REINTERPRETATION OF THE IGNACIO AND ELBERT FORMATIONS AS AN INCISED VALLEY FILL USING FACIES ANALYSIS AND SEQUENCE STRATIGRAPHY; SAN JUAN BASIN, SOUTHWEST COLORADO
Joshua T. Maurer
A Thesis
Submitted to the Graduate College of Bowling Green
State University in partial fulfillment of
the requirements for the degree of
MASTER OF SCIENCE
August 2012
Committee:
James E. Evans, Advisor Margaret Yacobucci Charles Onasch ii
ABSTRACT
James E. Evans, Advisor
The Ignacio Formation and the Devonian Elbert Formation of southwest Colorado
represent the lowest succession Paleozoic strata in the San Juan Basin of Colorado and New
Mexico. This study reinterprets the Ignacio Formation-Ouray Limestone as Devonian
(Famennian) in age and finds that the succession represented a laterally continuous depositional system of fluvial and estuarine environments (Ignacio Formation), prograding shoreline environments (McCracken Sandstone Member, Elbert Formation), tidal flats and shallow marine shales (Upper Member, Elbert Formation), and marine limestone and shales (Ouray Limestone).
The present study is based on lithofacies analysis, 137-m of measured stratigraphic sections at 11 outcrops, provenance analysis from 24 representative thin sections, 72 paleocurrent measurements, and photomosaics. Lithofacies analysis identified 14 lithofacies that are organized into fluvial channel, tidal channel, tempestite, and tidal flat sequences.
This study suggests that the Ignacio and Elbert formations may be reinterpreted an incised valley sequence. Evidence for this conclusion include variations in thickness, facies distribution, the conformable contact between the Ignacio Formation and the McCracken
Sandstone Member, lithofacies associations and ichnology, which confirm the shift from fluvial to estuarine to marine environments, and the onlap of these Paleozoic sedimentary rocks onto
Precambrian basement paleohighs. It is suggested that initial accommodation space was provided by paleotopography on the Precambrian basement surface. Later accommodation space was provided by relative sea-level rise, and the paleovalleys were backfilled by fluvial – estuarine sediments and later overtopped by marine sediment. iii
Ignacio Formation type section at Electra Lake iv
This thesis is dedicated to Jerry Calendine.
v
ACKNOWLEDGMENTS
The ideas presented here have resulted from the research of Joshua T. Maurer and Dr.
James E. Evans in ancient incised-valley and estuarine systems in Southwest Colorado. This
work was supported by the Colorado Scientific Society and the Ogden Tweto Memorial Fund in
conjunction with funding from Bowling Green State University as well as the Geological Society
of America for travel grants to present our research at the Northeastern region GSA meeting.
I, Joshua T. Maurer, thank the valuable input and support from the following people:
Colleagues within Bowling Green State University (Richard R. Hoare Research Scholarship) as well as several friends and family. Thanks to Dr. James E. Evans who served as my advisor and constant source of guidance and knowledge. Without Jim, this research would not have been possible. Thank you to my committee members Charles Onasch and Peg Yacobucci for being open to my numerous questions and providing amazing input and assistance. Specifically, thank you Charlie for your help in sample preparation and thin section making, and Peg for your help with the paleontological work and identification. Thanks to my field assistant Bharat Banjade for continual motivation in the field and a positive attitude when conditions looked bleak while scurrying around the San Juan National Forest. Thank you to my longtime friend Robert Tolley for logistical aid including the procuring our rental car. Thanks to Benjamin Nelson for your constant support and motivating me to accomplishing my goals throughout our time at BGSU.
Thank you Udita Datta for being a continuous source of inspiration and competition while completing this project. Finally, thank you to my entire family for the support.
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TABLE OF CONTENTS
Page
INTRODUCTION ………………………………………………………………………… 1
Sequence Stratigraphy ……………………………………………………….…… 1
Incised Valley Sequences ………………………………………………...………. 6
Facies Analysis ……………….………………………………………………….. . 16
Purpose and Objective ……………………………………………………………. 18
BACKGROUND……………………………………………………….………………… . 20
Regional Tectonic Setting……………………………………………………….... . 20
Precambrian Basement …………………………………………………………… . 20
Hiatus and Early Paleozoic Sedimentation.……………………………………… .. 21
Ancestral Rocky Mountain Uplift and Paradox Basin …………………………..... 22
Sevier and Laramide Orogenies …………………………………………………... 23
Regional Stratigraphy…………………………………………………………….... 26
Precambrian Basement ……………………………………………………………. 26
Ignacio Formation …………………………………………………………………. 28
Elbert Formation ………………………………………………………………… ... 33 vii
Ouray Limestone ………………………………………………………………… ... 36
Stratigraphy and Age ……………………………………………………………… 36
METHODS……………………………………………………………………………… ... 38
Stratigraphic Sections ……………………………………………………………... 38
Paleocurrent Analysis ………………………………………………………… ...... 41
Petrographic Analysis ……………………………………………………………. . 43
Contact Relationships ……………………………………………………………. . 46
RESULTS ………………………………………………………………………………… 47
Lithology…… ...... 47
Lithofacies Analysis ...... 58
Ichnofacies Analysis ……………...... 91
Paleocurrent Analysis ...... 99
Provenance Analysis……………………………………………………………… .. 100
Contact Relationships ………………………………..………………………….. ... 101
DISCUSSION …………………………………………………………………………….. . 114
Valley Incision …………………………………………………………………...... 114
Valley Fill …………………………………………………………………………… 123 viii
Depositional Model ………………………………………………………………… 130
SUMMARY AND CONCLUSIONS……...... 130
REFERENCES……………………………………………………………………………… 133
APPENDIX A. PALEOCURRENT DATA…………………………………………………. 148
APPENDIX B. COLUMNAR SECTIONS ...... 153
APPENDIX C. POINT COUNT DATA...……………………………………...………….. 168 ix
LIST OF FIGURES
Figure Page
1 Depositional Sequence Model………………………………………………….…… 5
2 Type-1 Incised Valley Sequence Model……………………………………….…… 10
3 Type-2 Incised Valley Sequence Model…………………………………………… 12
4 Incised Valley Sequence Longitudinal Facies Distribution…….…………………. 15
5 Shoreline Facies Model……………………………………………………………. 17
6 Ancestral Rocky Mountain Paleogeography………………………………………. 24
7 Coal Bank Pass and Snowdon Fault Block Cross-Section………………………… 25
8 Regional Stratigraphy……………………………………………………………… 27
9 Study Area Map……………………………………………………………………. 40
10 Paleocurrent Tilt Correction……………………………………………………….. 42
11 Dickinson Petrofacies Ternary Diagrams……………………………………...... 45
12 Ignacio Formation Outcrop Photographs………………………………………… ... 48
13 Ignacio Formation Photomicrographs……………………………………………… 49
14 McCracken Sandstone Member Outcrop Photographs…………………………….. 52
15 McCracken Sandstone Member Photomicrographs………………………………… 56 x
16 Lithofacies Sm …………………………………………………………………….. 62
17 Lithofacies Se …………………………………………………………………….... 64
18 Lithofacies Sp …………………………………………………………………….. . 66
19 Lithofacies St …………………………………………………………………….. .. 68
20 Lithofacies Sx …………………………………………………………………….. . 70
21 Lithofacies Sr …………………………………………………………………….. .. 71
22 Lithofacies Sh …………………………………………………………………….. . 73
23 Lithofacies Sl …………………………………………………………………….. .. 75
24 Lithofacies Sw …………………………………………………………………….. 77
25 Lithofacies Gm & Gt …………………………………………………………...... 80
26 Lithofacies Gl …………………………………………………………………….... 83
27 Lithofacies SSm ………………………………………………………………….. .. 86
28 Lithofacies SSl …………………………………………………………………….. 87
29 Lithofacies MSf ………………………………………………………………...... 89
30 Lithofacies MSb ……………….…………………………………………………... 90
31 Sultan Creek Ichnofacies ………………………………………………………….. 96
32 Mile Marker 54 of US 550 Ichnofacies ………………………………………….. .. 97 xi
33 Bioturbation Photomicrograph …………………………………………………….. 98
34 Regional Paleocurrent Data …………………………………………………...... 102
35 Petrofacies Data …………………………………………………………...... 103
36 Ignacio Fm. – Unnamed Conglomerate Contact …………………………………... 105
37 McCracken SS Mbr. – Baker’s Bridge Granite Contact ………………………...... 106
38 McCracken SS Mbr. – Unnamed Conglomerate Contact ……………………...... 107
39 McCracken SS Mbr. – Ignacio Fm. Outcrop Contact ……………………...... 109
40 McCracken SS Mbr. – Ignacio Fm. Contact Detail ……………...... 110
41 Longitudinal Hanging Stratigraphic Sections ……………………...... 119
42 Cross-Sectional Hanging Stratigraphic Sections ………………………………… .. 121
43 IVS Depositional Model …………………………………...... 128
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LIST OF TABLES
Table Page
1 Outcrop Geographic Coordinates …………………………………………….…… 39
2 Summary of Point Count Data ……………………………………………….…… 53
3 Fluvial Lithofacies ………………………………..………………………….…… 59
4 Marine and Estuarine Lithofacies …….…………………………………………… 61
5 Summary of Ichnofacies ………………………………………………………….. 93
6 Summary of Lithostratigraphic Contacts ………………………………………….. 113
1
INTRODUCTION
Sequence Stratigraphy
The method of sequence stratigraphy was developed and refined by Exxon in the 1970’s and continues to be an important tool for oil and gas exploration. Sequence stratigraphy was first introduced in the AAPG Memoir 26, Seismic Stratigraphy and Global Changes in Sea Level
(Vail et al., 1977), which discussed the role of global sea level changes as a major control on the stratigraphic record, along with basin tectonics and sediment supply. One of the most significant findings was that seismic reflectors represent chronostratigraphic surfaces. The field of seismic stratigraphy allowed for the prediction of sediment distribution based on seismic lines, and sedimentologists and stratigraphers began contributing to the field of sequence stratigraphy
(Jervey, 1988).
Sequence stratigraphy is the study of genetically related facies within a framework of chronostratigraphically significant surfaces (Van Waggoner et al., 1990). In the last fifty years the definition and principal methods have been evolving. Embry (2002) discussed a new updated version of the definition, that “Sequence stratigraphy is the recognition and correlation of stratigraphic surfaces that represent changes in depositional trends in sedimentary rocks. Such changes were generated by the interplay of sedimentation, erosion, and changing accommodation space and are now determined by sedimentological analysis and geometric relationships.” Three types of depositional systems are typically discussed in terms of sequence stratigraphic techniques; nonmarine, coastal, and marine (Catuneanu, 2006). Classically, sequence stratigraphy has been applied to coastal systems but it is also a technique that is wide reaching, for example, sequence stratigraphy can also be used to interpret lake-level fluctuations. 2
The most rudimentary units involved in sequence stratigraphy are depositional
sequences. These are defined as a relatively conformable successions of genetically related strata
bounded at the base and top by unconformities and/or their correlative conformities (Abreu et al.,
2010). Each depositional sequence represents one cycle of sea level change, or a transgression- regression cycle. All depositional sequences are contained between sequence boundaries (SB).
Each depositional sequence can be internally subdivided into systems tracts, including lowstand systems tract (LST), shelf margin systems tract (SMST), transgressive systems tract (TST),
highstand systems tract (HST), and falling stage systems tract (FSST). Important surfaces
include transgressive surfaces of erosion (TSE), maximum flooding surfaces (MFS), condensed
sections (CS), and nonmarine regression surfaces (NMRS) (Catuneanu, 2006) (fig. 1). Each systems tract can be composed of one or more parasequences. A parasequence is a conformable succession of genetically related beds or bedsets bounded by maximum-flooding surfaces (MSF)
and their correlative surfaces or deposits, such as condensed sections (CS). Stacking patterns
(aggradational, progradational, or retrogradational) of parasequence sets are used in conjunction
with bounding surfaces and their position within a sequence to define systems tracts (Van
Waggoner et al., 1988).
Sequence boundaries (SB) are important features to identify when using sequence
stratigraphy, yet they are not always easy to identify in the field. A sequence boundary can be an
unconformity that has experienced some subaerial or marine erosion, or their correlative
conformities. Sequence boundaries are often marked by an abrupt basinward shift in facies or
downlap surfaces suggesting a forced regression due to the falling base level irrespective of the
sediment supply (Boggs, 2001). 3
Two distinct types of sequence boundaries can be defined based on the conditions which
facilitate erosion of the surface. The first, type-1 sequence boundaries, are produced when the
rate of eustatic fall exceeds the rate of basin subsidence, resulting in relative sea level fall. This
relative sea level may lead to subsequent subaerial exposure and promote erosion. Generally, a
type-1 sequence boundary can be identified by fluvial incision and a basinward shift in facies.
Alternatively, type-2 sequence boundaries are produced when the rate of eustatic fall is less than the rate of basin subsidence, and there is no net change in relative sea level. Because there is not significant change in relative sea level, no subaerial exposure would exist and therefore no fluvial erosion would not take place. Also, type-2 sequence boundaries may not display the pronounced basinward shift in facies that type-1 sequence boundaries display (Dalrymple, et al.,
1994a).
