian Quartermaster Group and the Te­ consist largely of massive, green to red lenticular channel- covas and Trujillo Formations. fill sands and conglomerates. Scour and fill structures as Analysis of cross-bedding of the Dockum sandstones well as local cross-bedded pods are prevalent throughout was undertaken by Kiatta (1960), Cazeau (1962), and the sections. At locality 5 a large filled channel some 50 yd Cramer (1973), and they generally agree that paleocur- wide and 15-20 ft deep is exposed, cutting the bedded rent patterns show a definite preferred orientation to the sands below (Fig. 32). At the north end of the exposure west-northwest. Figure 36 is a map and compass diagram the channels are in a southeast-northwest direc­ showing the orientation of cross-beds in the Dockum tion. The outcrops also contain some highly friable sand sandstones. The mean cross-bed direction is N W, and clay pebble conglomerates, many of which occupy indicating that the depositing streams flowed generally to the basal portion of the channels (Fig. 33). Figure 34 is an the west-northwest. example of the festooned cross-bedding present within In localities where good cross-bedding was evident, the channel fill at locality 5. Other good exposures of the (localities 2, 5, 6, 8, 9) the principal directional compo­ Trujillo Formation may be seen along the eastern edge of nent was also to the west-northwest. However, direc­ the Caprock Escarpment near the town of Post in Garza tional readings and sandstone percentage maps con­ County where the deposits are principally channel fin structed from outcrop and subsurface data by McGowen sands and conglomerates. et indicate that the basin was peripherally filled. The Trujillo Formation of Palo Duro Canyon consists In many areas the upper Trujillo Formation shows 26 BAYLOR GEOLOGICAL STUDIES

evidence of extensive erosion prior to deposition of the Upper Tertiary Ogallala deposits. However, in the south­ eastern part of the Texas High Plains Chinle rocks Trujillo sands beneath the Ogallala cover.

CHINLE FORMATION The Chinle deposits of west Texas differ from the Chinle rocks of western New Mexico and Arizona, and their stratigraphic continuity with the western deposits is not presently known. In west Texas, the formation con­ sists mainly of greenish-gray and maroon sandy shales (Adams, 1929, p. 1052) with small conglomeratic and micaceous sandstone lenses. Overall, the clays and shales, of the Chinle are of the same general rock type as Fig. 33. Locality 5. Outcrop of the Trujillo Formation (?) showing Tecovas and Trujillo Formations. characteristic scour and fill structure much of which contains a basal clay-pebble conglomerate.

Fig. 34. Locality Large sigmoidal foreset cross strata in the lower part of a distributary-filled channel.

N \

Each arrow represents the dip direction of one set of crossbeds O 10 20 30

Fig. 35. Locality 14. View of north rim of Palo Duro Canyon illustrat­ ing the prominant sandstone ledges of the Trujillo Formation. The contacts between the Permian Quartermaster Group (Q), Tecovas Fig. 36. Map showing orientation of cross-beds in the Dockum sand- Formation (Tec), Trujillo Formation (Tru), and Ogallala Group (Og) stone. The compass diagram shows the mean cross-bed direction to be are easily distinguished in the canyon. N From Kiatta, M.S. thesis, Texas Tech College, p. 22. CONTINENTAL TRIASSIC ROCKS OF TEXAS 11

DISTRIBUTION AND CHARACTER OF THE BISSETT FORMATION

INTRODUCTION posed of a basal unit of red shale with some interbedded conglomerate and an upper cobble-to-boulder conglom­ The Bissett Formation is a terrigenous unit consisting eratic unit. largely of limestone and chert conglomerate, red shale, sandstone, and some local limestone and marl deposits INTERPRETIVE GEOLOGY (Sellards and Baker, 1934, p. 154). The formation crops out along the northwest margin of the Glass Mountains The sedimentary history of the four areas of Triassic in southwest Texas varying in thickness from 10-500 ft rocks, the Eagle Mills, Sycamore, Dockum, and Bissett (Fig. 5). It unconformably overlies Permian carbonates outcrop areas is described individually in the section that of the Capitan Formation and is unconformably overlain follows. As the final part of this paper, the Triassic rock by Lower Cretaceous sandstones and limestones. Near groups will be combined and their similarities in lithology Bissett Mountain (Type Locality) the formation is com­ and sedimentary history described.

SEDIMENTARY HISTORY OF THE EAGLE MILLS FORMATION

DEPOSITIONAL ENVIRONMENT toward a period of aridity for the region. Aridity is indi­ cated by the presence of the extensive red beds of the The Eagle Mills Formation represents the initial de­ Eagle Mills Formation in the Gulf Coast region and in posits of the subsiding Gulf Basin in Texas, Arkansas, the Dockum Group in west Texas. Figure 37C shows and north Louisiana. Eagle Mills rocks are believed to further spreading of the plates with restricted marine have been deposited in deep troughs bounded by steep influx into an extensive evaporite basin. The deposits of faults (Figs. The Eagle Mills sediments were land this episode constitute the Werner anhydrite and Louann derived with no marine constituents. These sediments salt of Early to Middle Jurassic age. were predominantly alluvium with the coarser Figures 37D-F depict the Gulf Coast region from Mid­ representing probable alluvial fan and bed-load to mixed- dle Jurassic to Early Cretaceous time. This was a pro­ load stream deposits derived from streams that flowed longed period of intermittent transgressions and regres­ from the uplifted Ouachita Tectonic Belt. The mud- sions dominated by shallow marine carbonate and sand stones, shales, and some of the siltstones are products of deposition. The sediments of these episodes, the Smack- flood-plain deposition and include marshes, ephemeral over Formation and the Cotton Valley Group, were de­ lakes, streams, and shallow lakes. The presence of more posited in an arcuate trend over a wide portion of south sand in the lower portion of the formation and shale in Texas. With continued subsidence, the area of sedimen­ the upper part is believed to be the result of (1) decrease in tation expanded. Both sedimentation and subsidence basin deepening tectonics, (2) lowering of the topography rates remained relatively equal through most of Jurassic in the source areas, or (3) a trend toward increased aridity and Cretaceous time resulting in the domination of shal­ resulting in a more sporadic stream activity (Scott et low marine or deltaic deposits. 1961). Figure 37 shows generalized lithofacies-environment PALEOTECTONIC IMPLICATIONS OF ORIGIN maps for the Texas-Arkansas Gulf Coast region from Late Pennsylvanian to Early Cretaceous time. Figure There are numerous theories pertaining to the origin 37A depicts the area prior to Eagle Mills deposition. It and initial deposits of the Gulf Coast sedimentary basin. shows the Ouachita-Marathon trend with subsequent Early investigators concluded that the Gulf Basin did not deposition both to the north and south. Figure 37B exist prior to the Mesozoic Era. Branner (1897) and represents the region during deposition of the Eagle Mills Willis (1907) believed that a large continental landmass, Formation (Late Triassic). Eagle Mills sediments are Llanoria, occupied much of the present basin area. Suess continental in origin, derived principally from the erod­ (1904) and Schuchert (1935) suggested that the Gulf ing Ouachita Mountains to the north and west and depos­ Basin formed by collapse of a foreland. However, geolo­ ited in grabens associated with the initial rifting of the gists now generally reject the concept of Llanoria, and North American Plate from the Afro-South American presently two main schools of thought exist: (1) the Gulf Plate. Paleoclimatic indicators for the Late Triassic point Basin has existed since late Precambrian or early 28 BAYLOR GEOLOGICAL STUDIES

Fig. 37. Generalized maps showing Late to Early Cretaceous time. From Burgess, GCAGS Trans., 26, p. used with permission. zoic time, or (2) it was formed by Early Mesozoic sea- Jurassic time as a result of rifting, which was probably floor spreading in the Gulf, which created the Gulf of initiated in Late Triassic time. Rowett(1972) Mexico as a product of the general breakup of the old and Walper (1980) proposed a modified version of the supercontinent Pangaea into continental blocks (Woods BuUard et fit the reconstruction of Pangaea. and Addington, 1973). Paine and Meyerhoff (1970), This reconstruction, which is based on paleomagnetic Shurbet and Cebull (1975), and others believe that the data and the alignment of comparable orogenic belts Gulf Basin has existed since the Precambrian (Fig. 38). (Fig. 39), rotates Gondwanaland clockwise about Dietz and Holden explained the origin of the Gulf and completely closes the Gulf of Mexico. According to Basin in terms of plate tectonics, by relating it to the Walper (1980), convergence of North America with breakup of Pangaea at the end of Paleozoic Dickin­ Europe, Africa, and South America occurred during son and Coney (1980) believed that the Gulf of Mexico Late Paleozoic time. Late Triassic sea-floor spreading, did not exist at the end of Paleozoic time, and that the which separated North and South America, created the oceanic crust of the central Gulf of Mexico formed in Gulf and Caribbean basins. CONTINENTAL TRIASSIC ROCKS OF TEXAS 29

Postulated basin margin

Fig. 38. Precambrian basin margin of the Gulf of Mexico as postu­ lated by Paine and Meyerhoff. Paine and Meyerhoff, 1970, GCAGS Trans., v. 20, p. 22, used with permission.

Others have interpreted the Gulf Basin as the result of Fig. 39. Late sea-floor spreading of Pan- north-south extension and left-lateral shear between gaea, which separated North America from South America and Africa, North and South America as those continents separated creating the Gulf and Caribbean basins. From Walper and Rowett, from Africa. 1972, GCAGS Trans., v. 22, p. used with permission. The most widely accepted theory for the formation of the present Gulf Basin is that it was probably part of a Texas and northern Louisiana report non-commercial continental landmass until Late Triassic time when wide­ gas shows in the Eagle Mills section. The Hilliard No. 1 spread rifting split the North American Plate from the Pruitt, Marion County, Texas, found shows of gas in an South American Plate (Fig. 40A). This rifting formed a Eagle Mills sequence that was over 1600 ft thick. complex system of grabens, the orientation of which was In Early or Middle Jurassic time an arm of the sea related to old structural trends in the Appalachian and extended across the newly formed Gulf Basin (Fig. 40D). Ouachita Mountains, though the fault system may also The Jurassic section is composed of evaporites (anhydrite be independent of older structures outside the basin (Fig. and salt), carbonates, and deposited in restricted 40B). According to theory the entire graben system marine, shallow-marine, deltaic, and continental envi­ formed in Triassic time, but downfaulting ended first in ronments (Fig. 37). the more northern grabens, progressively shifting south­ After deposition of the Eagle Mills Formation the ward as the basin subsided to the south. Locally derived breakup of Pangaea continued, with further separation sediments from the north and west, represented by the of the Afro-South American Plate from North America. Eagle Mills Formation, were deposited in the grabens The grabens that formed in the early stages of continental under arid conditions (Fig. 40C). In addition, igneous rifting began to develop into a subsiding basin (Fig. 41). activity accompanied the downfaulting and formation A topographic ridge developed along the retreating plate of diabase dikes and sills are presently recognized in the margin, which became an effective barrier to free circula­ Eagle Mills section (Rainwater, 1968). Dip sections tion of sea water to the submerged area lying landward drawn along the northern limit of the Gulf Basin illus­ from it This low lying area became the evaporite trate the deposition associated with the complex block basin where the Werner anhydrite and Louann salt were faulting (Plates The Eagle Mills, Werner, and deposited (Wood and Walper, 1974, p. 39). Louann sediments were best developed in these buried The Eagle Mills Formation is an Upper Triassic con­ graben systems. Cross sections A-A' (Plate I) and CC tinental red bed sequence resting unconformably upon (Plate II) indicate thick sections of Eagle Mills as do the weathered Paleozoic sediments. The age of the Eagle Humble Shell and Anadarko 1- Mills Formation was not accurately determined until Bellieu wells. The sections also show the great thicknesses when Scott and others, identified plant macrofossils associated with the Interior Gulf Basin, the updip limits of Late Triassic age from cores in the Humble, 1 -Royston of which closely coincide the subcrop of the Eagle well in southwest Arkansas. This age was later confirmed Mills rocks. by radiometric dating of diabasic igneous rocks in the The environment of deposition of Eagle Mills red beds Eagle Mills section, which formed between 180 and 200 was, for the most part, unfavorable for the generation million years ago (Baldwin and Adams, 1971). Thus and preservation of organic material. However, there Eagle Mills Formation cannot be younger than the intru­ may have been rift valley lakes where petroleum source sions and most probably represents deposition during material was deposited (Rainwater, 1968). Though no Late Triassic time. Upper Triassic palynmorphs were significant occurrence of hydrocarbons has been found in found in the Eagle Mills shales and silts in pre-Jurassic the Eagle Mills to date, recent drilling in northeastern tests in Falls (Cockburn, and Gonzales Counties, 30 BAYLOR GEOLOGICAL STUDIES