A lowstand systems tract (LST) is the portion of the depositional cycle that is active
during relatively low sea level. The LST overlies a type-1 sequence boundary and is capped by a
transgressive surface of erosion (TSE). During lowstand, sediment accommodation is reduced
and sediment supply is high, resulting in stacking aggradational parasequences of a lowstand
wedge or a lowstand basin fan. There are several types of features in LST, including lowstand
deltas, lowstand wedge, basin-floor fan, and incised valley sequence (IVS) (fig. 1).
A shelf-margin systems tract (SMST) occurs as the lowermost systems tract associated
with eustatic sea level fall and overlies a Type-2 sequence boundary (Fig. 2). The SMST forms
above a sequence boundary and is capped by the TSE. In general, SMST are rare and particularly
difficult to identify in the field or in seismic data (Boggs, 2001).
A transgressive surface of erosion (TSE) is the first major flooding surface across the
shelf in a depositional sequence. The TSE defines the top of the lowstand systems tract, and 4
separates progradationally to aggradationally stacked parasequences below from retrogradationally stacked parasequences above. As part of the TSE, multiple ravinement surfaces may incise into the underlying LST, each representing different energy storm deposits
(Abreu, et al., 2010).
The transgressive systems tract (TST) consists of retrogradational parasequences which demonstrate an overall deepening-upward succession and represent coastal migration landward.
The TST is bound by the TSE and MFS. The maximum flooding surface overlies the TST and marks the transition from retrogradational parasequence stacking in the TST to aggradational stacking in the early highstand systems tract (HST). Condensed sections (CS) occur both within the LST and also in the distal parts of overlying highstand systems tracts. Condensed sections are one or several facies which are characterized by thin beds of clastic, chemical, or biological sediments deposited at very slow rates. Condensed sections mostly form during the time of regional transgression of the shoreline because clastic sediments are trapped near shore (Van
Waggoner et al., 1988).
The highstand systems tract (HST) consists of an aggradational to progradational parasequences that overlies the MFS and may be overlain either by the next sequence boundary or by a nonmarine surface of erosion (NMSF). Sediments typically show overall shallowing, and deltaic and wave-dominated coastline deposits are most prevalent. If the HST is bounded at the top by a sequence boundary, it onlaps the upper parasequence in a landward direction and downlaps in a basinward direction. Within the HST, progradational parasequences are bound by
MFS (Catuneanu, 2006, Posamentier and Allen, 2003). Basinward, MFS may be overlain by condensed sections, recognized by distinctive features such as the accumulation of glauconite, or unique forms of bioturbation, such as firmgrounds or hardgrounds. 5
Figure 1. Simplified longitudinal cross section of a depositional sequence. Black lines represent sequence boundaries separating highstand systems tract (HST) from lowstand systems tract (LST). The dashed blue line represents the transgressive surface of erosion, and the solid blue line represents the maximum flooding surface (MFS). The transgressive systems tract (TST) is located between the TSE and the MFS. Notice the absence in this example of the shelf-margin systems tract (SMST), the absence of the falling stage systems tract (FSST), and only one highstand systems tract (HST) parasequence emphasizing the variability of depositional sequences and the inherent omission of systems tracts (from Van Waggoner et al., 1990).
6
The falling-stage systems tract (FSST) consists of sediments deposited during a regression after relative sea-level fall. It is typically bounded by NMRS and a SB. The resulting deposits are progradational parasequences as in a forced regression with an erosional surface at the landward edge (Van Waggoner et. al., 1988; Posamentier and Allen, 2003).
Within each systems tract, several sub-environments can be classified. Nonmarine environments may include colluvial, fluvial, glacial, lacustrine, and eolian environments. Coastal depositional environments may include deltaic, beach/barrier island, tidal flat, and lagoon/estuary environments. Marine environments may include neritic (shallow) and pelagic
(deep) marine environments. Evaluation and designation of sequence stratigraphic features including depositional parasequences, sequence boundaries, transgressive surfaces of erosion, flooding surfaces, lowstand systems tracts, transgressive systems tracts and highstand systems tracts, can only be conducted after facies and depositional environments have been identified.
Incised Valley Sequences
Incised valley sequences (IVS) are a common occurrence on continental margins
(Talling, 1998), and their formation is frequently linked to both a fall in base level, creating a valley, and subsequent rise in base level causing infilling of that paleovalley (Allen and
Posamentier, 1994). Incised valley-fills typically are comprised of fluvial, coastal, and shallow- marine sediments emplaced during the latter part of base level fall or during the subsequent rise of base level (Allen and Posamentier, 1994). Many studies have been conducted on both late
Cenozoic (Chaumillon and Weber, 2006; Corner, 2006; Li et al., 2002; Payenberg et al., 2006) and ancient incised valley systems (Plint & Wadsworth, 2006; Garrison & Van Den Bergh, 2006;
Wroblewski, 2006). Incised valley sequences have been an important topic in recent literature and have sparked a series of special papers such as the 1994 book Incised-Valley Systems: Origin 7
and Sedimentary Sequences (SEPM Special Paper 51), and the 2006 book Incised Valleys in
Time and Space (SEPM Special Paper 85).
Incised valley sequences are important because they record information about external controls responsible for incision and infilling, because the infilling depositional environments provide significant paleogeographic and paleoclimate data, and because of their implications as petroleum reservoirs. It is estimated that nearly 25% of all off-structure clastic reservoirs containing conventional hydrocarbons formed as lowstand to early transgressive, incised valley deposits (Brown, 1993). Independent of their economic importance, IVS are also important because the regionally mappable unconformity at their base has strong application to sequence- stratigraphy. These surfaces are important for developing a chronostratigraphic framework to explain the distribution of petroleum reservoirs in shallow marine and non-marine depositional environments (Vail et al., 1977). Global rise of sea level due to global warming will flood low- lying and heavily populated areas, therefore understanding of the evolutionary changes that occur within an IVS may allow for the prediction of the environmental effects allowing for a better response to this imminent sea level change (Zaitlin et al., 1994a).
According to Zaitlin et al., (1994b) an IVS is a fluvially-eroded, elongate topographic low that is typically larger than a single channel form and is characterized by an abrupt seaward shift of depositional facies across a sequence boundary at its base, which is in turn mappable regionally. The fill typically begins to accumulate during the next base-level rise, and may contain deposits of the following highstand and subsequent sea-level cycles. In order to recognize an IVS four separate features must be identified. First, a paleo-valley must be present.
A paleo-valley is a negative paleotopographic feature where the surfaces that defines an incised valley truncates the underlying strata (Zaitlin et al., 1994a). Second, the erosion surface at the 8
base of the incised valley must be correlatable to an erosional surface of regional extent, such as
a sequence boundary (Dalrymple et al., 1994b). Thirdly, the fill of the valley must display a shift
in basinward facies, and not show a conformable depositional sequence with the sediments
directly below it (Van Wagoner et al., 1990). Finally, depositional marker beds within the paleo-
valley will onlap the valley walls (Zaitlin et al., 1994a).
Bates and Jackson (1984), define an incised valley as a hydromorphological feature in
which a plateau is incised by a river. Thus, incised valley sequences represent a period of
subaerial erosion followed by a vertical facies progression from fluvial to estuarine to marine
environments (Thomas, et al., 1987). A minor technical consideration is that not all valleys are
incised valleys. An incised valley requires downcutting by a river and therefore evidence of
erosional surfaces must be observed. In contrast, valleys are topographic features that can be
produced by different processes including faults or karst dissolution of carbonates or evaporites
(Dalrymple, et al., 2006a). This may not be determinable, however, because erosional surfaces
might be obscure, because not all rivers incise, and because changes in sea level such as a drop in
sea level or substantial changes in sediment supply may promote or discourage incision. In general, incision by rivers is promoted by increasing the slope of the river either due to base level fall, differential uplift, or due to climatically induced changes in of sediment supply (Dalrymple
et al., 2006b).
Incision of the Paleovalley
Facies relationships in an IVS are complex due to several factors including causes and
timing of incision, geographical location within the valley, valley shape and morphology, and the
valley-fill architecture (Dalrymple et al. 2006a). Incised valley sequences initiate with erosional
incision creating a paleovalley. Most frequently, incision is localized to the soft sedimentary 9
rocks of the continental shelf region it but can also take place in areas of crystalline rocks which
have noticeable paleo-relief, possibly due to tectonic uplift or fault block movement (Dalrymple
et al., 2006b). The paleovalley will then be subsequently backfilled with fluvial, estuarine, and marine sediment during relative sea level rise.
The sequence stratigraphic approach to incised valley sequence is as follows. The IVS begins during falling stage of a lowstand systems tract (LST). During this time incision and downcutting by rivers is at its peak as stream gradient is at its highest resulting in a basal sequence boundary. Also the fluvial sequence and associated sediments are deposited during the
LST. This is then followed by a succession of estuarine and marine sediments deposited during a
TST as well as fluvial deposits related to the rise in baselevel. The fluvial deposits of the LST and the estuarine deposits of the TST may be deposited at very different times and may represent a set of adjacent parasequences (Tandon et al., 2006).
Two types of incised valleys are noted in the literature. Type-1 IVS are those valleys which are eroded in response to a fall of relative sea level, either by a eustatic sea-level fall or by tectonic uplift. Type-2 IVS are those valleys that did not form due to sea-level change, but other factors such as erosion due to tectonic uplift of an inland area, or to an increase in fluvial discharge caused by climatic change (Dalrymple et al., 1994b). Type-1 incised valleys (fig. 2) are associated with sequence-bounding unconformities and are influenced by marine processes along some of their length, and therefore have sequence stratigraphic significance. The majority of IVS discussed in the literature are type-1 systems and are interpreted as erosional valleys formed by river action during a relative sea-level fall, with valley fill beginning to accumulate near the end of the lowstand, but typically containing sediments deposited during the succeeding base-level rise (Zaitlin et al., 1994b). Type-2 incised valleys (fig. 3) are harder to identify in the 10
Figure 2. Type-1 IVS characterized by a basal sequence boundary (SB) overlain by lowstand sequence tract (LST) fluvial deposits, transgressive systems tract (TST) fluvial and estuary deposits, and highstand systems tract (HST) marine deposits. The LST is capped by the transgressive surface of erosions (TSE) marked in blue. The maximum flooding surface (MFS) marked with the green dashed line separates TST and HST deposits and is estimated in this particular example. Modified from Zaitlin et al., (1994b).
11
literature because they occur in fluvial successions and are filled by terrestrial deposits, which
makes them difficult to recognize. Also type-2 incised valleys did not create significant sequence
boundaries, although some changes in fluvial style may occur during filling due to cyclic
changes in accommodation space caused by tectonics or climate (Dalrymple et al., 1994a).
The location of a paleovalley may be controlled by several factors including pre-existing topography which may channelize drainage, the effect of erosion or nondeposition generating compound valleys, the presence of depositional topography such as delta lobes, or the effects of tectonics. Valley shape may be controlled by erodability and topographic relief where the rivers and tidal currents are focused to the path of least resistance. This generates localized scour at valley confluences and bends. The intersection of two fault zones also may localize scour, or active fault movement may cause offsets along the valley length (Blum, 1994). The shape of the
valley can also be attributed to the length of time the stream occupied the valley. Valleys with complex cross sections may have had a river present through falling stage, whereas simple cross sections are formed by rivers which occupied a valley for a short time.
Incised Valley Deposits
The valley-fill architecture and facies distribution of an IVS are controlled by several factors. First, the position of the study area relative to the trajectory of the shoreline during the filling of the valley must be considered, because deposits near the lowstand mouth and deposits near the transgressive limit may be very different. Second, the ratio of sediment supply to accommodation space is important because this difference reflects external variables such as rate of subsidence or uplift, rate of climate change, or the rate of eustatic sea level change. Third, the 12
Figure 3. Cross section of a type-2 incised valley fill characterized by fluvial and estuarine deposits above a sequence boundary (SB). Between the basal sequence boundary and the transgressive surface of erosion (TSE), lowstand sequence tract (LST) fluvial channels are common. Alternative to type-1 IVS, type-2 are mostly filled with terrestrial sediments such as lacustrine, or palustrine deposits. (Modified from Zaitlin et al., 1994a)
13
slope of an incised valley can greatly impact the fill architecture. An area with a flat slope will
allow for greater transgression distance than a steeper slope, which affects the in-fill volume.
IVS are composite features that can span both the LST and TST. The LST portion can include preserved fluvial deposits from the initial stage of valley incision, including lag deposits and channel deposits, such as planar-tabular cross-bedded sandstone and conglomerate, and thin heterolithic beds. Characteristic fluvial deposits can include bedload gravels and cross-bedded gravel bars.
The TST portion is characterized by sediments fining-upward from fluvial deposits to estuarine deposits, such as interbedded sandstone and mudrock deposited in the tidal-fluvial transition zone of the inner estuary, and eventually to marine deposits. The upper part of the sequence may show an overall coarsening upward, with fine-grained, parallel laminated sand flat sediments capped by cross-bedded, medium-grained sandstone. Notable discontinuities include erosional surfaces formed by tidal channel erosion that may bound deposits associated with sand bar migration and tidal flat progradation. Estuarine lithofacies can include flaser bedded sandstone and mudstone, wave and current rippled sandstone, and hummocky stratified sandstone. Sedimentary structures and lithofacies of nearshore environments include fining- upward sequences with cross-bedded sandstone overlain by parallel-laminated sandstone (sand flat), and by flaser and lenticular bedded sandstone and mudstone with bioturbation (mixed flat), and by tidal rhythmites and bioturbated mud (mud flat). Wide variations in the sediments in an
IVS are common. Type-1 IVS, for example, can lack fluvial sediment at their base resulting in the fill to be completely comprised of estuarine and marine sediments. In addition, incised valley fills can be simple fills (containing one parasequence and represent a single depositional sequence), or they can be complex (containing several parasequences and represent many 14
depositional sequences). These differences may depend on the rate of sedimentation, the overall
geometry of the valley, or the ratio between rate of baselevel rise and rate of fluvial
sedimentation (Dalrymple et al., 1994b). Type-2 incised valleys may not be completely
developed as regional erosion is less common. The fill may be dominantly terrestrial and the
omission of estuarine or marine sediments is possible. A type-2 IVS may not display a fully fill
succession, yet may contain evidence for a cyclic climate (Pienkowski, 1991). Figure 4 shows a longitudinal cross-section of an idealized Type-1 IVS (Li, et al., 2006). Figure 4 A displays the proximal terrestrial portion of the IVS. Notice the presence of several stacked channels in this complex fill. Figure 4 B is an example of the distal fluvial portion of the IVS, in which stacked channels are interbedded with fluvially-dominated fill in which gravel lag deposits are
interbedded with floodplain sequences. Figure 4 C shows the transition from tidally-influenced
fluvial to estuarine facies. Figure 4 D shows the transition from estuarine to fully marine
sequences. The presence of specific lithofacies associated with these three depositional
environments coupled with identification of flooding surfaces and bounding unconformities are
the basis for the sequence stratigraphic analysis being conducted in the study area. 15
Figure 4. Idealized longitudinal distribution of facies successions along an incised valley. The complexity of IVS-fill is due to the interplay of two distinct sedimentary environments (fluvial and marine). This interplay allows for the intermixing of sediments and lithofacies as described in the text. (From Li et al., 2006) 16
Facies Analysis
One of the most important concepts used throughout this paper is Walther’s Law of
Facies Succession. The law states that, in the absence of an unconformity, the material stacked successively on top of one another at one location formed in environments that were laterally adjacent to one another at the same time in geologic history. Facies models are constructed from the complex relationships between the modern and the ancient record. From the modern record, the lateral relationships of environments and deposits can be observed. From the ancient record, vertical stacking of environments and deposits can be observed. As an example, Figure 5
(Weller, 1960; Shaw, 1964) shows a coast line succession is an example of Walther’s Law of
Facies Succession in which the lateral migration of facies results in one being deposited on top of another. Over time, the sedimentary environments at any one location shifted from marine, to mud flat, to coal swamp, to water logged mudflat, to terrestrial environments. The vertical stacking of facies reflects the shifts of environments as the coastline migrated offshore.
Both the ancient record and modern record contribute a wealth of information. First, the ancient record allows viewing change at a particular geographic location through time. For example, the succession of rocks may show evidence for climate change, such as evidence for
Milankovich cycles. On the other hand, the ancient record filters the geologic record emphasizing large-scale events such as storm or tsunami deposits, allowing for great insight into the importance of low-frequency, high-magnitude events in the evolution of the Earth. In contrast, the modern record shows spatial relations over large geographic regions at one particular time. In addition, the modern record shows the effects of high-frequency (e.g., daily) but low-magnitude events with very high stratigraphic resolution. 17
Figure 5. Example of Walther’s Law of Facies Succession for shoreline environments. The diagram displays the relationship between adjacent sedimentary environments and the inherent stacking pattern of sediments found in the sedimentary record. The adjacent sedimentary environments form superimposed sedimentary rocks, as environments move through time
(modified from Weller, 1960; Shaw, 1964).
18
Purpose and Objective
Although other studies have reported examples of Paleozoic IVS (Aitken and Flint 1994,
Archer et al., 1994, Kvale and Barnhill, 1994), there has been little discussion about the role of paleotopography on crystalline basement rocks providing the accommodation space for incised valley fills. This reinterpretation of the Ignacio Formation and Elbert Formation as a IVS significantly alters the previous ideas about Paleozoic history and paleogeography of the area, as well as provides a new model for IVS above a major bedrock unconformity.
This study evaluates the depositional environments and internal structures of the Ignacio
Formation and Elbert Formation of southwest Colorado by examining outcrops and conducting lithofacies analysis. The age of the Ignacio Formation is suspect, ranging from Late Cambrian -
Early Ordovician (Eastman, 1904; Cross et al., 1905a&b) based on scant fossil evidence, lithologic correlation, and the presence of an unconformity, to Devonian (Barnes, 1954) based on gradational contact relationships and continuous successive sedimentary deposits. Also, there is a wide variation in composition and thickness for both units, including notable local omission of the Ignacio Formation and McCracken Sandstone Member (Elbert Formation), in specific areas across the study area. Several attempts to explain this (Baars, 1966; Rhodes and Fisher, 1957;
Thomas, 2007) have resulted in complex interpretations based on recurring fault block movement with different sense of offset, in some cases.
Evans (personal communication, 2010) suggested that the Ignacio Formation-Elbert
Formation contact does not display evidence for an unconformity, but is gradational in places across the study area. Evans also described evidence for fluvial and estuarine deposits in the
Ignacio Formation. This interpretation is consistent with the findings of Baars (1966) who 19
interpreted the Ignacio Formation as terrigenous clastic littoral and upper-neritic in origin.
Finally, Evans observed possible firmgrounds in the McCracken Sandstone Member.
This study reinterpreted the depositional environment of the Ignacio Formation and
Elbert Formation. Because understanding the depositional environment allows for the correct placement of the formations within the regional stratigraphic framework, the goal was to restate the history of these units in a sequence stratigraphic framework. Analysis of the biostratigraphic data within the Ignacio Formation allows for the determination of the formations age in the context of the surrounding lithologic units. The geographic distribution of the Ignacio Formation and Elbert Formation combined with the evidence for a sequence boundary at its base and a conformable contact with the McCracken Sandstone Member and evidence for changes in relative sea level associated with the deposition of the formations suggests the two units should be understood as part of a Devonian incised valley sequence.
20
BACKGROUND
Regional Tectonic Setting
The study area has experienced a long multiphase tectonic history including five orogenies; the Boulder Creek orogeny, Silver Plume orogeny, Colorado orogeny (Ancestral
Rocky Mountain Uplift), Sevier orogeny, and the Laramide orogeny. Collectively these major tectonic events have shaped much of the geology and topography of Colorado. Between the Late
Precambrian to Early Mesozoic, North America was part of Laurentia (Karim, 2008). Parts of this continent provided the clastic sediments that were shed westward into the proto-Pacific ocean. A portion of this former passive margin is preserved in the Colorado Plateau, which is located in the Four Corners region of Colorado, Utah, New Mexico, and Arizona. This passive margin basin changed dynamically throughout its evolution, and experienced numerous transgressive/regressive cycles (Tweto, 1980a).
Precambrian Basement
Boulder Creek Orogeny – 1.7-1.6 Ga
The Boulder Creek orogeny occurred between 1.7 and 1.6 Ga and was characterized by amalgamation of multiple juvenile arc terrains culminating in a regional deformation and metamorphism (Gonzales and Van Schmus, 2007). A result of the deformation and metamorphism was the intrusion of synorogenic plutonic rocks including the Baker’s Bridge
Granite into the protolith of the Twilight Gneiss (andesite and volcaniclastics) (Gonzales and
Van Schmus, 2007). Metamorphism of the andesite and volcaniclastics to pressure and temperature conditions indicative of greenschist and amphibolite facies allowed for the formation of gneiss and schist now termed the Twilight Gneiss (Tweto and Sims, 1963). After 21
the Boulder Creek orogeny, the crust underwent uplift and subsequent erosion with pervasive
siliciclastic deposition including the deposition of the Uncompahgre Formation (coastal deposit).
Silver Plume Orogeny – 1.4 Ga
During the Silver Plume orogeny of ~1.4 Ga felsic magmas intruded into the crust
resulting in local melting and thermal metamorphism throughout the study area (Gonzales and
Van Schmus, 2006). During this period, intracratonic deformation of the continental shelf aided
in folding twice and metamorphosing the Uncompahgre Formation (sandstone and shale) to
quartzite and slate. Later, between 1.48-1.35 Ga, the Twilight Gneiss was thrust faulted over the
Uncompahgre Formation. This was followed by the intrusion of the Eolus Granite at 1.44 Ga and a period of erosion for about 1 billion years (Gonzales and Schmus, 2006).
Hiatus and Early Paleozoic Sedimentation
Following the Silver Plume orogeny, a major depositional hiatus occurred. During this
time, basement rocks of the Precambrian were exposed in the form of a rocky shoreline
(Campbell and Gonzales, 1996). Much of the continent was composed of Precambrian plutonic
and volcanic rocks emplaced during the Boulder Creek and Silver Plume orogenies. Sometime
during this hiatus an unnamed conglomerate formed on the margin of a rocky shoreline.
Campbell and Gonzales (1996) identified this unit as the Weasel Skin Conglomerate but
considered it part of the Cambrian (?) Ignacio Formation. Based on compositional and textural
difference in the matrix between the unnamed conglomerate and the sandstones of the Ignacio
Formation, the conglomeratic unit has been reinterpreted to be a Precambrian high-energy
shoreline deposit (Evans, 2007).
There is no other record of sedimentation or magmatism until the Early Paleozoic (Tweto
1980a) when passive margin tectonics allowed the deposition of shore face and terrestrial 22
sandstones and shales, specifically, the basal most Paleozoic units, the Ignacio Formation
(various clastic deposits), Elbert Formation (sandstone and gray shale), Ouray Formation
(limestone), and Leadville Formation (limestone) (Baars and See, 1968). This succession of
sedimentary rocks serves as the basis of this study and each formation will be discussed in detail later.
Ancestral Rocky Mountain Uplift and Paradox Basin
During the Pennsylvanian, the Colorado Orogeny or the Ancestral Rocky Mountain
(ARM) uplift occurred. During this period (320-290 Ma), two island ranges (Frontrangia and the
Uncompahgria) were uplifted in response to the collision of Africa with North America (De
Voto, 1980). The structures of the ARM coincide with Precambrian basement faults, and display a history of reactivation on both Precambrian faults as well as Paleozoic faults (Baars and See,
1968). This faulting resulted in the uplift of the Grenadier fault block and the Sneffels fault block
(Fig. 6) located between the Uncompahgre uplift and the Paradox Basin (Thomas, 2007). The
Sneffels fault block is bounded by the Coal Bank Pass fault and the Snowden fault (Fig.7).
According to Baars and See (1968), the faulting deformed the pre-Pennsylvanian sedimentary rocks into flat bottom synclines and north-east trending asymmetric anticlines and exposed
Precambrian basement rock. The Baars and See (1968) interpretation in Figure 7 shows the erosional removal of the Ignacio Formation and McCracken Sandstone Member north of the
Coal Bank Pass fault. The Sneffels fault block uplifted the Uncompahgre Group juxtaposing it against older Precambrian rocks. Sedimentation during the ARM included deposition of thick successions of carbonates (Hermosa Group) along the continental margin during the
Pennsylvanian (Baars and See, 1968). The ARM experienced heavy erosion at the end of the
Paleozoic, and tectonic quiescence into the early Mesozoic (Thomas, 2007). 23
The Paradox Basin is a structural trough that was formed as part of the crustal deformation during the Ancestral Rocky Mountain Uplift. It has been interpreted as a pull-apart basin with right-lateral extension toward the northwest and southeast along a series of master faults (Stevenson and Baars, 1986).
Sevier and Laramide Orogenies
The Sevier orogeny occurred between 140 and 50 Ma in the western margin of Colorado and Utah as the Farallon plate subducted under the North American plate (Armstrong, 1968).
The collision resulted in an east-trending thrust belt which stacked up thrust sheets. Rocks of
Colorado do not show evidence of Sevier deformation, instead the orogeny is best displayed in the sedimentary record (Berman et al., 1980). The convergence of these two plates resulted in the opening of the Western Interior Seaway, a shallow sea extending from the Arctic Ocean to the Gulf of Mexico (Kaufman, 1977). The Western Interior Seaway was fed by at least seven phases of sedimentation from both the east and the west resulting in thick successions of marine and marginal marine sedimentary rocks in western Colorado and Utah (Berman et al., 1980).
The Laramide orogeny occurred in the Late Cretaceous to Tertiary (72-40 Ma) and was characterized by block faulting (Sims et al., 2001). Some faults active during the Laramide orogeny may have been re-activated the ARM or even Precambrian structures (Tweto, 1980c).