Fig. 40. Paleogeologic and tectonic reconstruction of the Gulf Coast region during (A) Pre-Mesozoic time, (B) Early Triassic time, (C) deposition of Eagle Mills Formation (Upper Triassic), and (D) deposition of Werner Formation (Middle Jurassic). Wood and Walper, GCAGS Trans., v. 24, p. 34-37, used with permission. CONTINENTAL TRIASSIC ROCKS OF TEXAS 32 BAYLOR GEOLOGICAL STUDIES

The Rotliegende Formation is divided into the lower Slochteren Member and the upper Ten Boer Member. The Slochteren Member is composed of up to 600 ft of coarse to fine, poorly to well-sorted brown-gray sand­ stone and coarse red conglomerate interbedded with sev­ eral thin shale lenses (Fig. 43). The Ten Boer Member consists of up to 250 ft of reddish-brown, sandy claystone, with white to green-gray anhydrite nodules (Stauble and Milius, 1970, p. 359) (Fig. 43). The Rotlie­ gende sediments are characteristic of fluvial, eolian, and Fig. 41. Generalized west to east cross-section across the East Texas sabkha deposition during a time of relative aridity. These basin, showing f ormational units from Late Triassic to Middle Jurassic rocks are overlain by the Upper Permian Zechstein evap- time. Todd and Mitchum, AAPG Mem. 26, p. orites. Figure 43 shows the lithology of the Permian formations of the North Sea, which is remarkably similar Texas (Mobil, Henry Kelsey, In to the Triassic formations of the Gulf Coast. addition, Exxon palynologists have dated Eagle Mills sediments as Upper Triassic from cores at 9,685-9,975 ft in the Texas Gulf Sulfur Co., well in County, Texas (Zingula, EASTERN U.S. TRIASSIC BASINS Rocks that fill the Triassic graben of the eastern United ANCIENT DEPOSITIONAL MODELS States are primarily continental and consist mostly of Ancient depositional models for the Eagle Mills For­ conglomerates, sandstones, arkoses, siltstones, shales, mation can be found in the Lower Permian of the North and argillites (Eardley, p. 128). The lithology and Sea (Rotliegende Formation) and the Upper Triassic sedimentary history of the rocks deposited in the New graben basins of the eastern United States. Jersey-Pennsylvania-Maryland-Virginia (Newark Group) and Connecticut Valley (Meriden Formation) basins are NORTH SEA PERMIAN SEQUENCE similar to those found within the Eagle Mills Formation. Lithology and sedimentary history of North Sea Per­ The of the Newark Group and Meriden Forma­ mian rocks (Rotliegende-Zechstein Formations) is not tion are characteristic of deposition by coalescing alluvial that of the lower Mesozoic (Eagle fan, fluvial, and lacustrine systems during a semiarid to Louann) stratigraphic sequence of the Gulf Coast area. arid climatic era.

NORTH AMERICA

Fig. 42. Paleogeologic reconstruction of the Gulf Coast region during deposition of the Louann salt. From Wood and Walper, 1974, GCAGC Trans., V. 24, p. 39, used with permission. CONTINENTAL TRIASSIC ROCKS OF TEXAS 33

MARINE MARINE Anhydrite and halite Anhydrite and halite EVAPORITE A A EVAPORITE BASIN BASIN Copper Shale - base Zechstein Copper Shale - base Zechstein MARINE DESERT (non-laminated) sandstones RE-WORKING Halite and red clay (only minor some slump structures ar LAKE anhydrite) Haselgebirge facies T Bedded clays, silts and sands with mud-cracked clays, sandstone dikes INLAND and adhesion ripples in varying degrees SABKHA of importance. Anhydrite nodules Increasing in importance upward

Well-sorted horizontal and large-scale MAINLY planar cross-bedded sandstones, EOLIAN adhesion ripple beds increasing in importance upward Horizontal and planar cross-bedded Some beds of sandstone with sandstones with beds adhesion MIXED small-scale cross-bedding SABKHAS ripples, alternating with sandstones EOLIAN (some conglomeratic) that have small-scale, commonly irregular cross- bedding and which grade up into clays that may be mud-cracked, or penetrated by sandstone dikes. The conglomerates contain pebbles of both quartz and clay

Homogenized sandstones; sandstones with small-scale cross-lamination, locally MAINLY developed beds of conglomerate, mud-cracked clay or adhesion ripples Conglomerates and conglomeratic sand­ stones with both quartz and clay pebbles Well-sorted horizontal and planar Horizontal and planar cross-bedded cross-bedded sandstones, some MIXED sandstones. Sandstones with small-scale beds of adhesion ripples EOLIAN cross-bedding, clays and some WADI quartz and clay pebble Sandstones (small-scale cross-bedding Sandstones, (argillaceous, some MIXED WADI or homogenized), clay-pebble with small-scale cross- & conglomerates and mud-cracked clays bedding; clays, usually mud- cracked or MINOR with sandstone dikes. penetrated by sandstone dikes; some EOLIAN Homogenized sandstones or sandstones horizontal or planar cross-bedded sandst. WADI with planar at base Quartz (and clay) pebble conglomerates. Dark shales, cross-bedded sandstones. Volcanic lavas with interbedded conglo­ VOLCANIC/ HUMID ID Coal seams - Carboniferous, mainly PARALIC merates {metamorphic and volcanic Westphalian components), lower Rotliegendes, or PARALIC dark shales and cross-bedded sandstones Fluvial curled (Carboniferous - commonly Westphalian) sand clay flakes, Ss dikes

Adhesion ripples Quartz and clay-pebble Homogenized and in eoUan sand conglomerates slumped sands

Fig. 43. Facies development of the upper Rotliegendes in northwest Europe. After Glennie, 1972, AAPG Bull. v. 56, no. 6, p. 1055. Used with permission.

SEDIMENTARY HISTORY OF SYCAMORE ROCKS

Much of the historical significance of Sycamore rocks rocks appears to follow closely southwest-northeast has already been discussed. However, here an effort will trending Paleozoic fault blocks extending northward and be made to establish the sequence, processes and the eastward from the Llano Uplift. These fault trends continuity of Sycamore deposition. roughly parallel the buried Ouachita Foldbelt (Fig. 44), Rocks of the Sycamore Formation were deposited and faulting may be related in age to that tectonic feature upon an erosional surface characterized by a series of though it appears most closely related to the uplift of the valleys and divides that extended north, east, and south Llano itself. After creation of these fault structures, ero­ of the Llano region (Fig. 44). Thus, the Llano Uplift sion carved the shale-filled blocks into a sequence of exerted principal control on Sycamore deposition, for it subparallel valleys, which were then partially filled with provided both the source of water and sediments. While it Sycamore sediments. Thus the of the Sycamore is generally thought that the principal uplift of the Llano Formation appear to have been deposited upon an ero­ region occurred during Pennsylvanian time, evidence of sional surface consisting of fault valleys extending Sycamore deposition suggests that this uplift may have radially from the Llano region. Along these valleys, the continued or again become active in Early Triassic time. gravels of the Sycamore conglomerate originated in the Thus the area of the Llano region underwent erosion Llano region and were transported to their place of depo­ throughout Triassic time. Distribution of Sycamore sition (Damon, 1940, p. 88). On the divides that separated 34 BAYLOR GEOLOGICAL STUDIES

bedding types require transitional and upper flow regime conditions. The semiarid to arid climate is suggested by the character of Sycamore deposits, which consist of thick structureless beds of conglomerate overlain by thin sand drapes indicative of individual torrential sedimen­ tary events. The conglomerate consists typically of large well-rounded cobbles in a matrix of sand and caliche. Distribution of sediments in sedimentary structures strongly suggests that earliest Cretaceous flow was directed eastward across the northern end of the Llano region while lithology and sedimentary character of Syc­ amore rocks indicate that they are derived almost exclu­ sively from the Llano region and that during their deposi­ tion the flow direction was principally northward. Thus following deposition of Sycamore rocks, sufficient time passed for the Llano region to be worn away, for the climate to change from warm arid to more tropical humid conditions, and for the directions of drainage to change from north to east. Therefore, following deposition of Sycamore rocks sufficient time had to pass for all these events to take place. This fact also strongly suggests that Sycamore deposits are far older than the oldest typically Cretaceous Fig. 44. Map of Llano Uplift region showing probable transport of deposits in the study area to which they have been Sycamore rocks along major fault valleys. assigned. Correlation of the Sycamore conglomerate with the Dockum Group, Bissett and Eagle Mills Forma­ tions is suggested by a common character of deposition. The sedimentary history of the overlying Trinity the valleys. Sycamore were not deposited. When Group as suggested by sediment type and formation the Llano Uplift attained sufficient height during Triassic geometry was a product of a complex relationship time, rock fragments were transported down stream val­ between such variables as physiography, topography of leys and alluvial fans into the lower area to the east, the the sedimentary platform, subsidence rates, possible cli­ northeast, and the south. Boulders, cobbles, and pebbles matic variations and the influx of terrigenous sediment were transported by high-energy streams issuing from (Dreyer, 1971, p. 47). At the beginning of Cretaceous the Llano Uplift and were dumped in the proximal por­ time the Llano Uplift had been worn down by erosion to tions of alluvial fans as sieve deposits. Such deposits are such an elevation that it contributed little sediment to the characteristic of source areas composed primarily of earliest Cretaceous streams. Low-to-medium energy coarse materials, much like that of the Llano region. streams, which originated far west of the Llano Uplift, Sieve deposits occur when water infiltrates debris lobes transported sands and pebbles and deposited them in formed when the water can no longer transport material. fluvial sequences, including braided stream and point- As the water passes through the debris lobes, the fine bar deposits in which cross-bedding is emphasized by material is carried off, leaving behind poorly sorted grav­ pebbles of chert and quartz (Whigham, 1978, p. 65) (Fig. els and sandstone (Hooke, 1967, p. 454). These streams ended in deltas on the east where they The climate at the time of Sycamore deposition met the transgressing earliest Comanchean sea. As the appears to have been semiarid to arid, differing from the sea continued to transgress eastward, the fluvial deltaic more humid climate suggested by the character of the deposits were reworked and overlain by more marine overlying Cretaceous deposits, which are typically those shales and limestones of the Glen Rose Formation. The of mixed-load streams together with clean marginal slow fluctuating advance of the sea continued throughout marine sands and shales. Alluvial fans are characterized Fredericksburg time, and the Llano Uplift was finally by intermittent surge flow and give rise to structureless covered during Edwards time (Upper Comanchean). sedimentary blankets where the entire bed load is depos­ ited simultaneously. In arid to semi-arid climates an SUGGESTED DEPOSITIONAL MODELS increase in rainfall will increase sediment yield. In semi- FOR SYCAMORE SEDIMENTATION arid regions during Sycamore deposition, violent flood transport of Sycamore sediments apparently took place, Evidence suggests that the Sycamore conglomerates though the flow was normal for transporting agents were deposited by alluvial fan and ephemeral (wadi) responsible for the development of wadi channels and stream systems. The wadi type channel-fill deposits wadi fans. Evidence for high stream velocities during formed nearest the Llano Uplift, while the alluvial fan Sycamore time is found in the dominance of plane bed­ deposits formed in the distal portions. Therefore ancient ding and low-angle cross-beds in the deposits. These and modern models for Sycamore deposition are those CONTINENTAL TRIASSIC ROCKS OF TEXAS 35

characteristic of semiarid climates, typically those of alluvial fans and ephemeral streams.

ANCIENT DEPOSITIONAL MODELS There are many ancient examples of coarse-grained terrigenous clastic fades interpreted as alluvial fan and ephemeral stream deposits. Some of these ancient se­ quences are: 1) the Precambrian Hazel Formation of west Texas, 2) the Cambrian Van Horn sandstone of west Texas, 3) the Devonian Old Red Sandstone of Norway, 4) the Permian Rotliegende of northwest Europe, 5) the Triassic Eagle Mills Formation in the Gulf Coast Basin, and 6) the Triassic Mount Toby Formation of northern (McGowen, Galloway, and Kreitler, 1978, p. 50). These ancient deposits are similar in rock type and structure to the conglomeratic deposits of the Sycamore Formation suggesting that all such deposits originated under similar climatic and tectonic conditions by swiftly flowing streams and alluvial fans. Two of these stratigraph- ic sequences, the Mount Toby conglomerate and the Van Horn sandstone, best typify terrigenous clastic fades and Fig. 45. Schematic cross-section of the Van Horn alluvial fan serve as useful ancient depositional models for Sycamore system showing the general succession of stratification types from the sedimentation. coarse, massive gravels of the proximal fan facies to the trough and The Van Horn sandstone was deposited by high- foreset cross-bedded sand of the distal fan facies. From McGowen and gradient, short-duration, high-energy streams, which Groat, Texas Bureau of Economic Geology Rept. of Invest. 72, p. transported gravel, sand and lesser amounts of silt and 8, 39. clay through steeply dissected canyons onto an alluvial erate argues for swift, probably intermittent streams plain (McGowen and Groat, p. 8). The Van Horn flowing down steep grades in what was probably a semi- alluvial fan consists of proximal, mid, and distal fan arid to arid climate (Hand, 1969, p. The structure facies (Fig. 45). The proximal fan consists mostly of of the Mount Toby conglomerate is predominantly well-rounded cobble and boulder conglomeratic depos­ planar bedding characteristic of upper flow regime its, similar to those of the Sycamore Formation. Both conditions. gravel and sand accumulated in the mid-fan area while Both the Mount Toby and Van Horn conglomerates the distal fan consists primarily of coarse to very coarse are similar in composition and structure to the Sycamore sand. These coarse conglomerates, which probably rep­ conglomerate, and therefore deposition probably occurred resent surge deposits, grade vertically and laterally into under similar environmental and tectonic conditions. alternating conglomerate and thin sandstone beds. The sandstone developed as thin sheets overlying the con­ MODERN DEPOSITIONAL MODELS glomeratic beds, representing waning stages of current flow (McGowen, Galloway, and Kreitler, 1978, p. 52). Alluvial fan deposits tend to be poorly sorted coarse­ Sandstone drapes similar to these are found in the Syca­ grained sediments composed primarily of gravel cobble­ more Formation at locality 25 (Elliot Creek). Although stones and boulders and are most common where coarse the Van Horn Formation has been classified as a "wet" detrital material is most abundant (Reineck and Singh, alluvial fan (McGowen, Galloway, and Kreitler, 1978, p. p. 234). Alluvial fans are the product of sporadic, 50), the sedimentary processes involved were not unlike high energy, short duration depositional events (Mc­ those which deposited the Sycamore conglomerate. Gowen et 1978, p. 44). The Gorak Shep fan of the The second ancient depositional model is the Mount desert region of central California (Eureka Valley) is an Toby conglomerate of Triassic age, which crops out in example of an alluvial fan with sediments similar to those northern Massachusetts where it is part of a continental of the Sycamore Formation. Deposits of the Gorak Shep sequence of fan conglomerate, fluvial sandstone, mud- fan consist of pebble to cobble-sized gravel, with little or stone, and lake sediments occupying a fault valley no fine material. The source area of the Gorak Shep fan is flanked by higher areas of metamorphosed Paleozoic underlain by a thick section of resistant carbonate rocks, rock. The Mount Toby conglomerate consists of fine to principally dolomite (Hooke, 1967, p. 455). coarse sandstone with tongues of poorly sorted pebble to Deposition occurs mainly in sieve lobes, formed when boulder conglomerate. The conglomerates become coarser water is unable to effect further transport (Hooke, and thicker toward the higher source areas to the west. p. 454). Figure 46 shows the distribution of the sieve The regional paleogeography is one of coalescing alluvial deposits on the Gorak Shep fan. This distribution of fans spreading westward from a border fault. The sediment types may explain the diversity and discontinu­ coarseness of the sediment of the Mount Toby conglom- ous nature of Sycamore rocks from outcrop to outcrop. 36 BAYLOR GEOLOGICAL STUDIES

calcareous formation of considerable thickness and volume, is found a few inches below the surface soil, upon the broad, dry gravelly plains, mesas and alluvial fans. While the processes of formation of caliche are not yet clearly understood, it occurs most commonly within the K horizon (horizon of carbonate accumulation) of the soil profile and is formed as a product of soil-water geochem­ istry. In a gravelly sequence of caliche development, the first stages are characterized by thin discontinuous peb­ ble coatings and interpebble fillings. In the later stages there are many interpebble fillings, and eventually the plugging of the horizon with overlying horizontal laminae (Rightmire, 1967, p. 5). The origin and climatic significance of caliche is a point of much debate upon which most authors agree in general but for details are uncertain. The overall concensus is that caliche signifies a semiarid climate where average evapo­ ration exceeds average precipitation (Mikels, 1976, p. 12). The existence of thick massive caliches is of consid­ erable paleoenvironmental interest and, while some argument may exist concerning exact climatic implica- tions, it is almost universally agreed that such caliches deposits ' represent near-surface accumulations during prolonged

, ' exposure (Amsbury, 1967, p. 4; Dunham, 1972; I Recent channel deposits - \ . r " " ' Reeves, 1976, p. 5). Regardless of the mechanism of caliche formation, its common association with semiarid channel V deposits makes the occurrence of widespread caliches surface both at the base and at the top of the Sycamore conglom- Older fan surface erate strongly suggestive of semiarid land deposition. Oldest fan material \ Amsbury (1967, p. 4) found evidence of ancient caliches ' in Lower Cretaceous and Sycamore rocks of central The caliches are best developed in the Upper por- inferred from air j tions of depositional Sequences, unconformities. of fan At the Nix Cemetery and Elliot Creek there are of channel extensive caliche zones above the Sycamore conglomer- scarp, eroded; inferred ate (Fig. 47) where caliche sections have asymmetrical Hachures on down-thrown side . . . , , profiles sharp tops and bases. 1 hus, the Contour 40 feet acceptable modern models for Sycamore deposi- Contouring by Plotter . . , are those typical of semiarid lands: streams and alluvial fans. From this evidence it appears that of Fig. 46. Map of Shep alluvial fan, Eureka Valley region, Sycamore Formation were deposited in stream California. Note the area of active sieve deposition. From , c 1 , , fans extending the mountain uplands the Hooke, 1967, Journal of Geology, v. 75, no. 4, p. 442, used . permission. Uplift. Intermittent streams flowed from what may have been a pre-Sycamore pediment surface extending A second possible modern model may be seen at the from the mountain toe northwestward to the basins of base of the Upper Pleistocene sequence in Nubia (Egypt) accumulation of the Dockum Group. Over this pediment where the Wadi Or, a large Nubian tributary, enters the surface Sycamore sediments were transported and depos- on the east bank. The Wadi Or is usually a dry ravine ited in shallow channels and fans separated by indistinct except during rainy seasons. In hilly areas where occa- divides. torrential rain falls, become flooded the Following deposition of Sycamore sediments there flowing water can transport considerable amounts of appears to have been a reversal of the topography in sediments for short periods of time. Deposits within the many parts of the region. The valley fills armored by wadi channels are mostly conglomeratic and fanglom- caliche-cemented Sycamore conglomerate were more re- eratic (Glennie, 1970, p. 29-34). Deposits found at the sistant to erosion than the softer inter-stream divides. Wadi Or are coarse pebble to cobble-grade gravels of were a quicker rate ferruginated sandstone, with a white to pink matrix of leaving once lower, valley-fill deposits of small calcreted coarse sand (Butzer, 1968, p. 283). hills. Present day outcrops of the Sycamore conglomer- Caliche, common in many wadi deposits, while not in ate in Mills, McCulloch, and San Saba Counties occur on itself an indicator of semiarid chmates nevertheless is ridges and hills where the conglomerate forms a relatively most common in semiarid to arid regions. Caliche, a smooth surface a topography (Fig. 48). CONTINENTAL ROCKS OF TEXAS 37

Fig. 47. Extensive caliche zone developed in the upper portions of the Fig. 48. Locality Overall view of the gently rolling topography and Sycamore conglomerate in Lampasas County. flat upland surfaces characteristic of the present day Sycamore Forma­ tion. Sycamore rocks are exposed in the roadcut on the right.