For example, the Uncompahgre uplift (part of the ARM) was mostly reactivated during the
Laramide orogeny to form the San Juan Dome. Many large structural basins were formed due to the Laramide uplift including the San Juan Basin, which records coarse clastic sedimentation and volcanism (Tweto, 1980c).
24
Figure 6. Map of Colorado and adjacent region at the time of the Ancestral Rocky Mountain
Uplift, showing the location of the Front Range Uplift, Central Colorado Trough, Uncompahgre
Uplift, the Paradox Basin, and two Pennsylvanian fault blocks, the Sneffels Block and the
Grenadier Block. These major tectonic features provided a sediment source and structural basins for Paleozoic sedimentation (from Thomas, 2007). Shaded region is San Juan and La Plata
County Colorado.
25
Figure 7. Geologic cross section across the Coal Bank Pass fault and Snowdon fault along the
Sneffels fault block (Fig. 6). The Uncompahgre Group and Paleozoic sedimentary rocks are juxtaposed against older Precambrian rocks. According to the interpretation in the Figure, the
Ignacio Formation tends to thin as it approaches the fault boundary. The McCracken Sandstone
Member has nearly equivalent thickness across the area but thins near at the Coal Bank Pass fault. The Upper Member is continuous across the Coal Bank Pass fault but is slightly offset. Red lines represent faults. Section is approximately 400 m long (from Thomas, 2007).
This interpretation calls for erosional removal of the Ignacio Formation and McCracken
Sandstone Member north of the Coal Bank fault prior to the deposition of the Upper Member.
The implication would result in an intraformational unconformity in the Elbert Formation which is not observed in the field, and significant Devonian tectonic uplift which is poorly constrained.
Alternatively, the Ignacio Formation and the McCracken Sandstone Member were never deposited in the region north of the Coal Bank Pass fault as it was outside the incised valley.
26
Regional Stratigraphy
The stratigraphy of the area shows the thin succession of Paleozoic sedimentary rocks on
lapping Precambrian igneous and metamorphic rocks of the Twilight Gneiss, Baker’s Bridge
Granite, and Uncompahgre Formation (Evans, 2007). This study focuses on the oldest Paleozoic
units, including the Cambrian(?)–Devonian Ignacio Formation, Devonian Elbert Formation, and
the Devonian Ouray Limestone (Fig.8). These three formations display a typical deepening
upward shift in facies, from terrestrial sediments through marine limestones and shales. They are
unconformably overlain by thick carbonates of the Mississippian Leadville Formation.
Precambrian Basement
The protolith of the Twilight Gneiss was an intermediate composition volcanic rock
(andesite) that formed by partial melting of a basaltic source derived from a depleted-mantle
system (Barker, 1969). Subsequently, the andesite was subjected to greenschist and amphibolites
facies dynamothermal metamorphism at ~1.7 Ga. The unit consists of medium grained, largely
homogenous, pink- gray, or red hornblende-plagioclase gneiss with foliated quartz monzonite veins and local bodies of amphibolite (Barker, 1969). The Baker’s Bridge Granite and Eolus
Granite both intrude the Twilight Gneiss throughout the study area. The 1.7 Ga Baker’s Bridge
Granite is a homogenous, coarse-grained, reddish granite of the Routte Plutonic Suite (Tweto,
1987). It cross-cuts structural fabric and layering in the Twilight Gneiss (Gonzales and Von
Schmus, 2007).
The Uncompahgre Formation is a metasedimentary unit consisting of up to 2500 m of
quartzite, slate, and phyllite which was metamorphosed at about ~1.4 Ga (Tweto, 1980b). The
1.4 Ga Eolus Granite is a medium- to coarse-grained, equigranular to porphyritic, granite to 27
Figure 8. Simplified regional stratigraphy of the study area in southwest Colorado. Precambrian
basement includes the Twilight Gneiss, Baker’s Bridge Granite, and the Uncompahgre
Formation. A major unconformity (~1 b.y.) separates the Precambrian basement and the base of the Ignacio Formation. Paleotopography is indicated by areas where the Ignacio Formation is
absent and the Elbert Formation overlies the Precambrian Basement. Where present, the Ignacio
Formation is conformably overlain by the McCracken Sandstone Member or the Upper Member
of the Elbert Formation.
28
monzonite with minor granodiorite and diorite (Gonzales and Von Schmus, 2007). The Eolus
Granite cross cuts both the Twilight Gneiss and the Uncompahgre Formation in the study area.
Capping the Uncompahgre Formation is an unnamed stratified, clast-supported, pebble to boulder conglomerate believed to be Precambrian in age (Evans, 2007). This lithostratigraphic unit is pervasive throughout the study area and has been interpreted to be part of the Ignacio
Formation (Weasel Skin Conglomerate Member of Campbell and Gonzales, 1996) although there is no evidence the units are related. During the Paleozoic, erosion resulted in a paleotopographic surface on the basement rocks that created up to 65-m of relief (Evans, 2007).
This paleotopography served as the accommodation space for the later deposition of the Ignacio
Formation. Because the unnamed conglomerate was not stratigraphically related to the Ignacio
Formation or the McCracken Sandstone Member, no lithofacies analysis was conducted on the unit. The Precambrian conglomerates are ~95% quartzite clasts derived from the Uncompahgre
Quartzite with minor vein quartz, schist, greenstone, and banded iron formation. These deposits are organized into clast supported boulder-cobble conglomerates usually several meters thick, with clasts up to 1.4 m in diameter. There are relatively few interbedded sandstone units. The sandstones show line grain contacts and embayed grain contacts (Evans, 2007).
Ignacio Formation
The Ignacio Formation is a siliciclastic unit with a weathered color of reddish brown- grayish brown, and an unweathered color of light brown, white, or light pink. The unit is mostly friable, dolomitic, or ferruginous quartz arenite, with sandy shale partings and rare conglomerate lenses (Gonzales et al., 2003a). Wiggin (1987), examined the conglomerates of the Ignacio
Formation and incorrectly identified the Precambrian conglomerate as part of the Ignacio
Formation and concluded it was Cambrian in age. In places, the Ignacio Formation contains 29
interbedded poorly sorted pebble conglomerate and laminated, red arenaceous shale. The formation is largely unfossiliferous, with only minor beds of lingulid brachiopods at one outcrop and Devonian fish fossils found at the top of the unit (Eastman, 1904). Thickness is variable, with a maximum of 60-m in the Animas Valley of southwest Colorado (Rhodes and Fisher,
1957).
When present, the Ignacio Formation overlies Precambrian basement and underlies the
Elbert Formation; typically, the McCracken Sandstone Member. However, variations in this trend can be found throughout the study area. At Vallecito Reservoir the Ignacio Formation is composed of 12 fluvial lithofacies directly oain by the Upper Member (this study). Outcrops of the Ignacio Formation at its type section along the shoreline of Electra Lake do not expose the basal contact. The rocks exposed at Electra Lake mostly resemble the McCracken Sandstone
Member, however it is possible that the contact has been flooded with the damming of the previously named Ignacio Reservoir to create Electra Lake. At this location the McCracken
Sandstone Member was overlain by the Upper Member. Rhodes and Fisher (1957) identified the basal Paleozoic sandstone at Baker’s Bridge as the “Ignacio Quartzite”. Petrographic analysis shows that the sandstone composition at Baker’s Bridge is much more similar to the McCracken
Sandstone Member across the study area than the composition of the Ignacio Formation (this study).
Much controversy is present in the literature about the age of the Ignacio Formation
(Cross et al., 1905a and b; Barnes, 1954, Baars and Knight, 1957; Rhodes and Fisher, 1957;
Baars, 1966; Baars and See, 1968). The first major attempt to date the Ignacio Formation was based on the presence of the lingulid brachiopod Obolus sp.. Brachiopods found by Cross et al.,
(1905a) at what now is the U.S. 550 mile marker 54 outcrop, are preserved in highly friable 30
sandstone, and identification to the species level was not possible. These brachiopods may have
been misidentified (Eastman, 1904; Rhodes and Fisher, 1957). Subsequently, the age range of
this genus has been reinterpreted as Cambrian to Devonian (Barnes, 1954). Trace fossils in the
Ignacio Formation include representatives of the Glossifungites ichnofacies found at the same
Coal Bank Pass outcrop as well as at the outcrop south of Molas Lake (this study). Barnes (1954)
noted that the Ignacio Formation grades upward into the Elbert Formation and noted the presence
of Devonian fish remains belonging to genus Bothriolepis, including B. coloradensis, B.
canadensis, and B. major, of Late Devonian in age in the top portion of the Ignacio Formation at
Coal Bank Pass (Eastman, 1904). It is widely known that both Bothriolepis and Holoptychius are
found in Late Devonian freshwater, estuarine, and coastal deposits. Benton (1997), suggests that
Bothriolepis was Middle to Late Devonian and had a worldwide distribution and was able to live
in several types of environments because it may have breathed air. This study was unable to find
examples of these fish remains.
In summary, the Ignacio Formation is controversial for two reasons. First, the age is
poorly constrained because of the general absence of fossils. Second, there is a disagreement
about the contact relationships between the Ignacio Formation and underlying or overlying units.
Three major studies (Cross et al., 1905a &b; Barnes, 1954; and Rhodes and Fisher, 1957)
produced three different interpretations of the depositional environment, stratigraphy, and age of
the Ignacio Formation. These will be discussed next.
Cross et al. (1905a &b) believed that the Ignacio Formation always overlies older
metamorphic and igneous rocks in the area and that an erosional unconformity separates the
Ignacio Formation from the underlying rocks. They believed that the erosion on the Precambrian
surface during Precambrian-Late Cambrian was followed by sedimentation of the Ignacio 31
Formation. After the deposition of the Ignacio Formation, there was a lengthy hiatus from the
Ordovician to Middle Devonian. According to them, neither the Ignacio Formation nor the
Devonian aged Elbert Formation can be found continuously throughout the study area, specifically the Ignacio Formation is absent from certain localities of the Silverton quadrangle, and the Elbert Formation is absent from the Uncompahgre Valley near Ouray. They therefore concluded that a significant disconformity exists between the Ignacio and the Elbert Formations.
Based on the only identifiable fossil in the Ignacio Formation (Obolus, an inarticulate brachiopod), they concluded that the Ignacio Formation was Cambrian in age. The fossil was collected from Overlook Point at Mountain View Crest in the Needle Mountains Quadrangle.
The inarticulate brachiopod was identified by Charles Walcott (Eastman, 1904) who based his classification of the Obolus? sp. sample on related forms which occur in Middle or Upper
Cambrian rocks at various locations in the western United States.
Barnes (1954) believed the Ignacio Formation, Elbert Formation, and Ouray Limestone are Devonian units that could be interpreted as related depositional environments. He interpreted the Ignacio Formation as littoral zone deposits, including coarse-grained siliciclastics, while the
Ouray Limestone was interpreted as a combination of fine-grained clastics and carbonates deposited in quiet water offshore, and the Elbert Formation was interpreted as a lithological transition between the two. This conclusion was based in part on a reinterpretation of the
Obolus? sp. fossils in the Ignacio Formation as of Cambrian to Devonian age range, and on the presence of Devonian placoderm fish fossils B. coloradensis, and B. major, found in the uppermost part of the Ignacio Formation.
Barnes (1954) determined that examples of oboloid brachiopods in the field are scanty at best and that with only few samples, accurate identification is difficult. He concluded that the 32
fossil evidence for a Late Cambrian age of the Ignacio Formation was inconclusive, and focused on the better described Devonian fish remains as indicative that the Ignacio Formation is
Devonian in age. He claimed that the sandstone beds containing the placoderm remains were part of the upper portion of the Ignacio Formation and not the basal portion of the Elbert Formation.
Upon examination of contacts, Barnes (1954) found neither an eroded surface nor a distinct lithologic change between the Ignacio Formation and the Elbert Formation. Also, he noted that a similar regional variation in thickness, lithofacies, bedding parallelism, and geographic distribution of the Ignacio and Elbert formations exists. For example, the Ignacio
Formation and Elbert Formation are both present at Molas Lake, Coal Bank Pass, and near
Rockwood, and that both thicken to the west into the Paradox Basin. Based on this evidence he concluded that the Ignacio Formation, Elbert Formation, and Ouray Limestone represent a single depositional sequence.
Rhodes and Fisher (1957) examined the Precambrian contact with the Ignacio Formation at several of the outcrops examined in this study, including Baker’s Bridge and Coal Bank Pass.
They found up to eight meters of paleorelief on the Precambrian surface, and that the Ignacio
Formation was deposited unconformably above the surface. They also examined the contact between the Ignacio Formation and the Elbert Formation and saw no clear evidence for a transition in lithology between the two units. Accordingly they interpreted a major unconformity between the Ignacio Formation and the Elbert Formation. They did not offer an explanation for parallelism of bedding of the Ignacio Formation and the Elbert Formation or for the lithologic transition between the formations.