SEDIMENTARY HISTORY OF DOCKUM ROCKS

The Dockum Group is composed of a complex suite of glomeratic lenses. The fine material accumulated in small continental clastic sediments deposited in a broad shal­ ephemeral lakes. The lacustrine sediments are character­ low fluvial-lacustrine basin extending across most of west istic of a slow continuous form of sedimentation: an Texas. The Dockum Group includes sediments that environment beneficial to the emplacement and preserva­ range through the entire sequence of continental clastic tion of fossil remains. Thus the Dockum Group contains deposition from braided to meandering streams, alluvial faunal and floral assemblages representative of lacustrine fans, constructive lobate deltas, fan deltas and lakes deposits. The faunal assemblages and plant associations (McGowen et 1979, p. 16). suggest an environment in which one or more streams The sedimentary history of the Dockum Group is emptied into lakes or ponds (Green, 1954, p. 92). Verte­ extremely complex and interpretations pertaining to the brate fossil remains include bones and teeth of reptiles depositional environments vary widely. Presently, there (mostly Phytosaurs) and amphibian (labyrinthodonts) as exist two contrasting environmental interpretations. well as some fish scales and teeth. Invertebrate remains Kiatta (1960, p. 12) and Cazeau (1962, p. 81) believed include those of the mollusk Unio, a fresh water bivalve, that the poorly bedded clays and silts of the Tecovas also characteristic of lake environments. Outcrops of Formation were essentially flood plain and meandering laminated siltstone and fine sandstone of the Tecovas stream deposits whereas the overlying Trujillo sands and Formation exposed along the eastern edge of the Cap- shales are characteristic of braided stream channel de­ rock Escarpment contain thin layers of light-green clay posits. and sparry gypsum characteristic of lacustrine deposi­ On the other hand, McGowen and others (1979, p. 16) tion. The presence of banded clay and some fine-grained analyzed the sedimentary history of the Dockum Group limestone masses indicates deposition in quiet undis­ in terms of genetic facies that compose depositional sys­ turbed waters (Green, 1954, p. 20). In addition, the muds tems. They concluded that the Tecovas Formation is and silts are interbedded with lenticular sands and con­ characteristic of ephemeral stream, fan-delta and lacus­ glomerates believed to have been associated with fan trine deposition (low-stand depositional systems) where­ delta systems. These sand and conglomeratic lenses are as the Trujillo Formation represents deposition by probably indicative of the waxing and waning stages of meandering stream and prograding lobate delta systems stream deposition when stream channels became deeply (high-stand depositional systems). incised then rapidly filled with detrital material. The method and interpretations established by McGowen and others (1979, p. 16) have proven to be of most use to TRUJILLO FORMATION this study and confirm field observations made of Dockum outcrops throughout the area of investigation. The Trujillo Formation consists predominantly of dis- continous lenticular sandstones and intraformational conglomerates interbedded with red mudstone and silt- TECOVAS FORMATION stone. The sediments are characteristic of prograding The Tecovas Formation consists essentially of poorly delta and meandering fluvial systems. The sand and bedded clays and silts with lenticular sandstone and con­ conglomerate lenses are highly variable in thickness and 38 BAYLOR GEOLOGICAL STUDIES

Fig, 49. Locality 5. Distributary channel-fill, dissected in lenticular cross-bedded sandstone pods. Fig. 50. Locality 5. Well developed cut-and-fill structure characteris­ tic of fluvial Dockum sedimentation can be seen at Locality 5 where large channels of cross-bedded sand have cut into the surrounding horizontal extent, are commonly cross-bedded, and mudstone and siltstone deposits. exhibit well developed structures. Most of the outcrops reveal the lenticular nature, cross-bedding and prograding deltaic sequence. The thickly bedded sand­ scouring of the Trujillo sediments. Many of the filled stone developed at the top of the outcrop may represent a channel deposits have clay- and sand-pebble conglom­ distributary channel fill. Large sigmoidal foreset cross- eratic lenses at their bases (Fig. 33), while other channels strata shown in Figure 34 are characteristic of the basal were dissected into lenticular crossbedded sandstone part of a distributary channel fill sequence. pods before being filled (Fig. 49). Characteristic Trujillo deposits are shown in Figure 50 (locality 5) where highly Low STAND DEPOSITIONAL SYSTEMS cross-bedded channel-fill sands have cut into red mud With a shift toward arid conditions, base level dropped, and siltstone layers. lake size decreased, and valleys were scoured. Local braided streams became the dominant type of fluvial DEPOSITIONAL SYSTEMS system replacing most of the meandering streams (McGowen et al., 1979, p. 4). Low-stand deposits in­ The depositional environment for Dockum rocks can cluded lacustrine, fan delta, ephemeral streams, and be analyzed in terms of depositional systems, under the interdeltaic mudflat systems (Fig. These are pri­ influence of base-level oscillations (McGowen et marily the products of sporadic, high-intensity, short- 1979, p. 16). Dockum sedimentation was influenced duration depositional events (McGowen et al., 1979, p. greatly by rifting associated with the opening of the Gulf 23). The fan deltas formed along the margins of ephem­ of Mexico, which probably caused a change in climate, eral lakes and were fed by high-gradient headward- 2) subsidence of the Dockum Basin, and 3) an uplift in eroding braided streams. Many older Dockum deposits of the Ouachita Tectonic Belt. McGowen and others were scoured by these streams forming numerous valleys, (ibid.) noted that: which were then filled by a complex sequence of sedi­ with a slight increase in precipitation Permian sabkha environ­ ments ranging in texture from mudstone to cobble con­ ments were replaced by expanding lacustrine and fluvial-deltaic glomerate (ibid., p. 31). These intrabasinal sediments environments. were the major constituents of fan-delta depositional Late Triassic time in west Texas was characterized by systems deposited in the and delta-platform alternating pluvial and arid conditions, which caused the facies. The cross-stratified intrabasinal conglomerates lake area and depth to fluctuate. Lake level was highest and sandstones shown in Figure 33 (locality 5) probably and most stable during wet periods, when water depths represent sedimentation in a feeder channel for a small reached about 30 ft. fan-delta system. The lake sediments probably accumu­ lated by the settling of suspended particles and by bottom HIGH STAND DEPOSITIONAL SYSTEMS currents. The high-stand facies systems, characterized by mean­ Sedimentation characteristic of both depositional sys­ dering streams and high constructive lobate delta sys­ tems can be seen at Palo Duro Canyon where fan-delta tems, developed when climatic conditions were more and lacustrine deposits of the Tecovas Formation (low humid and lake areas and level were at their maximum stand) are overlain by prograding deltaic deposits charac­ 1979, p. (Fig. 51 A). The high-stand teristic of the Trujillo Formation (high stand) (Fig. 35). sediments are composed of a basal prograding, upward During drier periods, base level was lowered and many of coarsening deltaic sequence of mudstone and sandstone the Trujillo deltaic sediments were removed by erosion to overlain by an upward fining, meandering fluvial se­ form valleys, which were later filled when base level rose quence of thick sandstone and conglomerate. Figure 26 because of accumulating fluvial, deltaic, and lacustrine shows a section of red mudstone and siltstone inter- deposition. bedded with thin sandstone layers overlain by more thickly bedded sandstone. This exposure is probably SEDIMENTARY HISTORY characteristic of lacustrine type deposition overlain by a Lower and Middle Triassic sediments are not recorded Fig. Major depositional elements present during (A) the high-stand, humid phase (high-stand depositional systems) and (B) the low-stand, arid phase (low-stand depositional systems). From McGowan et 1979, Texas Bureau of Economic Geology Rept. of Invest. 97, p. 16. 40 BAYLOR GEOLOGICAL STUDIES

in west Texas, an indication that this time period was one depositing coarser sand and gravel on the northeastern of relative The climate was unchanged from that margin and finer silt and clay in the deepest portions of of Permian time, remaining for the most part arid. Ero­ the basin. Multiple provenance areas for the Upper Tri­ sion of Permian evaporites and red beds was widespread, assic sediments of west Texas were first postulated by but no significant deposition occurred within the area of Kiatta in 1960 (p. 50-51). He concluded that most of the this investigation. It was not until Late Triassic time, depositing streams flowed to the north and west from probably as a direct result of the rifting associated with upland regions in central Texas (Llano Uplift and Bend the opening of the Gulf of Mexico, that tectonic instabil­ Arch) and southern Oklahoma (Wichita and Arbuckle ity affected the region. Slight rejuvenation of some older Mountains). Paleozoic structural features as well as a change from an p. 74) concluded from paleocurrent and arid to more semiarid climate initiated sedimen­ heavy mineral analysis that the immediate source of tation. These sediments were deposited in a gently subsid­ Dockum rocks was from a sedimentary terrain of Penn- ing basin roughly coinciding in area with that of the sylvanian and Permian age lying to the southeast, prob­ Midland Basin of Permian age. This structural ably the Llano Uplift region of central Texas. tion resulted in increased erosion of Permian and older Cramer (1973, p. 26) assumed from grain size Triassic red beds, and sediments from this erosion were and the presence of vein quartz and chert that the transported from the highlands by braided to meander­ Ouachita-Marathon Fold Belt was the source area for ing streams, which formed alluvial fans and constructive most of the Dockum Group. Figure 4 is a map of Texas and destructive delta systems. Deposition during alter­ showing suggested source areas for the Dockum sedi­ nating pluvial and arid conditions existed throughout ments. The areas represented are mostly relict Paleozoic most of Late Triassic time. Most of the rainfall and the structural features and indicate that the basin was periph­ dominant vegetation occurred in the highlands to the east erally filled as the result of a complex mixture of and southeast, decreasing in abundance westward (Mc- sediments derived from numerous provenances. Included Gowen et 1979, p. 4). Lake area and water depth in the provenance areas are the: 1) Llano Uplift, 2) Bend fluctuated with changes in climate and sedimentation. Arch, 3) Wichita and Arbuckle Mountains, 4) Amarillo During more humid climatic conditions progadational Uplift, 5) Sierra Grande Arch, 6) Sacramento Uplift, 7) deltas and associated meandering streams constituted the Diablo Platform, and 8) Ouachita-Marathon Tectonic principal depositional systems. The progradational delta Beh. sequences are composed of extrabasinal sediments de­ rived from the surrounding upland regions. Erosion of DEPOSITIONAL MODELS neighboring Dockum sediments accompanied a shift toward arid conditions and a lowering of base level (ibid., Because of the complex nature of the sediments no p. 1). These deposits of intrabasinal sediments range from single depositional model describes the variety of mudstone to conglomerate. Dockum depositional systems (McGowen et al., 1979, p. 4). However, the Dockum Group displays characteristics common to several ancient and recent fluvial-deltaic- PROVENANCE AREA lacustrine systems. The Dockum sediments were derived chiefly from FLUVIAL-LACUSTRINE SYSTEMS older sedimentary rocks lying to the east, west, and south of the basin (Fig. 4). Heavy mineral assemblages give Fluvial-lacustrine depositional sequences are governed evidence of metamorphic and igneous source rocks, but by numerous physical and biological factors best devel­ their contribution to the Dockum sediments was rela­ oped during lake level fluctuations (Picard and High, tively (Kiatta, 1960, p. 49). This probably indicates 1972, p. Because most lakes are eventually filled that more than one sedimentary cycle was involved. regression dominates the history of the lake. The ideal Although the preferred cross-bed direction in Dockum fluvial-lacustrine sequence consists of fine-grained lacus­ rocks was to the west-northwest, there is a considerable trine clays and muds overlain by coarser deltaic and spread in calculated values. In addition, directional read­ fluvial sands and conglomerates (Twenhofel, 1932, p. ings and sandstone dispersal patterns described by Mc- 827). Most large-scale closed lake systems form regional Gowen and others (1979, p. 4) indicate that the Dockum base levels that rise and fall in response to Basin was peripherally filled. Its seems probable, there­ changes. If significant enough, these changes can produce fore, that the Dockum sediments were deposited by com­ alternating transgressive and regressive depositional cycles plex depositional systems, which derived their sediments similar to those found within the Dockum Group. A not from a single localized source but from multiple delicate balance exists between erosion and deposition in provenance areas. These source areas are probably response to these base-level fluctuations. An ancient related to slight rejuvenation of old Paleozoic structural example of such base level cycles is found within the elements. Green River Formation (Eocene) of Wyoming, Colo­ Green (1954, p. believed that the Permian red rado, and Utah (Picard and High, 1972, p. 134). beds served as the major source for the Dockum shales Transgressive-regressive cycles formed in shallow water and clays. He suggested that the sediment was trans­ areas as the base level and area of Lake Gosiute and Lake ported by westerly and southwesterly flowing streams Uinta rose and fell. Alternating cycles of channel cutting CONTINENTAL TRIASSIC ROCKS OF TEXAS 41

and filling also occurred on the surrounding floodplain and regressive cycles. (Picard and High, 1972, p. 134). Lake Eyre in south Australia, a large shallow basin Two possible modern depositional models for the covering some 3000 is rarely filled with water. Sedi­ Dockum Group are the Omo Delta in Ethiopia and Lake ment supply is relatively low and the lake margin fluctu­ Eyre in Australia (McGowen et 1979, p. 47). The Omo ates over great distances (Reading, 1978, p. 61). Lake Delta, a distributary delta along the shores of Lake Eyre, like the Dockum Basin, is peripherally filled receiv­ Rudolph, is characterized by alternating transgressive ing sediment from more than one provenance.

SEDIMENTARY HISTORY THE BISSETT FORMATION

AGE DETERMINATION DEPOSITIONAL ENVIRONMENT Age of the Bissett conglomerate has been subject to The nature and character of Bissett rocks suggest much debate, having been classed variously as Upper deposition not that of the Dockum Group and Permian, Lower to Middle Triassic, and Lower Creta­ Eagle Mills Formation. The coarse limestone, dolomite, ceous. However, plant and vertebrate remains found and chert gravels were probably derived from various within the Bissett Formation by P. B. King and E. H. nearby Permian formations and the Ouachita Tectonic Sellards strongly suggest a Triassic age. The fossils, stud­ Belt. Furthermore, McGowen and others (1979, p. 8) ied by Case and Read (King, 1935, 1545), both flora believed that the Bissett Formation records initial Trias­ and vertebrate species, are typically early Mesozoic sic alluvial fan and fan-delta sedimentation immediately types. Thus, the physical and paleontological evidence north of the Ouachita Foldbelt. indicate an Early Triassic (pre-Dockum) age for the Bis­ sett conglomerate (King, 1935, p. 1546).

COMPARISON THE CONTINENTAL TRIASSIC ROCKS OF TEXAS

The Triassic rocks of Texas are represented by four The conglomerates of the Bissett and Sycamore For­ distinct but similar rock groups. They are all primarily mations both contain rounded pebbles and cobbles of terrigenous in nature, and many are derived from the limestone, dolomite and chert. They are typically homo­ same tectonic elements. Therefore, a similarity exists genous masses with some interbedded sand layers repre­ between the lithological character and sedimentary his­ senting waning stages of stream surges (Fig. 20). tory of these rock types, and a useful paleogeography of Another comparison exists between the of Texas during the Middle to Late Triassic Period can be the Dockum Group and Eagle Mills Formation (Table constructed based upon the distribution of these deposits. 4). They are both red-bed clastic sequences composed of The lithologies of the Bissett Formation, Dockum interbedded layers of sandstone, siltstone, mudstone and Group, Eagle Mills Formation, and Sycamore conglom­ conglomerate. The sandstones range from white to red in erate are much alike. They are clearly of nonmarine color and are locally highly cross-bedded, and in places origin derived principally from older Paleozoic sedimen­ finely micaceous. The silts and shales of the Eagle Mills tary rocks. The rock groups are composed in part or Formation and Dockum Group are sometimes mottled entirely of mudstone, siltstone, medium- to coarse­ with light green clay or mud inclusions indicative of grained sandstone, and pebble-to-boulder conglomerate reducing lake conditions. (intrabasinal and extrabasinal). Taking into account the All of the Triassic coarse sandstones and conglomer­ diversity of the provenance areas, a unique interrelation­ ates are products of high energy short duration deposi­ ship exists among lithologies of the formations. Table 4 tional events. The coarse detritus was transported when lists the four rock groups and their associated character­ sporadic heavy rainfall, characteristic of semiarid to arid istics. It illustrates the compatibility of the rock charac­ regions, fell in the surrounding upland regions. The teristics, particularly between those of the Bissett and cobble-to-boulder conglomerates of the Bissett and Syc­ Sycamore Formations and the Dockum Group and amore Formations were deposited by high-gradient Eagle Mills Formation. Each of the rock units contains swiftly flowing streams on coalescing alluvial fan sys­ varying amounts of poorly sorted conglomerate and tems. These were highly localized deposits accumulating sandstone. close to their respective source areas. 42 BAYLOR GEOLOGICAL STUDIES

TABLE 4. THE FOUR TRIASSIC ROCK GROUPS OF TEXAS AND ASSOCIATED CHARACTERISTICS. (All are Continental Red-bed Sequences Similar in Composition, Texture, and Color)

BISSETT SYCAMORE EAGLE MILLS FORMATION GROUP FORMATION FORMATION

Sandstone Sandstone Conglomerate Sandstone red. red. fine poorly coarse brick red, white, and fine grained. to coarse grained. dolomite. green, fine to coarse finely micaceous. and chert pebbles. grained. Shale quart/itic. and boulders thinly bedded, in a coarse quart/ Siltstone reddish brown to Shale red. argillaceous. sand and maroon. unlithified, variegated, finely matrix. micaceous. mottled by light green Conglomerate caliche occurs as indu­ clay. rounded pebbles Conglomerate rated /ones both at the and cobbles of intrabasinal and Conglomerate base and top of the limestone and extrabasinal; pebbles and cobbles of formation. dolomite with intrabasinal-clasts quart/ and ortho- minor amounts of mud- quart/ite in a matrix stone, siltstone sand­ of fine to coarse­ and chert in a cal­ and caliche; grained argillaceous careous sand extrabasinal-clasts red sandstone. matrix. of chert, inclusions quartzite, and vein quart/ with some minor amounts of sid- wood fragments erite. anhydrite vein in a red sand matrix. calcite, and dolomite.

In contrast, deposits of the Dockum Group and Eagle nental clastic sediments deposited in an inland fluvial- Mills Formation are arealy extensive accumulating in a lacustrine basin encompassing much of west Texas. The variety of terrigenous clastic depositional environments. thinned edge of the Dockum red beds may have reached Both of these rock groups are composed of alternating almost to the Llano region in central Texas before being alluvial fan, fluvial, deltaic, and lacustrine deposits, eroded back to its present position during the Jurassic which reflect local climatic and tectonic fluctuations. and Cretaceous Periods (Barnes et 1972, p. 41). The Figure 8 is a paleogeographic reconstruction of Texas Dockum Basin was filled by streams entering around its during Triassic time illustrating the environments present margins, and thus the sediments reflect a variety of prov­ during deposition of Triassic rocks. The Bissett Forma­ enance areas (Fig. 4). Some of the chert and quartz found tion was deposited in alluvial fans and braided-to- in some of the extrabasinal conglomerates may have been meandering streams, which extended northward from derived from the Llano Uplift region (Roth, 1961, p. 51). the southern end of the Ouachita-Marathon Foldbelt. Dockum sediments were deposited in alluvial fans, chan­ These are the earliest Triassic deposits in Texas. The nels of braided-to-meandering streams, on prograding Triassic deposits in eastern Texas of mudstones, silt- lobate deltas, on fan deltas, and in lacustrine systems. stones, sandstones and conglomerates of the Eagle Mills Sedimentation was greatly affected by alternating cli­ Formation were derived from the northern extent of the matic conditions, which produced changes in base level, Ouachita Tectonic Belt. Erratic thickness patterns and water depth, and areas of lakes and in the character of geophysical data suggest that the Eagle Mills Formation streams that flowed into the basin (McGowen et al., 1979, was deposited in subsiding grabens formed as extensional p. 46). The low-stand deposits are products of sporadic, features associated with the initial opening of the Gulf high-energy, short-duration depositional events similar Coast during Late Triassic time adjacent to the Ouachita to those that formed the Sycamore and Bissett Forma­ Foldbelt. The sediments were transported by southeast­ tions. The alternating fluvial deltaic, and lacustrine ward flowing streams and deposited in alluvial fans orig­ sequences of the Dockum Group are not unlike those inating from the Ouachita Mountains. found within the Eagle Mills Formation and were proba­ The Sycamore Formation was a similar localized con­ bly deposited in similar depositional environmentals. glomeratic deposit surrounding the Llano Uplift of cen­ The character and distribution of the continental Tri­ tral Texas. These poorly sorted conglomerates were de­ assic rocks of Texas were directly influenced by the cli­ posited by high-gradient streams, which formed alluvial matic regime existing during deposition. The character of fans adjacent to the Llano region. There is a pronounced these formations strongly suggests semiarid deposition decrease in size away from the Llano Uplift indicating typical of the low latitude desert regions of today. During that it was the principal source area for Sycamore Triassic time arid climates dominated a broad region of deposits. the global landmass, as is indicated by paleogeography The Dockum Group is a complex sequence of conti­ and paleoclimatology. This arid region included the Tri- CONTINENTAL TRIASSIC ROCKS OF TEXAS 43

implications

Fig. 53. Distribution of certain Upper Triassic climate-sensitive sedi­ mentary rocks. Arrows show wind direction obtained from aeolianites, Fig. 52. Sketch of probable course of the Intertropical Convergence evaportites in black. A aeolian, C coals, D deltaic, F fluviatile, Zone for July and January and major precipitation climatic regions in L lacustrine, R red beds. From Robinson, 1973, Imphcations of the Triassic Period. Vertical dashed lines indicate region with year- continental drift to the Earth Sciences, p. 467, Academic Press, used round dry chmate. From Robinson, of continental with permission. drift to the Earth Sciences, p. 466, Academic Press, used with permission. assic depositional area of Texas. Because of the north­ conditions are suggested. Thus, Sycamore, Eagle Mills, ward deflection of the Intertropical Convergence Zone Dockum, and Bissett rocks appear to be products of (Fig. 52) the Trade Winds travelled over the land- deposition during a single maj or arid to semiarid climatic mass, bringing a dry semiarid climate to the region of episode, such as that of Late Triassic time. These deposits Triassic deposition (Robinson, 1973, p. 464-467). In Fig­ are most closely related, not in terms of stratigraphic ure 53 the distribution of Upper Triassic evaporites, aeo­ continuity but in terms of the mechanics of their origin: lian sands and red beds has been plotted. The semiarid to continental clastic sediments derived primarily from arid type deposits occur in the intertropic latitudes of the older sedimentary rocks during a period of semiaridity. western and central portion of the landmass where dry