Rhodes and Fisher (1957) re-evaluated the fossil evidence for the Lower Cambrian age of the Ignacio Formation. After extensive field investigations, no fossils were found at the localities 33
examined by Cross et al. (1905a and b), or Barnes (1954). They did find beds within the Ignacio
Formation which did contain Obolus ? brachiopod fossils just south of Coal Bank Pass. The fossiliferous bed is located 10-m from the base of the formation. Approximately 200 samples were collected in varying states of preservation. This particular bed has been re-examined in this study as well and it has been found that because of the friability of the sandstone, identification of fossils to the species level is not possible. However, Rhodes and Fisher (1957) had their specimens examined by G. Arthur Cooper who verified the brachiopods as the genus Obolus but admittedly described his comparisons to specimens of Cambrian and Ordovician obolids as
“equivocal”. Cooper went on to note similarities between Rhodes and Fisher’s specimens and O. ion and O. anceps of Lower Ordovician and Cambrian age. This equivocal evidence, was enough for Rhodes and Fisher (1957) to reaffirm the Ignacio Formation as “probably Late Cambrian to
Early Ordovician” (Eastman, 1904).
Elbert Formation
The Elbert Formation (Upper Devonian) is subdivided into the McCracken Sandstone
Member and the Upper Member (Gonzales, et al., 2003b). Collectively the Elbert Formation consists of interbedded red to maroon shale, siltstone, fine-to medium-grained quartz arenite, and sandy dolomitic limestone that either overlies on the Ignacio Formation or is directly above
Precambrian basement. This study found that at Sultan Creek and Coal Bank Pass, the
McCracken Sandstone Member overlies the Ignacio Formation and is overlain by the Upper
Member. At Molas Lake, the McCracken Sandstone thins and pinches out, so that the Upper
Member lies directly above the Ignacio Formation. At Baker’s Bridge and along SR. 250 near
Rockwood, the Ignacio Formation is absent and the McCracken Sandstone and Upper Member sit directly above the igneous and metamorphic Precambrian surface. 34
McCracken Sandstone Member
The McCracken Sandstone Member is a white to tan dolomitic quartz arenite with minor
pebbly conglomerate (Gonzales, et al., 2003b). Dolomite cements vary locally with the study
area. Previous workers found that dolomite cement varied between 0% and 61% (Gonzales, et
al., 2003a) while Cole and Moore (1996) note up to 90% locally. Common sedimentary
structures include planar-tabular and festoon cross-stratification, horizontal lamination, glauconite horizons and peloids, as well as Cruziana and Glossifungites ichnofacies (Cole and
Moore, 1996). Cole and Moore (1996) have interpreted the depositional environment of the
McCracken Sandstone Member as an upper intertidal setting with periodic supratidal conditions prevailing.
Upper Member
The Upper Member is a gray shale interbedded with thin sandy dolomitic limestone beds.
The unit is moderately fossiliferous, containing horizons rich in marine fossils. In most areas it is non-fossiliferous, with variable thickness from nonexistent to 30 m (Barnes, 1954). Notable features in the Upper Member include iron staining, hopper crystals, trace fossils, stromatolites, and desiccation cracks (Barnes, 1954; Gonzales et al., 2003b).
Biostratigraphy of the Elbert Formation
The Elbert Formation has been discussed extensively in regards to its biostratigraphy
(Eastman, 1904; Cross et al., 1905a and b, Barnes, 1954; Baars, 1966; Rhodes and Fisher, 1966).
The first attempt to date the unit based on biostratigraphy was conducted by Cross et al. (1905a).
They collected many specimens of fish remains in the Durango Quadrangle, and Needle
Mountains Quadrangle. The remains were sent to the Harvard University Library and were examined by Charles Eastman (1904). 35
Specimens from the Durango Quadrangle were collected from the Elbert Formation near
Rockwood although it is not clear of which specific outcrop the sample was taken. This
uncertainty stems from the fact that the sample was collected from a piece of sandstone float
determined to be from a bed 30-m above the basal conglomerate, which rests on granite
(Eastman, 1904). Also, Cross et al. (1905a) state that a very careful search did not suffice to
detect the layer from which the slab in question came from, but that it was their opinion that it
was from Silurian beds intermediate between the Cambrian and the Devonian (Eastman, 1904).
In this case it seems that the Ignacio Formation is referred to as the Cambrian rocks, while the
Devonian rocks are likely to be the Elbert Formation, more specifically the McCracken
Sandstone Member. Silurian rocks are not found in the study area (Tweto, 1980a), so it is
questionable what Cross et al. (1905a) meant. Eastman (1904) identified three specimens in the
piece of float collected by Cross et al. (1905a) as the genus Bothriolepis, but the species could
not be determined as the specimens were larger than B. canadensis and smaller than B. major.
Eastman also noted a different style of superficial ornamentation, general proportions of the body
and appendages, and differences in the outline and structure of the ventral plates. He then went
on to name the specimen as a new species to be known as B. coloradensis.
Specimens from the Needle Mountains Quadrangle were also collected near Endlich
Mesa by Cross et al. (1905a). These particular outcrops of the Elbert Formation were not visited
during this study, but they are important due to their biostratigraphic significance. At this locality
Cross et al. (1905b) found fish remains distributed throughout beds at the base of the section as
well as near the top of the section. Eastman (1904) noted that although many remains are present,
very few species are represented including Bothriolepis coloradensis, B. leidyi, Holoptychius
giganteu, and H. tuberculatus. These species are known to exist in the Upper Devonian, and 36
interestingly B. leidyi is known to exist in a portion of the Catskill Formation of Pennsylvania,
which has now been reinterpreted as an IVS (Cotter and Driese, 1998). Thomson and Thomas
(2001), reviewed the taxonomic status of various Bothriolepis species, and concluded that the
species Bothriolepis minor, B. virginiensis, B. coloradensis, B. leidyi, and B. darbiensis cannot
reliably be distinguished from Bothriolepis nitida, which is known from the Late Frasnian and
Famennian age.
Ouray Limestone
The Ouray Limestone (Upper Devonian) is a brown to light gray dolomitic limestone,
micritic limestone, or sandy dolostone with a relatively consistent thickness in the study area of
45 m (Gonzales et al., 2003b). The oölitic dolomite layers contain minor thin, irregular partings of green shale locally (Knight and Baars, 1957). The unit is tabular, and is interbedded with calcareous shale and minor calcareous quartz arenite. The Ouray Limestone contains some fossiliferous beds (Barnes, 1954). Topographically, the Ouray Limestone forms the lower part of limestone cliffs above the “Elbert bench” of the San Juan Mountains (Baars and Knight, 1957).
In this study, the Elbert Formation to Ouray Limestone contact is used as a stratigraphic marker horizon to assess the thickness and contact relationships of the underlying Ignacio and Elbert
Formations. The Ouray Limestone is considered to be Late Devonian, specifically mid-
Famennian in age based on brachiopod fauna (Armstrong and Mamet, 1976).
Stratigraphy and Age
The examination of drill cores across the region, as compiled by (Baars, 1966), interpreted the Ignacio Formation to be the lateral equivalent to the Tapeats Sandstone-Bright
Angel Shale sequence of the Grand Canyon and the Tintic Quartzite-Ophir Shale sequence of central Utah. This conclusion was based on the hypothesis that the Ignacio Formation was 37
Cambrian in age. Operating on these age assumptions, Rhodes and Fisher (1957) believe they observed an unconformity between the Ignacio Formation and the Elbert Formation. However, it was noted that the interpreted sedimentary environments of the Ignacio Formation and the Elbert
Formation would be feasible as a depositional sequence (Cole and Moore, 1996). Evidence for gradational contact will be presented later in this paper.
It is believed that the Elbert formation is Devonian in age based on the fish fossils mentioned above. Concordantly, the Ouray Limestone is considered to be Famennian in age based on brachiopod fauna. Fish fossils of Holoptychius are found in the Elbert Formation and perhaps from the top of the Ignacio Formation. The only known occurrence of Holoptychius outside of the Late Devonian are found at the Devonian-Carboniferous boundary (Blom et al.,
2007). If any of these rhipidistian type fish are indeed in the Ignacio Formation, they must have been from sometime during the Devonian, probably Frasnian or Famennian.
38
METHODS
Stratigraphic Sections
The study was outcrop based, and involved measured stratigraphic sections at locations in the Electra Lake, Hermosa, Engineer Mountain, Snowdon Peak, and Vallecito Reservoir 7 ½ minute quadrangles of southwest Colorado. A total of 11 outcrops were examined across the study area during July of 2010 (Fig. 9). The area is primarily in the San Juan Mountains of southwest Colorado, with the majority of the outcrops of the Ignacio Formation and the Elbert
Formation found along U.S. Highway 550 between Durango, Colorado and Silverton, Colorado.
Large exposures of the Elbert Formation are also found along County Road 250 near Rockwood,
Co. An additional outcrop of the Ignacio Formation was examined north of Vallecito Reservoir.
Several locations display the continuous Ignacio Formation – Ouray Limestone section, such as at Coal Bank Pass, Molas Lake, and Sultan Creek. At the southern outcrop at Sultan
Creek, the Ignacio Formation contacts the Upper Member directly and the McCracken Sandstone
Member is absent.
At all 11 locations the coordinates were recorded using a handheld GPS (Table 1), and stratigraphic sections were measured from the top of the Precambrian basement through the
Elbert-Ouray Formation contact. Other information that was collected includes paleocurrent data
(Appendix A), measured stratigraphic sections (Appendix B), hand specimens collected for petrographic analysis (Appendix C), and outcrop photographs.
Measured stratigraphic sections were measured using standard sequence stratigraphic techniques as described by Catuneanu (2006). Stratigraphic sections were recorded in the field by measuring thicknesses perpendicular to bedding and noting changes in lithofacies, as well as the presence of fossils, and secondary sedimentary features (Appendix B). 39
Table 1: Outcrop locations
Outcrop Zone Easting Northing
Baker's Bridge 13S 252340 4149250 Coal Bank Pass 13S 255067 4175391 Electra Lake 13S 251352 4161429 Mile Marker 54 US 550 13S 250272 473836 Molas Lake 13S 263972 4180538 Rail Road at Shalona Lake 13S 251661 4152476 Rockwood 13S 252161 4150244 Sultan Creek 13S 264402 4182387 Vallecito Reservoir 13S 274168 4153701
40
Figure 9. Location map of 11 outcrops measured of the Ignacio Formation and the McCracken
Sandstone Member along U.S. 550 and La Plata County Road 250. The Ignacio Formation and the McCracken Sandstone Member are mapped as DeCi. For other geologic unit descriptions see
Gonzales et al., (2003 a&b). 41
Facies analysis including facies associations were used to discuss the depositional history
of the region. Lithofacies were identified based on grain-size and sedimentary structures present.
Distinctions between fluvial, estuarine, and marine depositional environments based on
lithofacies and lithofacies assemblages will be discussed in greater detail later in this paper.
The contact between the Ignacio Formation and Elbert Formation was examined in detail
with special interest in the geometry of the contact and the correlative/noncorrelative nature of
the contact surfaces.
Paleocurrent Analysis
Paleocurrent data were collected from cross-bedding, current and wave ripples and clast imbrications, using a Brunton Compass (Appendix A). Paleocurrent analysis is conducted in order to describe the direction of flow or orientation of flow during a specific time in the past
(Boggs, 2001). Directional structures measured are those primary sedimentary structures formed during deposition. The structures represent the orientation of currents along the bottom or a stream or the seafloor such as those formed by traction deposition of sand or by the erosion of cohesive muddy bottoms (Potter et al., 1979). Features may vary in sizes with scales ranging from less than one centimeter to over a meter in size (Ali Shah et al., 2009).
Paleocurrent data were collected with a Brunton compass by measuring the downstream orientation of a structure. Each paleocurrent measurement is a bearing. Ideally several measurements were taken from each outcrop. A combined total of 72 paleocurrent measurements were taken from the 11 outcrops measured in the study. The location of each measurement was recorded in UTM, as well as the attitude of bedding. After the data were collected, they were
corrected for tectonic tilt (Fig. 10) as outlined in Ali Shah et al. (2009). Although it is common 42
A) B)
C) D)
Figure 10 . Steps for adjusting paleocurrent data for tectonic tilt. The blue circle represents the pole of the measured bedding and the red x represents the cross-bedding measurement. The dip of the bedding is adjusted out by rotating the stereogram until the pole to bedding lies on the XY axis, and then the pole is moved to the origin of the stereogram. The paleocurrent measurement also moves the same number of degrees along the nearest small circle. Finally the overlay is rotated to the original position and the new paleocurrent is the azimuth located 90° from the final position of the pole to cross-bedding (from Ali Shah et al., 2009). 43
practice to only adjust the paleocurrent for tectonic tilt if the structural dip is greater than >15°,
all of the paleocurrent data present in Appendix A have been adjusted. This correction was done
by rotating the pole of each measurement about the horizontal axis using a stereographic
projection.
Petrographic Analysis
A total of 36 oriented hand specimens were collected from the outcrops within the study
area. These hand specimens were chosen to be representative of lithofacies, or specific
sedimentary features, or fossils. Of the 36 hand specimens collected, 24 thin sections were made
for provenance analysis. Thin sections were prepared using standard techniques in the Bowling
Green State University thin section laboratory. Billets were treated with blue epoxy to highlight porosity.