CONCLUSIONS

Four Upper Triassic rock units exist in Texas: with the coarser representing alluvial fan 1) Eagle Mills Formation, 2) Sycamore Formation, deposits derived from streams that flowed from the 3) Dockum Group, and 4) Bissett Formation. Ouachita Tectonic 2. The Triassic rocks occur both in outcrop and in the 9. Mudstones and of the Eagle Mills Forma- subsurface encompassing large portions of north- tion are products of deposition, which east, central, and northwest Texas. includes marshes, ephemeral lakes, and meander- 3. The pre-Triassic surface was a continental ero- ing streams. sional surface of moderate relief transecting a 10. The environment of deposition during Eagle Mills number of structural provinces. time was unfavorable for the generation and pres- 4. The Eagle Mills Formation exists in discontinuous ervation of organic material. However, there may occurrence in the subsurface from south Texas have been rift valley lakes where petroleum source northeastward across southern Arkansas and material was deposited. northern Louisiana into western Mississippi. The Late Triassic age for the Eagle Mills Formation 5. The eroded north edge of the Eagle Mills Forma- was based on the identification of Upper Triassic tion follows the southern boundary the Ouachita palynmorphs and plant macrofossils as well as Tectonic Belt and represents the initial deposits of radiometric dating of diabasic igneous rocks in the the Interior Gulf Basin. Eagle Mills section. 6. Eagle Mills deposition was controlled by normal The Sycamore conglomerate is the lowermost coarse faulting associated with the initial rifting of the Gulf terrigenous conglomeratic deposit immediately be- Coast. neath Cretaceous rocks unconformably overlying 7. The Eagle Mills Formation is composed of inter- Paleozoic rocks and distributed in discontinuous bedded layers of sandstone, siltstone, shale and outcrop along the northern, eastern, and southern conglomerate. flanks of the Llano 8. Eagle Mills sediments are predominantly alluvium 13. The Sycamore Formation is composed of 44 BAYLOR GEOLOGICAL STUDIES

cobbles and boulders of hard limestone, dolomite, which were deposited in a broad, shallow fluvial- and chert in a matrix of coarse quartz sand and lacustrine basin by alluvial fans, braided and me- caliche derived primarily from erosion of Paleozoic andering streams, lobate deltas, and fan sedimentary rocks of the Llano region. The Dockum Group is presently divided into three 14. In areas of good Sycamore rocks typi- formations, which can be traced for considerable consist of thin structureless beds of caliche- distances and include, from oldest to youngest, the cemented conglomerate capped sand drapes, Tecovas, Trujillo, and Chinle which represent the waning stages of separate 22. patterns from cross-beds the Dockum stream flows. sandstones show a preferred orientation to Evidence of Sycamore deposition suggests that the west-northwest indicating that the depositing streams Llano Uplift may have continued or again become east of the basin flowed generally to the west-north- active in Early Triassic time. west. 16. Distribution of Sycamore rocks follows closely 23. Lake area and water depth of the Dockum basin southwest-to-northeast trending Paleozoic fault fluctuated with changes in climate and sedimenta- extending north the Llano Uplift. tion. When climatic conditions were more 17. When the Llano Uplift attained sufficient height and lake level and areas were at their maximum, during Triassic time, rock fragments were trans- deltas and associated meandering streams ported by high-energy streams and dumped in the constituted the principal depositional systems. With proximal portions of alluvial fans as sieve deposits. a shift toward more arid conditions base level and 18. Distribution of sediments and sedimentary struc- lake size decreased and valleys were scoured. Local tures strongly suggests that earliest Cretaceous flow braided streams and fan deltas became the domi- was directed eastward across the northern end of nant type of depositional system. the Llano region while lithology and sedimentary 24. The Bissett Formation is a terrigenous unit com- character of Sycamore rocks indicate that they are posed primarily of limestone and chert conglomer- derived from the Llano region and that during their ate, red shale and sandstone, which crop out along was principally northward. the northwest margin of the Glass Mountains. 19. Regardless of the mechanism of caliche formation, 25. The coarse limestone, dolomite and chert gravels of its common association with semiarid deposits the Bissett Formation were deposited in alluvial makes the occurrence of widespread caliches both fans and fan deltas by streams following northward at the base and at the top of the Sycamore Forma- from the Ouachita Fold Belt. tion strongly suggestive of semiarid land deposition. 26. The nature and character of the Triassic rock units 20. The Dockum Group of west Texas is a complex strongly suggest semiarid to arid deposition typical red-bed sequence consisting of interbedded mud- of the low latitude desert regions of stones, siltstones, sandstones, and

LOCALITIES 7. Outcrop of Dockum Group along the Caprock Escarpment about 2 mi north of Fluvanna, Scurry County, Texas. 1. Outcrop of Dockum rocks just west of Sterling City cemetery on 8. Railroad cut exposed along U.S. Highway 84, 3 mi south of Jus- State Highway 163, Sterling County, Texas. ticeburg, Garza County, Texas. 2. Dockum rocks exposed along State Highway 24 north of 9. Dockum rocks exposed along U.S. Highway 84, 2 mi north of Sterling City, Mitchell County, Texas. Justiceburg, Garza County, Texas. 3. Roadcut on State Highway 163 at east end of Mountain, Triassic Dockum Group exposed along U.S. Highway 84, approxi­ 14 mi south of Sterling City, Mitchell County, Texas. mately 3 mi south of Post, Garza County, Texas. 4. Outcrop of Dockum rocks just north of Camp Creek on State Triassic Dockum Group, mi south of Post on Farm Road 669. Highway 163, 11 mi south of Colorado City, Mitchell Outcrop of Dockum rocks exposed along Highway 38,1 mi west of County, Texas. Post, Garza County, Texas. 5. Dockum rocks exposed in a roadcut along State Highway mi 13. Dockum rocks exposed along the Caprock Escarpment on State south of Colorado City, Mitchell County, Texas. Highway 207 just north of Courthouse Mountain, approx­ 6a. Triassic Dockum conglomerate found at the intersection of two imately 2 mi north of Post, Garza County, Texas. unpaved farm roads just north of Interstate Highway 20, 14. Outcrop of Permian (Quartermaster Group), Triassic (Dockum city limit, Loraine, Texas (Mitchell County). Group), and Tertiary (Ogallala Group) rocks exposed at 6b. Stream just off Interstate Highway 20 Loraine, Texas (Mitchell Palo Duro Canyon State Park, mi east of Canyon, Ran­ County). dall County, Texas. 5. Sycamore Formation exposed above the steeply dipping Paleozoic rocks, Pedernales Falls area, Blanco County. Appendix 1, measured sections and described localities Dockum 6. Outcrop of Sycamore on the Pedernales River northern Hays Group, Sycamore Formation (77 p.), is available from the Department County. of Geology, Baylor University, for reproduction costs. 7. Sycamore Formation exposed at Cypress Creek, Blanco County. mi km 8. Sycamore conglomerate exposed at Hammett's Crossing, Peder­ 0.9144 m nales River, Travis County. CONTINENTAL TRIASSIC ROCKS OF TEXAS 45

9. Section of Sycamore conglomerate exposed at Cox's Crossing, Road 2822 with intersection of State Highway 190, 5 mi Pedernales River, Travis County. northeast of Rochelle, County. 10. Sycamore Formation exposed at Hickory Creek, Burnet County. 26. Rochelle Conglomerate and Paleozoic sandstone exposed at a 11. Travis Peak-Sycamore Formation exposed at Sycamore Creek, roadcut 4 mi east of Rochelle on State Highway 190, mi east of Marble Falls, Burnet County. McCulloch County. 12. Exposure of Paleozoic Walnut Clay, and Comanche Peak 27. Sycamore and Cretaceous outcrop 2 mi east of Rochelle on State Limestone 12.1 mi west of Lampasas on Farm Road 1478, Highway 190, McCulloch County. Lampasas County. 28. Rochelle Conglomerate exposed in a small hill at Soldiers Water- Sycamore conglomerate exposed in roadcut (Bend Locality), 3 mi hole (historical monument) 2 mi east of New Sweden on southwest of Bend on State Highway 501, San Saba unmarked county road, McCulloch County. County. 29. Massive caliche section resting on Paleozoic sands along unmarked 14. Basal Cretaceous and Sycamore contact 0.25 mi south of Nix, on county road 4 mi from intersection with U.S. Highway 283 country road at Lynch Creek, Lampasas County. (approximately 22 mi northwest of Brady), McCulloch 15. Sycamore conglomerate exposed in stream cut at Nix Cemetery, County. just west of Nix, Lampasas County, Texas. 30. Sycamore conglomerate scattered about Paleozoic sandstone along Exposure of Travis Peak Formation 7 mi east of Lampasas at the Farm Road 502, 4 mi west of Mercury. intersection of Farm Road 580 and Lucy Creek, Lampasas Brown County. Series of two road cuts on Farm Road 845 south of County. Brownwood, approximately 2 mi north of Brown-Mills 17. Sycamore conglomerate in contact with Paleozoic rocks 6.7 mi County line. Sycamore Formation overlying Pennsylvanian west of Lometa on Texas Highway 190 (Dobbins, 1974, p. shale. 32. Sycamore Sand exposed southwest of road, 5.3 airline mi north- Massive Sycamore conglomerate outcrop exposed at Creek, northeast of Ebony Cemetery, Mills County. Mills 1000 south 33. Sycamore-Strawn outcrop exposed on hilltop (roadcut) 7.6 mi of old Lometa Highway (Boone, 1968, p. 61). southwest of Mullin on Texas Highway 573, Mills County. 19. Sycamore-Strawn outcrop exposed in roadcut 3 mi west of Elliot 34. Sycamore Conglomerate exposed in roadcut 5.3 mi southwest of Creek locality on unmarked county Mills County Mullin, on State Highway 573, Mills County. (1 mi east of the Colorado River. Base of Sycamore rocks). 35. Brown County. Road cut on county road immediately north of the 20. exposed on a hill 12 mi northeast of San Brown-Mills County line, approximately 6 mi south of Saba on unmarked county road (1.5 mi north of Locality 10) Zephyr. Sycamore Formation overlying Pennsylvanian Mills County. shale. 21. Mills County. In road cut of county road. Section about 1 mi west 36. Paleozoic and Sycamore contact, 0.8 mi from Jones Chapel on of intersection of two county roads. Begins about half way Highway 67-377, Brown County. down hillside and continues west to crest of hill. Contact 37. Cretaceous-Pennsylvanian contact, on county road at intersection elevation ft. with a second county road, 9 airline mi west of Brownwood, 22. Sycamore conglomerate on Farm Road 500, 9.3 mi north of San Brown County. Saba, San Saba County. 38. Paleozoic and Sycamore contact on Farm Road 586, 3 mi south- 23. Sycamore conglomerate found atop ridge on Farm Road 500, west of Bangs at the Ford Holt Brown County. mi north of San Saba, San Saba County. 39. Sycamore Formation exposed on hilltop 5 mi northwest of Bangs, 24. Basal Cretaceous and Sycamore conglomerate of Farm Brown County. Road 2997 about mi on dirt road approximately 1.5 mi 40. Sycamore conglomerate along Park Road mi south of its from Wilbarger Peak, San Saba County. intersection with Highway 2559, north of Lake Brown- 25. Massive outcrop of Rochelle Conglomerate 1 mi north on Farm Brown County.