The goal of provenance analysis is to deduce the characteristics of a source area based on the compositional and textural properties of sediments. Provenance analysis involves examination of all the factors related to the production of sediment, including the composition of the parent rock, the physiography and climate of the source area, to reconstruct the history of sediment from initial erosion to final burial (Weltje and Eynatten, 2004).
Point counting was conducted on the 24 thin sections representing 12 samples of the
Ignacio Formation and 12 samples representing the McCracken Sandstone Member. At least 300 grains per slide were identified using the Gazzi-Dickinson method (Ingersoll et al., 1984) and given in Appendix C. Grains were classified as polycrystalline, monocrystalline, or cryptocrystalline quartz, plagioclase or potassium feldspar, sedimentary, metamorphic, or igneous rock fragments, accessory minerals, unknown/ unidentifiable minerals, cement, matrix, 44
or porosity. Count statistics involved recalculating percentages to plot data on four petrofacies diagrams (Fig. 11).
Dickinson (1970) established a classification system of nine petrofacies to link sediment source areas to tectonic settings. For example, sandstone from continental provenances have abundant mature quartz, and few feldspars reflecting weathering of Precambrian shield complexes and platform sediments. In contrast, sandstones from basement uplifts are relatively rich in feldspar due to tectonic uplift of Precambrian basement providing sources of feldspar
(Miall, 1984). In contrast, sandstone derived from magmatic arcs (including dissected, transitional, or undissected arcs) are typically rich in volcanic lithic fragments and are typically immature in composition (Dickinson, 1970). Undissected arc sediments include abundant volcanic fragments and plagioclase feldspar. Dissected arcs reflect a more plutonic composition and mirror continental basement with higher feldspar content (Miall, 1984). Finally, sandstones derived from recycled orogens (including subduction complexes, collisional orogens, or foreland uplifts) typically contain significant amounts of polycrystalline quartz, sedimentary and metasedimentary lithic fragments (Dickinson, 1970). Alternatively, subduction complex sediments can consist of sediment derived from ophiolites (volcanic and metamorphic lithics) mixed with relatively high amounts of sedimentary lithics (chert, argillite, and lithic arenite).
Finally, sediments derived from foreland uplifts may include a mixture of sediment derived from ophiolites and continental margin sediments, such as a significant amount of monocrystalline quartz or a high amount of chert, (Miall, 1984). The ternary diagrams were used to classify petrofacies are given in Figure 11. The desired components were selected based on the
Dickinson (1970) classification system. 45
Figure 11. Four petrofacies diagrams from Dickinson (1970). Q = total quartz; Qm = monocrystalline quartz; Qp = polycrystalline quartz; F = total feldspars; P = plagioclase feldspar; K = potassium feldspar; L = total lithics; Lt = L + Qp; Vrf = volcanic rock fragments;
Srf = sedimentary rock fragments.
46
Contact Relationships
Finally, examination of the Precambrian contact and overlying sandstone geometry was mapped to identify areas of paleotopography. Photographs and photomosaics were used to display examples of paleotopography, particularly in areas in which the transition between the
Ignacio Formation or McCracken Sandstone Member over the Precambrian units was visible within the outcrop scale. Careful examination of contacts, particularly between the Ignacio
Formation and the McCracken Sandstone Member were noted with heavy emphasis on gradational relationships and sharp boundaries between the units. Although Barnes (1954) considered the succession of the Ignacio Formation through the Ouray Limestone as representing a single depositional sequence, Rhodes and Fisher (1957) did not see a clear transition between the Ignacio Formation and the Elbert Formation. Examination of thin sections across the Ignacio
Formation and McCracken Sandstone Member was conducted to alleviate any such controversy.
47
RESULTS
Lithology
Ignacio Formation
The Ignacio Formation is mostly a fine-to medium-grained, moderately sorted, quartz arenite with minor shale and conglomerate. Common sedimentary structures include planar- tabular cross-bedding, flaser bedding, wavy bedding and bioturbation. At Vallecito Reservoir, the Ignacio Formation also consists of a planar-tabular cross-bedded, poorly sorted, well rounded, oligomict paraconglomerate (Fig 12 b). Pebbly coarse-grained sandstones are common, mostly in the basal portion of the Ignacio Formation. Mudstones are also rare in the Ignacio
Formation. Thicker section of shales are limited to Vallecito Reservoir and to near mile marker
54 of U.S. 550 (Fig. 12c) south of Silverton. The mudstones are red in color and contain ferruginous cement surrounding fine-grained quartz sand and clay minerals. The mudstones are very friable and are finely laminated.
Petrographically, the unit contains abundant monocrystalline quartz (Fig. 13a,b,c) with lesser amounts of polycrystalline quartz and sedimentary lithics (Table 2). Table 2 shows the summarized point count data for the Ignacio Formation and the McCracken Sandstone Member and displays the maximum, minimum, and average percentages for each component measured in the 12 representative samples. Figure 13d is an example of a 5mm diameter metamorphic rock fragment in the Ignacio Formation. Alternation between laminae of coarse and fine-grained quartz is common in the unit (Figure 13e). Feldspar content is typically low, with plagioclase values ranging from 0%-8.3% and an average of 1.3% (Table 2). Potassium feldspar values tend to be more consistent than plagioclase with values ranging from 0%-4.7% and an average of
48
a) b)
c) d)
Figure 12. Outcrop photographs of the Ignacio Formation displaying common lithologies. A)
The Ignacio Formation from the Sultan Creek outcrop displaying a thin bed of dolomitized sandstone interbedded with a fine-grained, tan siliceous quartz arenite. (Hammer = 30.5-cm) B)
The Ignacio Formation from Vallecito Reservoir displaying a mostly clast supported conglomerate characteristic of fluvial lithofacies association. (Hammer = 35-cm) C) Interbedded red sandstones and mudstones of the Ignacio Formation at US 550 mile marker 54 near Coal
Bank Pass. (Hammer = 35-cm) D) A purple and tan, fine grained, well sorted, quartz arenite from the Ignacio Formation at its type section on the shore of Electra Lake. (Cup = 18-cm)
49
Figure 13. Photomicrographs of the Ignacio Formation with samples at 2.5 x magnification.
50
Figure 13 cont. A) Sub-angular to sub-rounded quartz grains with siliceous cement. (B)
Calcareous cement in the Ignacio Formation at Sultan Creek has overgrown much of the original sedimentary framework. (C) The Ignacio Formation at Vallecito Reservoir displaying quartz overgrowths. (D) A metamorphic rock fragment from the Ignacio Formation at Vallecito
Reservoir. (E) An example of backfilling of a burrow with fine sand and mud in the Ignacio
Formation from Coal Bank Pass. (F) A sample of a strained polycrystalline quartz clast in from
Coal Bank Pass mile marker 54.
51
2.0%. Cement consisted of pore-filling silica (Fig. 13a), pore-filling calcite (Fig. 13b), and quartz overgrowths (Fig. 13c), with bulk percentage ranging from 0.3% to 13% for the 12 samples.
Porosity in the Ignacio Formation ranges from 4.9%-15.3% with an average of 7.1% (Table 2).
In samples collected from the upper portion of the Ignacio Formation at Sultan Creek show that
much of the pore space and cement has been replaced with dolomite as well as a portion of the
quartz (26.3%–46.8% of the total sample).
Thin section analysis, the Ignacio Formation shows a multiphase cementation history
including multiple cement types including quartz overgrowths. Based on point count data
(Appendix C) of 12 samples of the Ignacio Formation, cement typically represent 4.9%-15.3% of
the total sample with an average 9.2% per sample (Table 2). The first phase of cementation was
typically siliceous (60%)(Fig. 13a) with a second phase of calcareous and dolomitic cementation
(30%) (Fig. 13b). Ferruginous (5%), and argillaceous (5%) cements exist locally and typically
represent a third phase of cementation. The argillaceous component of cement is most frequently
found in fluvial components of the Ignacio Formation as in the base of Sultan Creek. Quartz
overgrowths are common, as in Figure 13c where a rounded quartz clast of 3-mm is surrounded
by diagenetic quartz cement up to 0.5 mm thick. Quartz overgrowths are frequently partially
eroded, and pores are filled with calcareous cement indicating a first phase of siliceous cement
on quartz and feldspar grains followed by a later, second phase of calcareous cement.
McCracken Sandstone Member
Hand samples of the McCracken Sandstone Member are typically white or purple,
laminated, medium-coarse grained, well sorted, siliceous quartz arenite. Other sedimentary
structures observed include planar-tabular cross-bedding, hummocky stratification, massive 52
a) b)
c) d)
Figure 14. Examples of different lithologies in the McCracken Sandstone Member. A)
Glossifungites ichnofacies at the McCracken Sandstone Member-Upper Member contact at
Sultan Creek (Field book = 127-cm x 89-cm). Beds at the Sultan Creek outcrop characteristically have more dolomite cement. B) An example of a coarse-grained, well-sorted, white quartz arenite at Sultan Creek. C) Coarse-grained, well-sorted, purplish and tan, quartz arenite from Baker’s Bridge (Hammer = 30.5-cm). D) Clast supported quartz pebble conglomerate and coarse-grained, poorly-sorted, white, siliceous quartz arenite at CR. 250 outcrop near Rockwood.
53
Table 2. Summary of Point Count Data
Ignacio Formation McCracken Sandstone Member
Average Standard Deviation Average Standard Deviation
Monocrystalline Quartz 40.2% 16.41% 52.4% 13.09%
Cryptocrystalline Quartz 0.3% 8.39% 0.3% 3.17%
Polycrystalline Quartz 12.6% 0.73% 5.1% 0.46%
Plagioclase Feldspar 1.3% 1.16% 4.5% 2.17%
Potassium Feldspar 2.0% 2.09% 2.6% 2.59%
Sedimentary Lithic 7.1% 0.00% 3.3% 0.00%
Volcanic Lithic 0.0% 0.00% 0.0% 0.00%
Metam. Foliated Lithic 1.1% 1.64% 3.6% 3.25%
Metam. Non-Foliated Lithic 0.0% 0.00% 0.0% 0.00%
Accessories 3.7% 2.63% 2.8% 2.16%
Cement 9.2% 1.69% 8.7% 1.58%
Unknowns 1.3% 2.39% 1.5% 3.92%
Porosity 7.1% 1.69% 2.5% 1.62%
Carbonate 12.8% 4.35% 11.4% 2.19%
Note: Carbonate value represents both carbonate cement and replaced material. Cement is all other cement types including siliceous, argillaceous, and ferruginous.
54
bedding, or wave ripple lamination. The McCracken Sandstone Member is very well cemented
with silica, thus the unit is a ridge former. Horizons of Glossifungites ichnofacies are common
but are most frequently located in the upper portion of the unit specifically in the white portion.
Figure 14 shows four photomicrographs from the McCracken Sandstone Member displaying the
variation in lithology. Horizontal burrows can be found in the purple portion as well (Fig. 14a).
Some of the white portions of the unit (Fig. 14b) have dolomite cements, such as at Sultan Creek,
and are thinner (~1-5m thick) than the purple portion which is between 10 and 25 meters thick.
At Baker’s Bridge, the McCracken Sandstone Member is a red-purple, coarse-grained, well
sorted, siliceous quartz arenite which is most often contains significant cross-bedding and
hummocky stratification (Fig. 14c). Local beds of the McCracken Sandstone Member contain a
layer of coarse-grained, poorly-sorted, pebbly red quartz arenite. These layers are typically found
at the bottom of the section in outcrops at CR. 250 (near Rockwood) and at Baker’s Bridge.
In thin section analysis, the McCracken Sandstone Member displays similar composition
and clast composition as the Ignacio Formation. The total percentage of cement in the
McCracken Sandstone Member, based on 12 thin sections, is between 1.3%- 11.7% with an
average of 8.7% (Table 2). The first phase of cementation was siliceous cement as quartz
overgrowths. The second phase of cementation was calcite cement. The third phase of
cementation was dolomite replacing phase two calcite cement (Fig. 15a). Percentages of
carbonate cement are much higher in the McCracken Sandstone Member than the Ignacio
Formation, with samples containing between 0.0%-61.8% and an average of 11.4%. Carbonate
porosity is typically slightly lower in the McCracken Sandstone Member than in the Ignacio
Formation, with samples ranging between 0.6%-8.0% and an average porosity of 2.5% (Table 2). 55
Point counting determined the McCracken Sandstone Member is comprised of on
average 3.6% foliated metamorphic rock fragments (schist or gneiss). Plagioclase feldspar found with partly abraded quartz overgrowths (Fig. 15b) likely originate from second cycle recycling of sandstones. Grain boundaries are sharp and clasts are tightly packed in the McCracken
Sandstone Member (Fig. 15c) and total volume of cement is low in many of the samples
(Appendix C). The McCracken Sandstone Member contains both monocrystalline and polycrystalline quartz (Fig. 15 D) with some minor amounts of cryptocrystalline quartz which is interpreted as chert (sedimentary rock fragments). The chert may represent reworking of basement rocks or the Ignacio Formation or (according to Walther’s Law) from the Upper
Member or Ouray Limestone.