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(1951) Structural geology of North America: Harper& Panhandle: Texas Bureau of Economic Geology Report of Inves­ Brothers, New York, 624p. tigations 97, 60p. FISHER, WILLIAM L. and RODDA, PETER R. Lower Cretaceous McGowEN, JOSEPH H. and GROAT, C. G. (1971) Van Horn Sandstone, sands of Texas: Stratigraphy and resources, Texas Bureau of west Texas; An alluvial fan model for mineral exploration: Texas Economic Geology, Report of Investigations 59, Bureau of Economic Geology Report of Investigations 72, 57p. FOWLER, P. T. (1964) Basement faults and Smackover structure: Gulf MIKELS, J. Origin and significance of caliche, central Texas: Coast Association of Geological Societies, v. 14, p. Unpublished Student paper no. 995, Baylor University. GETZENDANER, F. M. (1956) Geology of the Trinity division of the PAINE, R. and MEYERHOFF, ARTHUR A. 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(1906) The geology and water resources of the lacustrine rocks in Recognition of Ancient Sedimentary Envi­ eastern portion of the Panhandle of Texas: U.S. Geological Sur­ ronments: Society of Economic Paleontologists and Mineralo­ vey Water-supply Paper 64p. gists Special Publication p. CONTINENTAL TRIASSIC ROCKS OF TEXAS 47

PLUMMER, F. B. Lower Pennsylvanian strata of the Llano region: 7, p. Journal of Paleontology, v, 21, no. 2, p. TAFF, JOSEPH A. Reports on the Cretaceous area north of the RAINWATER, E. H. Geological history and oil and gas potential Colorado River: Texas Geological Survey Annual Report, v. 3, of the central Gulf Coast: Gulf Coast Association of Geological p. 267-379. Societies Transactions, v. 18, p. 124-165. TARLING, D. H. and RUNCORN, S. (eds.) (1973) of (1957) Physiographic map of Texas from Landforms of continental drift to the earth sciences, v. 1: N.A.T.O. Advanced the United States. Study Institute, The University of Newcastle upon Tyne, Aca­ READING, H. G. (1978) Sedimentary environment and facies: Elsevier demic Press, London, New York, 622p. Co., New York, 557p. ToDD, R. G. and MITCHUM, R. M., JR. (1975) Seismic stratigraphic REEVES, C. C. Origin, classification, and geological history of identification of eustatic cycles in Late Triassic, Jurassic, and caliche on the southern High Plains, Texas and eastern New Early Cretaceous Gulf of Mexico and West Africa: Gulf Mexico: Journal of Geology, v. 78, p. 352-362. Coast Association of Geological Societies Transactions, v. 25, REEVES, C. C. classification, morphology, and p. 41-43. uses: Estacado Books, Lubbock, Texas, 167p. ToDD, R. G. and MITCHUM, R. M., JR. (1977) Seismic stratigraphy and REINECK, H. E. and SINGH, L B. (1973) Depositional sedimentary global changes of sea level, part 8: Identification of Upper Triassic, environments: Springer-Verlag, New York, 439p. Jurassic, and Lower Cretaceous seismic sequences in Gulf of C. T. A radiocarbon study of the age and origin of Mexico and offshore West Africa in Seismic caliche deposits: Unpublished M.S. thesis, University of Texas, applications to hydrocarbon exploration (Payton, C. E., ed.): Austin, 67p. American Association of Petroleum Geologists Memoir 26, ROBINSON, P. L. (1973) and continental drift in p. 145-163. Implications of continental drift to the Earth Sciences (Tarling, TWENHOFEL, W. H. (1950) Principles of sedimentation, 2nd. ed.: D. H. and Runcorn, S. K., eds.): N.A.T.O. Advanced Study McGraw-Hill, New York, 673p. Institute, The University of Newcastle upon Tyne, Academic VAIL, P. R. and MITCHUM, R. M., JR. Seismic stratigraphy and Press, London, New York, 622p. global changes in sea level in Stratigraphic interpretation of seis­ ROTH, ROBERT (1932) Evidence indicating the limits of Triassic in mic data: American Association of Petroleum Geologists Memoir Kansas, Oklahoma, and Texas: Journal of Geology, v. 40, no. 8, 26, p. p. 688-725. VAN DER VOO, R.; MAUK, F. J.; and FRENCH, R. B. 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Africa 28. 29 Hazel Fm. 35 Panhandle?, 14,22, 23 Mts. Hempstead Co, Ark. 9 Pecos Co. 8, 22 Uplift 40 Hensell sand 10 Pecos R. 22 Anticline 14 High Plains 12, 22, 26 Pedernales Appalachian Mts. 29 Hill Co. 7 Pedernales Falls 18, 20, Arbuckle Mts. 40 Hopkins Co. 7 Pedernales R. Arizona 26 Howard Co. 7 Pennsylvania 32 Arkansas 5, 7, 9, 12, 15, 27, 29 Hunt Co. 7 Pennsylvanian age 9, 20, 27, 33, 40 Armstrong Co. 7 Hydrocarbons 29 Permian age 8,9, 22, 23, 24, 25. 27, 32, 38,40, Austin. Tex. 7. 14 Igneous activity 29 Petroleum 8, 29 Australia 41 Igneous rocks 14, 29 Plate tectonics 9, 27, 28, 29 Bend Arch 40 Interior Gulf Basin 29 24 Bissett Fm. 5, 6, 22. 27, 34, 37, 42, 43 Intertropical Convergence Zone 43 Pleistocene age 36 Bissett Mt. 27 Iron 23, 24, 25 Pliocene age 23 Ponds 37 Blanco Co. 7. Jurassic age 5, 9, 27, 28, 29 Borden Co. 7, 22 Post, Tex. 24, 25 Kansas 6, Bowie Co. 5, 7 7, 11,23 Kaufman Co. 7 Bravo Dome Precambrian age 27, 28, 35 Kent Co. 7 Brazos R. . Quartermaster Gp. 22, 23, 24, 25 Kimble Co. Co. 8 Rainfall 34, 36 Brisco Co. 8 Lake Eyre Randall Co. 7 Brown Co. 7, 18, 20, Lake Gosiute 40 Red beds 35, 40 Brownwood, Tex. 14, 20, 21 Lake Rudolph 41 Red R. Co. 7 Burnet Co. 7, 9 Lake Uinta 40 Red R. Gp. Lakes 6, 27. 37, 38, 40, 42 Co., Ark. 9 Red R. 14 Lampasas area 21 California 35 Reeves Co. 22 Lampasas Co. 7, 20 Cambrian age 14, 35 Robertson Co. 5, 7 Lampasas Cut Plain 7 Camp Co, 7 Rochelle. Tex. Lampasas, Tex. 18, Canadian R. 22 Rockwall Co. 7 Limestone Co. 7 Capitan Fm. 27 Rocky Mts. Llano area 14, 18. 20. 33, 34, 42 Caprock Escarpment 7, 22, 25, 37 Rotliegende Fm. 32, 35 Co. 7 Caribbean Basin 28 Round Mt. Province Llano Estacado 7, Cass Co. 7 Llano 12, Sabkha 38 Cenozoic age Llanoria 27 Sacramento Uplift 6. 40 Central Basin Platform Louann deposits 27, 29, 32 Sangre de Cristo Mts. Chinle Fm. 26 Louisiana 15, 27, 29 San Saba Co. 7, 36, 37 Climate 34, 35, 36, 38, 43 Lubbock Co. 7 San Saba Province Colorado 6, 40 Lynn Co. 7 "Santa Rosa" sand Colorado R. Scurry Co. 7. 22 Ccmanche Co. 7 Marathon Sea-floor spreading 28 Comanchean age 34 Marble Falls Era. 19, 20, 21 Seas 22. 34 Concho Arch 12, Marble Falls Province Sierra Grande arch 40 Connecticut Valley 32 Marion Co. 29 Sieve deposits 34, 35 Cotton Valley Gp. 15, 27 Marshes 27 Sills 29 Cow Creek beds Martin Co. 7 Slochteren Mbr. 32 Cretaceous age 5,9, 20, 21, 23, 34, 41 Maryland 32 Smackover Fm. 27 Crosby Co. 7 Mason Co. 7 Smithwick Fm. 14. 20, Massachusetts 35 Dallas, Tex. 7 South America 28 Matador Arch 13, 23, 24 Dawson Co. 7 Sterling Co. 7, 22 McCulloch Co. 7, 18, 36, 37 Deaf Smith Co. 7 Strawn ss. 20 McLennan Co. 7 Delta Co. 7 Streams 6, 21. 37, 38,40, 42 Meriden Fm. 32 Deltas 6, 21, 34, 37, 38, 40, 41,42 braided 34, 37, 38,40. 42 Mesozoic age 9, 27, 28, 32. 41 Desert Cretaceous 34 Metamorphic rocks 16, 17, 23 Diablo Platform 6, 40 ephemeral 35. 37 Midland Basin 7, Dickins Co. meandering 6, 21, 37, 38, 40, 42 Co. 32 Dikes 29 mixed-load 5, 21, 34 Mills Co. 7. 37 Dockum Basin 23, 38, 40 Wadi 6, 36 Minerals 16, 23, 24, 25, 40 5,6,7,8, Swisher Co. 7 Mississippi 15 42,43 Sycamore Creek 20. Mississippian age 13, 14 Sycamore deposition 13, 14, 18,33 Eagle Mills, Ark. 9 Mitchell Co. 22, 24, 25 Sycamore Fm.5,6,7,9. 17, 18, Eagle Mills Fm. 5, 7,8, 9, 16, 27, 34, 43 Morehouse Fm. 9 35. 36. 37,41,42. 43 Edwards time 34 Morrison Fm. 23 Synclines 23 Ellenburger Gp. 19, 21 Motley Co. 7 Elliot Creek 18, 20, 35, 36 Mount Toby Fm. 35 Tecovas Creek 23 Ellis Co. 7 Tecovas Fm. 23, 24, 25, 26, 37. 38 Navarro Co. 7 Ethiopia Ten Boer Mbr. 32 Newark Gp. 32 Eureka Valley 35 Titus Co. 15 New Jersey 32 Europe 28, 35 Trade winds 43 New Mexico 6, 13, 21, 22, 26 Travis Co. 18 17, 23, 24, 27, 29, 32, 37, 43 Nile 36 Travis Peak deposition 10, Falls Co. 5, 7, 29 Nix, Tex. 20. 21, 36 Travis Peak Fm. 19, 20 Fans 5, 6, 27, 34, 35, 36, 37, 38, 40, 42 Nolan Co. 7 Triassic age 32 Fault blocks Fm. 15 Triassic Peak 25 Fisher Co. 7 North America 28. 29 Trinity Gp. 9. 18. 34 Florida 12 North Sea 32 Trujillo Creek 25 Floyd Co. 7 Norway 35 Trujillo Fm. 23, 24, 25, 26, 37, 38 Fluvanna, Tex. 24 Nubia 36 Twin Mt. Fm. 19 Fossils 9, II, 23,29, 37,41 Ogallala Gp. 23, 26 Unconformity 22 Franklin Co. 7 Oklahoma 6, II, 40 Uranium 8 Fredericksburg Gp. 18 Oldham Co. 25 Utah 40 Fredericksburg time 34 Delta 41 Freestone Co. 7 Ordovician age 14 Van Horn Fm. 35 Garza Co. 11. 24, 25 Ouachita disturbance 10 Virginia 32 Gillespie Co. Ouachita Foldbelt 13. 14, 33, 41 Waco, Tex. 7 Glass Mts. 6, 8, II, 27 Ouachita-Marathon Foldbelt 6, 42 Wadi Or 36 Glen Rose Fm. 34 Ouachita-Marathon trend 27 Walnut Fm. 18 Gondwanaland 28 Ouachita Mts. 15, 27, 29, 42 Werner Fm. 17, 27, 29 Gonzales Co. 29 Ouachita Tectonic Belt 5, 6, 7, 8, 12, 27, 38 Western Cross Timbers 7 Gorak Shep fan 35 Wichita Mts. 14,40 Grabens 5, 9, 27, 29, 32, 42 Paleoclimates 27 Wichita System 6, 12, Green R. Fm. 40 Paleozoic age 13, 18,20,27,28,29,32,33, Wyoming 40 Gulf Coast Basin 5, 27, 28, 29, 35 Gulf Coast region 8, 9, 27, 32, 42 Palo Duro Basin 7, 24 Zechstein Fm. 32 Gulf of Mexico 9, 28, 38, 40 Palo Duro Canyon 24, 25, 38 Palo Duro Canyon State Park 25 Hays Co. 7, Pangaea.28, 29 BAYLOR GEOLOGICAL