Upper Member
In the field the Upper Member typically forms slopes and is covered with soil and vegetation. The formation ranges in thickness from 3.6-m to approximately 21-m across the study area. When exposed, the Upper Member is a fine-grained, well-sorted, tan, argillaceous clay shale with local hopper crystals or alternatively thin bedded dolomite with stromatolite lamination. Samples were described in the field and no petrological analysis was conducted on rocks of the Upper Member.
Ouray Limestone
In the field the Ouray Limestone overlies the Upper Member and is often a ridge former.
It is between 3-m to 15-m thick in the study area and lies directly below the Leadville Formation.
In hand samples it is a laminated, medium-to-dark gray, limestone with an elephant skin texture.
As the Ouray Limestone is considered a marker or datum for this study, only field observations 56
Figure 15. Photomicrographs of the McCracken Sandstone Member from the study area at 2.5 x magnification. Entire scale bar represents 5 mm A) The upper portion of the McCracken
Sandstone Member at Sultan Creek were recorded. Several other studies have been… 57
Figure 15 (cont.) …conducted which discuss the mineralogy of the Ouray Limestone in great
detail (Baars and See, 1967; Baars, 1966; Gonzales et al., 2003a & b). containing abundant
carbonate cement and large sub-angular monocrystalline quartz clasts (Fig. 14a) (Sample
10JTM18). B) The top of the McCracken Sandstone Member at Baker’s Bridge outcrop
containing metamorphic rock fragments and abraded feldspars. (Sample 10JTM1) C) The bottom
of the McCracken Sandstone Member at the Baker’s Bridge section containing coarse-grained,
poorly sorted, monocrystalline quartz in a siliceous cement. (Sample 10JTM2) D) The
McCracken Sandstone Member from County Road 250 near Rockwood containing an outsized quartz clast in fine-grained, calcareous quartz arenite. (Sample 10JTM4) E) The contact between the Ignacio Formation and the McCracken Sandstone Member is displayed in sample 10JTM23.
The gradational contact of sediments between the coarse-grained quartz arenite of the
McCracken Sandstone Member and fine-grained quartz arenite of the Ignacio Formation is evidence for a conformable contact between the units. F) An example of the McCracken
Sandstone Member from Molas Lake which contains mixed sediment related to a burrow. Fine- grained, calcareous quartz arenite at the top represents the burrow infilling material and the coarse-grained quartz arenite at the bottom of the image is the host rock. (Sample 10JTM24)
58
Lithofacies Analysis
Lithofacies were determined based on primary lithology and a sedimentary structure.
Four lithologies are identified in the study; conglomerate (G), sandstone (S), mudstone (M), and heterolithic (mudstone and sandstone) (MS). Accompanying sedimentary structures include; massive bedding (m), intraclasts (e), planar tabular cross-bedding (p), trough cross-bedding (t), ripple cross-lamination (x), planar lamination (l), oscillation ripples (r), hummocky cross- stratification (h), flaser bedding (f), and lenticular bedding (b). Lithofacies were interpreted to indicate a depositional environment and grouped accordingly. In total nine separate lithofacies were interpreted to represent a fluvial depositional environment (Table 3), and an additional 11 lithofacies were interpreted to represent marine or estuarine depositional environment (Table 4).
Sandstone Lithofacies
Lithofacies Sm (Massive Sandstone)
Facies Sm is composed of massive, fine to coarse-grained, moderate to well-sorted, quartz arenite often containing burrows (Glossifungites) and lithoclasts (Fig. 16). The bed thicknesses vary considerably (2-cm to 120-cm) locally; however, most frequently thickness ranges between (20-cm to 30-cm). Massive sandstone is interpreted to be found in both marine and fluvial depositional environments.
Lithofacies Sm is common in outcrops of both the Ignacio Formation and the McCracken
Sandstone Member. Lithofacies Sm in the Ignacio Formation is widely dispersed, with beds in the basal, middle, and upper sections (Appendix B). At Molas Lake, beds below Lithofacies Sm in the Ignacio Formation are most frequently flaser or wave ripple laminated sandstones
(lithofacies Sf or Sx). The beds above lithofacies Sm are either wave ripple laminated sandstones or wavy bedded sandstones. Upper and lower contacts in lithofacies Sm of the Ignacio Formation 59
Table 3. Fluvial Lithofacies
Lithofacies Lithology Sedimentary Interpretation Code Structures
Sm Very fine-to medium- Massive Destratified grained sandstone
Se Very fine-to medium- Pebbles Channel lag grained sandstone
Sp Very fine-to medium- Planar tabular Bars and dunes grained sandstone cross-bedding
Sx Fine to medium-grained Ripple cross Unidirectional ripples sandstone lamination
SSm Siltstone Massive Overbank fines (Destratified)
SSl Siltstone Planar laminated Overbank fines
Mm Mudstone Massive Overbank fines (Destratified)
Ml Mudstone Planar laminated Overbank fines
Gm Pebble conglomerate Massive Bar platform
Gp Pebble conglomerate Planar tabular Bar margin avalanche face cross-bedding
Ge Pebble conglomerate Intraclasts Channel lag / debrite (matrix supported)
Gl Pebble conglomerate Planar bedded Bedload sheets
60
are often scoured (60% bottom, 20% top) and contain either normal or inverse grading. In the
McCracken Sandstone Member, Sm is most frequent in the basal most beds and are typically overlying sandstone beds containing hummocky stratification and overlain by beds containing planar tabular cross-bedding. These beds tend to be normally graded throughout the McCracken
Sandstone Member and have sharp basal and upper contacts.
Destratification of sand may result from several processes including grain flow, quick deposition of homogenous material, or through the destruction of structures by bioturbation
(Boggs, 2001). In this study, massive beds of sandstone frequently overly beds which are planar laminated or wavy bedded sandstones, and are overlain by beds of planar tabular cross-bedding or hummocky stratification (Appendix B). An association between lithofacies Sm and these other lithofacies containing primary sedimentary structures allows for interpretation of depositional environments. Lithofacies Sm can occur in a wide range of depositional environments including fluvial, mudflats, estuary, shoreline, foreshore and backshore (Boggs, 2001). Distinction between
Sm of marine/estuarine and fluvial deposition was determined based on lithofacies associations.
Massive sandstone itself is not indicative of a specific depositional environment, therefore modifiers such as ichnofacies, body fossils, organic material, or desiccation cracks can help constrain the environment of deposition (Boggs, 2001). No organic material or root casts were found in the study area.
Lithofacies Se (Sandstone with pebbles)
Beds containing individual intraclasts or pebbles or stringers of pebbles are found frequently within the bottom portion of massive sandstone beds in the McCracken Sandstone
Member as well as less frequently in massive sandstone beds of the Ignacio Formation. 61
Table 4. Marine and Estuarine Lithofacies Lithofacies Lithology Sedimentary Interpretation Code Structures
Sm Very fine-to medium- Massive Destratified grained sandstone
Se Very fine-to medium- Pebbles Bar lag (Upper grained sandstone shoreface)
St Very fine-to medium- Trough cross-bedding Offshore bar grained sandstone
Sp Very fine-to medium- Planar tabular cross- Offshore bar grained sandstone bedding
Sr Very fine-to coarse-grained Oscillation ripples Wave ripples sandstone
Sh Very fine-to coarse-gained Hummocky Tempestites sandstone stratification
Sl Very fine-to coarse-grained Planar laminated Upper plane bed sandstone or fallout from suspension
Sw Very fine-to coarse-grained Wavy bedded Wave ripples sandstone
SSm Siltstone Massive Tidal rhythmites
Mm Mudstone Massive Destratified
Ml Mudstone Planar Laminated Laminated mud
MSf Heterolithic mud and very Flaser bedded Tidal rhythmites fine to coarse-grained sandstone
MSb Heterolithic mud and very Lenticular bedding Tidal rhythmites fine-to coarse-grained sandstone
62
Figure16. Lithofacies Sm, massive sandstone from Molas Lake. The lack of bedding structures is interpreted to be a result of depositional process as opposed to biological process because there was no evidence of bioturbation in the bed. The upper contact and lower contacts are both scoured. The massive sandstone is overlain by hummocky stratified sandstone and wave rippled, fine-grained sandstone. This example is interpreted as sand being deposited in an estuarine setting.
63
Lithofacies Se is typically massive sandstone beds of the Ignacio Formation. Clasts are typically
associated with fine to coarse-grained, moderate to well-sorted, planar tabular cross bedded,
quartz arenite. Pebbles are composed of rounded clasts of quartzite and quartz sandstone and
range in size from 0.3 cm-to- 2.5 cm (Fig. 17). Lithofacies Se is found in the base of the Ignacio
Formation at Sultan Creek and Molas Lake, as well as throughout outcrops at Coal Bank Pass and mile marker 54 along US 550 (Appendix B). The lithofacies is common in the McCracken
Sandstone Member and can be found in outcrops at Cr. 250 near Rockwood, Baker’s Bridge, and the railroad outcrop at Shalona Lake.
Isolated pebble horizons (stringers) in sandstones are common in marine shoreface/breaker zone depositional settings. Generally, the deposits are characterized by rapid transport and deposition under upper regime flow conditions (Boggs, 2001). Lithofacies Se is found overlying wave ripple laminated sandstone (lithofacies Sx) or climbing ripple laminated sandstone (lithofacies Sl) and is overlain planar-tabular cross-bedded sandstone (lithofacies Sp), massive sandstone (lithofacies Sm), or hummocky stratified sandstone (lithofacies Sh). Both a fluvial and marine interpretation for the lithofacies is determined in this study based on lithofacies association. In the Ignacio Formation, lithofacies Se is found associated with planar- tabular cross-bedded sandstones (lithofacies Sp) and massive sandstones (lithofacies Sm).
Lithofacies Se is then interpreted as a lag deposit associated with a fluvial channel.
Lithofacies Sp (Planar- tabular cross-bedded sandstone)
Lithofacies Sp is found throughout the Ignacio Formation as well as the McCracken
Sandstone Member, and is composed of fine-to coarse-grained, moderate to well-sorted, planar-
tabular cross-bedded quartz arenite (Fig. 18). It is the most frequent lithofacies within the 64
Figure 17. Lithofacies Se, sandstone with quartz pebbles, from mile marker 54 of US 550. The
quartz pebbles are sub-round to sub-angular measuring 2-cm wide, and are from the base of a massive sandstone body within the Ignacio Formation. The contact between lithofacies Se and lithofacies Mm in this photograph is scoured with differentiation up to 3-cm. The association of
Se with massive sandstone is interpreted as occurring within high-energy flow conditions under
high sediment supply, possibly floods or storms. The stringer is formed as erosional lag
deposited under upper plane bed conditions. (Chisel = 14-cm)
65
McCracken Sandstone Member, with some outcrops (Baker’s Bridge and Rockwood outcrops)
containing up to 50% of lithofacies Sp (Appendix B). Planar-tabular cross-bedding was
measured in the field to determine a paleocurrent direction as summarized in Figure 34. The dip angle of lithofacies Sp varies locally with angles ranging between 6˚-36˚ to bedding. Bed of lithofacies Sp are typically thin ranging from 5-cm to 30-cm thick in both the Ignacio Formation as well as the McCracken Sandstone Member. Most (~75%) contacts between beds containing lithofacies Sp are sharp, however some instances of truncated and scoured contacts are found.
These scoured contacts are pervasive when lithofacies Sp is overlain by beds containing climbing ripples or hummocky stratification.
Boggs (2001) suggests that planar-tabular cross-bedding forms as a result of the migration of large scale, straight crested dunes during low flow regime conditions. The depositional environment interpretation for planar-tabular cross-bedding can be highly varied.
For example, in the Ignacio Formation lithofacies Sp may represent dunes that are part of fluvial point bar or bayhead delta deposits, while in the McCracken Sandstone Member lithofacies Sp may represent dunes that are generally representative of a prograding shoreline depositional environment. If present during storm events, planar-tabular cross-bedding is reworked and re- deposited as planar-laminated sandstone or hummocky stratified sandstone (Walker and Plint,
1992). In the event planar-tabular cross-bedding is found in association with the afore mentioned deposits, the depositional environment may be interpreted as shoreface deposits. Thus, lithofacies associations must be conducted before a specific depositional environment can be characterized.
66
Figure 18. An example of lithofacies Sp, planar-tabular cross-bedded sandstone of the
McCracken Sandstone Member from County Rd. 250 near Rockwood. Here many subsets of cross-bedding can be found vertically stacked. It is not uncommon to see reversals of current direction in the McCracken Sandstone Member, suggesting tidally-influenced marine depositional environments (Brunton compass = 7.5 cm).
67
Lithofacies St (Trough cross-bedded sandstone)
Trough cross-bedded sandstone (lithofacies St) is much less common than planar-tabular cross-bedded sandstone (lithofacies Sp) in the study area, with only local beds of the McCracken
Sandstone Member at Baker’s Bridge and Rockwood containing the lithofacies (Appendix B).
No examples of trough cross-bedding were found in the Ignacio Formation. Lithologically, lithofacies St is a fine-to coarse-grained, moderate to well-sorted, trough-tabular cross-bedded, quartz arenite (Fig. 19). Trough cross-stratification in the study area displays curved foresets lamination which tangentially contact the lower bounding surface. On average, beds of lithofacies St are thicker than wavy bedded or planar-tabular cross-bedded structures, with set thickness varying between 20-cm to 40-cm. The lowermost contact is frequently scoured and uppermost contact is sharp. Often (40%) lithofacies St overlies wavy bedded sandstone and is overlain by planar laminated or planar-tabular cross-bedded sandstone.