Baylor Geological Studies 26. Davis, Keith W. (1974) Stratigraphy and depositional environ­ Holloway, Harold D. The Lower Cretaceous Trinity aqui­ ments of the Glen Rose Formation, north-central Texas: Baylor fers, McLennan County, Texas: Baylor Geological Studies Bull. Geological Studies Bull. No. 26 (Spring). Out of print. No. 1 (Fall). Out of print. 27. Baldwin, Ellwood E. (1974) Urban geology of the Interstate 2. William A. The Lower Cretaceous Paluxy Sand in Highway 35 growth corridor between Belton and Hillsboro, central Texas: Baylor Geological Studies Bull. No. 2 (Spring). Out Texas: Baylor Geological Studies Bull. No. 27 (Fall). $1.00 per of print. copy. 3. Henningsen, E. Robert (1962) Water diagenesis in Lower Cre­ 28. Allen, Peter M. Urban geology of the Interstate Highway taceous Trinity aquifers of central Texas: Baylor Geological Stud­ 35 growth corridor from Hillsboro to Dallas County, Texas: ies Bull. No. 3 (Fall). Out of print. Baylor Geological Studies Bull. No. 28 (Spring). $ per copy. 4. Silver, Burr A. The Bluebonnet Member, Lake Waco 29. Belcher, Robert C. (1975) The geomorphic evolution of the Rio Formation (Upper Cretaceous), central lagoonal de­ Grande: Baylor Geological Studies Bull. No. 29 (Fall). $1.00 posit: Baylor Geological Studies Bull. No. 4 (Spring). Out of print. per copy. 5. Brown, Johnnie B. (1963) The role of geology in a unified conser­ 30. Carl Dean Origina and significance of the oyster vation Flat Top Ranch, Bosque County, Texas: Baylor banks in the Walnut Clay Formation, central Texas; Baylor Geo­ Geological Studies Bull. No. 5 (Fall). Out of print. logical Studies Bull. No. 30 (Spring). Out of print. 6. Arthur O., Jr. Stratigraphy of the Taylor Formation 31. Dolliver, Paul Noble (1976) The significance of Robert Thomas (Upper Cretaceous), east-central Texas: Baylor Geological Stud­ Hill's contribution to the knowledge of central Texas geology: ies Bull. No. 6 (Spring). Out of print. Baylor Geological Studies Bull. No. (Fall). $1.00 per copy. Spencer, Jean M. (1964) Geologic factors controlling mutation 32. Pool, James Roy (1977) Morphology and recharge potential of and review: Baylor Geological Studies Bull. No. 7 certain lakes of the Edwards Plateau of Texas; Baylor (Fall). Out of print. Geological Studies Bull. No. 32 (Spring). $1.00 per copy. 33. Bishop, Arthur L. Flood potential of the Bosque basin: Urban geology of Greater Waco. A series on urban geology in coopera­ Baylor Geological Studies Bull. No. 33 (Fall). $1.00 per copy. tion with Cooper Foundation of Waco. 34. Hayward, Chris (1978) Structural evolution of the Waco region: Part I: Geology Geology and urban development by Peter Baylor Geological Studies Bull. No. 34 (Spring). $1.00 per copy. T. Geology of Waco by J. M. Burket: Baylor Geological 35. Walker, Jimmy R. (1978) Geomorphic evolution of the Southern Studies Bull. No. 8 (Spring). $1.00 per copy. High Plains: Baylor Geological Studies Bull. No. 35 (Fall). $1.00 Part 11: Soils (1965) Soils and urban development of Waco by W. per copy. R. Elder: Baylor Geological Studies Bull. No. 9 (Fall). $1.00 36. Owen, Mark Thomas (1979) The Paluxy Sand in north-central per copy. Texas: Baylor Geological Studies Bull. No. 36 (Spring). Part Water (1966) Surface waters of Waco by Jean M. per copy. Spencer: Baylor Geological Studies Bull. No. 10 (Spring). $1.00 37. Bammel, Bobby H. (1979) Stratigraphy of the Simsboro Forma­ per copy, tion, east-central Texas: Baylor Geological Studies Bull. No. 37 Part Water (1976) Subsurface waters of Waco by Siegfried (Fall). Out of print. Rupp: Baylor Geological Studies Bull. No. (Fall). $1.00 per 38. Leach, Edward Dale (1980) Probable maximum flood on the copy. Brazos River in the city of Waco: Baylor Geological Studies Bull. 12. Part IV: Engineering (1967) Geologic factors affecting construc­ No. 38 (Spring). $ per copy. tion in Waco by R. G. Font and E. F. Williamson: Baylor Geolog­ 39. Ray, Bradley S. A study of the crinoid genus ical Studies Bull. No. (Spring). Out of print. in the Hunton Group of Pontotoc County, Oklahoma: Bay­ 13. Part V: Socio-Economic Geology Economic geology of lor Geological Studies Bull. No. 39 (Fall). $1.00 per copy. Waco and vicinity by W. T. Huang; Geology and community 40. Corwin, Linda Whigham Stratigraphy of the Fredericks- symposium coordinated by R. L. Bronaugh: burg Group north of the Colorado River, Texas: Baylor Geologi­ Baylor Geological Studies Bull. No. (Fall). Not yet published. cal Studies Bulletin No. 40 (Spring). $1.00 per copy. Part VI: Conclusions Urban geology of greater Waco- 41. Gawloski, Ted Stratigraphy and environmental signifi­ Summary and recommendations by Editorial Staff: Baylor Geo­ cance of the continental Triassic rocks of Texas: Baylor Geologi­ logical Studies Bull. No. 14 (Spring). Not yet published. cal Studies Bulletin No. (Spring). $1.00 per copy. Boone, Peter A. (1968) Stratigraphy of the basal Trinity (Lower Cretaceous) sands, central Texas; Baylor Geological Studies Bull. Baylor Geological Society No. (Fall). $1.00 per copy. 138. Out of print. For titles see earlier Baylor Geological 16. Proctor, V. (1969) The North Bosque Inventory Studies Bulletins. of a drainage basin: Baylor Geological Studies Bull. No. 134. The Black and Grand Prairies. A physiographic study of two (Spring). Out of print. central Texas Prairies, 1974. Out of print. 17. LeWand, Raymond L., Jr. (1969) The geomorphic evolution of 137. Structural geology of central Texas. A professional level guide­ the Leon River system: Baylor Geological Studies Bull. No. book. Out of print. (Fall). Out of print. Urban development along the White Rock Escarpment, Dallas Moore, Thomas H. Water geochemistry. Hog Creek basin, Texas (1978). $1.50 per copy. central Texas: Baylor Geological Studies Bull. No. 18 (Spring). 140. Paluxy Watershed. Geology of a river basin in north central Texas Out of print. (1979). $1.50 per copy. Mosteller, Moice A. (1970) Subsurface stratigraphy of the Geomorphic evolution of the Grand Prairie, central Texas (1979). Comanchean Series in east central Texas: Baylor Geological Stud­ per copy. ies Bull. No. 19 (Fall). Out of print. 142. The nature of the Cretaceous-precretaceous Central 20. Byrd, Clifford Leon (1971) Origin and history of the Uvalde Texas (1979). $4.00, a professional level guidebook. Gravel of central Texas: Baylor Geological Studies Bull. No. 20 The Geology of Urban Growth $1.50 per copy. (Spring). Out of print. 144. A day in the Cretaceous (1980). $1.50 per copy. 21. Brown, Thomas E. Stratigraphy of the Washita Group in 145. Landscape and Landuse (1980). per copy. central Texas: Baylor Geological Studies Bull. No. 21 (Fall). Out of print. 22. Thomas, Ronny G. The geomorphic evolution of the Pecos River system: Baylor Geological Studies Bull. No. 22. (Spring). Out of print. 23. Roberson, Dana Shumard (1972) The paleoecology, distribution and significance of circular bioherms in the Edwards Limestone of central Texas: Baylor Geological Studies Bull. No. 23 (Fall). Out of print. 24. Epps, Lawrence Ward The geologic history of the Brazos River: Baylor Geological Studies Bull. No. 24 (Spring). Out of print. 25. Bain, James S. (1973) The nature of the Cretaceous-pre- Publications available from Baylor Geological Studies or Baylor Geo­ Cretaceous contact in north-central Texas: Baylor Geological logical Society, Baylor University, Waco, Texas 76798. Studies Bull. No. 25 (Fall). Out of print. Texas residents add five cents per dollar for state tax.