Trough cross-bedding is formed by 3-D dunes which produce cross-bed sets from the migration of megaripples or dunes (Boggs, 2001). They can form in areas of lower flow regime when scoured troughs are filled with low-angle cross laminae indicative of a strongly tidal influenced upper shoreface (Walker and Plint, 1992). Based on the facies associations, lithofacies St are considered to be part of a marine set of lithofacies in this study.
Lithofacies Sx (Ripple asymmetric or current-laminated sandstone)
In the study area, lithofacies Sx consists of fine-to coarse-grained, moderate to well- sorted, ripple cross-laminated, quartz arenite (Fig. 20). Ripple cross-lamination is found regularly in the basal portions of the Ignacio Formation, but with decreasing frequency in the upper portions of the Ignacio Formation (Appendix B). In contrast, ripple cross-lamination is rarely found in the McCracken Sandstone Member, with the only examples being located at 68
Figure 19. An example of lithofacies St, trough cross-bedded sandstone of the McCracken
Sandstone Formation at Baker’s Bridge. This example is bound by planar-tabular cross-bedded sandstone (lithofacies Sp) and by planar laminated sandstone (lithofacies Sl). The deposit is interpreted as part of the upper shoreface environment (breaker zone) (Hammer = 30.5-cm).
69
Sultan Creek. The thickness of bedding is thin, between 10-cm and 25-cm. In the Ignacio
Formation, lithofacies Sx overlies planar-laminated sandstone (lithofacies Sl) or planar-tabular
cross- bedded sandstone (lithofacies Sp) and is overlain by heterolithic flaser bedded sand and
mudstones (lithofacies MSf) at the top. The lower contact is typically sharp, and the upper
contact is frequently gradational to mudstones that form the top of fining-upward sequences.
Ripple cross-lamination is most often observed in fluvial and estuarine depositional settings (Boggs, 2001). It forms when current or wave ripples migrate rapidly, producing cross- lamination of these superimposed ripples, as one ripple climbs up the crest of the ripple
downstream (Boggs, 2001). Deposition of ripple cross-laminated sand occurs when sediment
buries and preserves the ripples. The mechanism for deposition involves the lower flow regime
bedload transport of sediments (Boggs, 2001). Areas with large amounts of suspended sediments
and flow conditions which perpetuate rapid sedimentation include fluvial flood plains, point bars,
and river deltas (Greb and Martino, 2005) allowing for a fluvial to estuarine depositional
environment interpretation.
Lithofacies Sr (Symmetric or Wave Ripple Laminated Sandstone)
Lithofacies Sr consists of very-fine to coarse-grained, moderate to well-sorted, quartz
arenite with wave or oscillation ripples (Fig 21). Oscillation ripples are found frequently in the
McCracken Sandstone Member, but are also found locally in the upper portion of the Ignacio
Formation at Sultan Creek (Appendix B). Beds are thin with thickness ranging from 5-cm to 20-
cm. The uppermost beds of the McCracken Sandstone Member with lithofacies Sr also contain
Glossifungites ichnofacies. Frequently, the upper contact of a bed containing lithofacies Sr
preserves the bed form height and wavelength of the ripples (Table 5). Oscillation ripples can be
used to determine paleocurrents, as displayed in Figure 21.The symmetrical shape and uniform, 70
Figure 20. An example of current ripple-laminated sandstone (lithofacies Sx) in the Ignacio
Formation from Molas Lake. The lithofacies is bounded by flaser bedded heterolithic deposits on top and lithofacies St on the bottom, with both contacts being scoured. This example is interpreted as estuarine in origin.
71
Figure 21. An example of oscillation ripples preserved in quartz arenite (lithofacies Sr) of the
McCracken Sandstone Member at Electra Lake. Lithofacies Sr is bounded by planar laminated quartz arenite and both the upper and lower contacts are scoured. The deposits are interpreted to have formed in a lower shoreface environment. (Pencil = 14-cm)
72
straight crests of the ripple imply a sub-uniform, alternating current (Boggs, 2001). Lithofacies
Sr used in paleocurrent analysis measured in the Ignacio Formation had a wavelength between 6- cm and 26-cm with an average of 14.5-cm. The amplitude of the same ripples were between 0.5- cm and 3.8-cm with an average of 1.9-cm. The wavelength to amplitude index for wave ripples measured in the Ignacio Formation was 8.88. Lithofacies Sr in the McCracken Sandstone
Member had wavelengths between 8-cm and 16-cm with the average ripple having a wavelength of 12.1-cm. The amplitude of the ripples were between 0.5-cm and 3-cm with an average amplitude of 1.5-cm. The wavelength to amplitude index was 11.94. The most accepted environment which generates oscillation ripples is a shallow marine setting above storm wave base, under low flow regime conditions. Under these conditions, the lower shoreface is influence by wave action. Often oscillation ripples are destroyed by an increase in energy, such as a storms, and the sediment is re-deposited as part of a tempestite sequence (Boggs, 2001).
Lithofacies Sh (Hummocky stratified sandstone)
Lithofacies Sh is composed of very-fine to coarse-grained, moderate to well sorted, hummocky stratified, quartz arenite (Fig. 22). Hummocky stratification is found in the upper portion of the Ignacio Formation at Sultan Creek, and is found pervasively throughout the
McCracken Sandstone Member (Appendix B). In the Ignacio Formation, bed thickness is between 10-cm and 35-cm while in the McCracken Sandstone Member thickness ranges from
10-cm to 45-cm. In the study area, hummocky stratification overlies a scoured surface in massive or planar bedded sandstone at the base, and is overlain by planar-laminated sandstone (lithofacies
Sl). It is likely that a portion of the massive sandstone (lithofacies Sm) is actually well sorted hummocky stratified sandstone (lithofacies Sh), with the homogeneous fabric making hummocks unnoticeable in outcrop view. 73
Figure 22. Hummocky stratification in the McCracken Sandstone Member from Baker’s Bridge.
In this example hummocks and swales are contained within a sandstone bed which is bounded by trough cross bedded sandstone and planar laminated sandstone. The upper contact is scoured and the lower contact is sharp. This example is interpreted as part of a tempestite sequence in shoreface deposits. (Hammer = 30.5-cm)
74
According to Walker and Plint (1992), hummocky stratification occurs in upper flow regime
conditions in which the scouring of sand occurs and then is followed by rapid deposition of
suspended sediments after a high-energy event. Commonly, escape burrows will be contained
within beds of hummocky-stratified material composed of a series of swales and hummocks.
This is interpreted as a storm event in which an initial surge scours the surface and sediment is
deposited during the oscillatory flow as waves advance and retreat (Boggs, 2001). The
depositional environment most conducive to these conditions is a shallow marine setting above
storm weather wave base and below fair weather wave base.
Lithofacies Sl (Planar-laminated sandstone)
Lithofacies Sl is a very-fine to coarse-grained, moderate to well sorted, planar-laminated,
quartz arenite (Fig. 23). Planar-laminated sandstone bedding is the most common lithofacies in
the study area. It is found in throughout both the Ignacio Formation as well as the McCracken
Sandstone Member (Appendix B). The thickness of bedding is highly variable, with beds ranging
from 5-cm to 75-cm. Upper and lower contacts are both sharp and scoured. There are various
lithofacies above and below lithofacies Sl, ranging from hummocky stratified sandstone
(lithofacies Sh) to ripple-laminated sandstone (lithofacies Sx).
Planar-laminated sandstones are one of the most common lithofacies in the sedimentary
record (Boggs, 2001). This is because the beds form under upper flow regime, are easily
preserved, and can form in almost any conceivable depositional environment from fluvial to
marine. Planar-laminated sandstone can form in shallow flows with low-angle current ripples migrating in a downstream direction and slow aggradation of sediment from fallout from suspended sediment of the planar surface (Boggs, 2001). Planar lamination is easily preserved 75
Figure 23. Lithofacies Sl in the McCracken Sandstone Member at Electra Lake. Planar laminated sandstone is found above hummocky stratified sandstone with a scoured lower contact. The upper portion of the bed is considered massive as no structures could be identified. The varying colors are reduction features and not related to sedimentary structures or sandstone composition.
The sequence is interpreted as a tempestite sequence in a shoreface depositional setting (GPS =
12-cm).
76
because it forms in a high energy (upper flow regime) environment which becomes subsequently buried by lower energy (low flow regime) deposits.
Lithofacies Sw (Wavy Bedded Sandstone)
Lithofacies Sw consists of very-fine to coarse-grained, moderate to well sorted, wavy bedded, quartz arenite (Fig. 24). Wavy bedded sandstones are found in the upper portion of the
Ignacio Formation as well as throughout the McCracken Sandstone Member. In the Ignacio
Formation, wavy bedded sandstone is often found in association with heterolithic flaser bedded material and frequently overlain by lithofacies Sr (beds containing climbing-ripple lamination), and mud drapes (Appendix B). Specific outcrops of the Ignacio Formation have pervasive wavy bedded sandstones (Molas Lake, Sultan Creek, Coal Bank Pass) while other outcrops contain no beds of lithofacies Sw (Vallecito Reservoir). Typical bed thickness is between 15-cm and 40-cm.
In the McCracken Sandstone Member, beds of wavy bedded sandstone (lithofacies Sw) are frequently bioturbated by Glossifungites ichnofacies such as at Molas Lake. Every outcrop of the McCracken Sandstone Member contains beds of lithofacies Sw, and the beds are not intrinsically related to any specific surrounding lithofacies. Wavy bedding is most commonly found in tidal and estuarine environments (Dalrymple, 1992). Wavy bedding occurs when dynamic flow conditions through wave action combined with tidal effects facilitate sedimentation of fine particles during slackwater conditions (mud-drapes) over an irregular surface formed by wave ripples (Boggs, 2001). Packages of lithofacies Sw are often reworked through bioturbation or increased wave action resulting in the destruction of the depositional structure. Lithofacies Sw is often found associated with sediments of estuarine origin or upper shoreface.
77
Figure 24. An example of wavy bedded sandstone (lithofacies Sw) in the Ignacio Formation from Molas Lake. Thin mudstone lenses drape sandstone and are weathered out, resulting in the parting appearance in outcrop view. The bed overlies ripple laminated sandstone (lithofacies Sr) and is overlain by massive sandstone (lithofacies Sm) in this example (Pencil = 14-cm).
78
Conglomerate Lithofacies
In the study area conglomerate is found in the Ignacio Formation at Vallecito Reservoir and is found in the McCracken Sandstone Member at Rockwood and Coal Bank Pass (Appendix
B). The conglomerate in the Ignacio Formation mostly consists of thin stringers of pebble-to granule-size vein quartz that are interbedded with thick sandstones showing cross-bedding and hummocky stratification. The following conglomerate lithofacies are representative of the
Ignacio Formation across the study area, with a heavy emphasis on the rocks at Vallecito
Reservoir.
Lithofacies Gm (Massive Conglomerate)
Lithofacies Gm is a poorly sorted, well rounded, oligomict paraconglomerate (Fig. 25). In this study five examples of lithofacies Gm were found in both outcrops of the Ignacio Formation at Vallecito Reservoir, and all these beds have sharp upper and lower contacts and are bound by either planar-tabular cross-bedded conglomerate (lithofacies Gp) or planar-tabular conglomerate
(lithofacies Gl) (Appendix B). Bed thickness is between 35-cm to 75-cm. The clasts are primarily quartzite, presumably derived from the Uncompahgre Formation. The conglomerate is disorganized, with no apparent order to the fabric. The bedding has a variable thickness laterally as they tend to swell and pinch in outcrops over erosional basal contacts.
Although massive conglomerates can also be found in marine depositional environments as wave-worked material is deposited in a beach setting (Boggs, 2001), the majority of the conglomerates in this study are interpreted to be of fluvial origin because of facies associations.
Lithofacies Gm can form in high-energy upper flow regime conditions under either unidirectional or multidirectional flows. Unlike sandstones or mudstones, bioturbation is generally not deemed a mechanism for the disruption of bedding. Instead, the formation of 79
massive bedding in clast supported conglomerate is due clast-by-clast accretion of bedload gravel during high discharge events (Walker and Plint, 1992). As an alternative, matrix supported conglomerate can also form from the deposition of debris flow but there is an abundance of characteristic debrite fabrics in these deposits. In this study, massively bedded conglomerates are interpreted as longitudinal bars, channel lag, or sheet flow deposits associated with a meandering stream.
Lithofacies Gl (Planar bedded conglomerate)
Lithofacies Gp is a poorly sorted, well rounded, planar bedded, oligomict paraconglomerate (Fig. 26). In the study area 14 examples of planar bedded conglomerate were examined in both outcrops of the Ignacio Formation at Vallecito Reservoir accounting for 36% of the lithofacies at the location (Appendix B). The lithofacies is comprised of alternating layers of matrix supported, quartz pebble conglomerate and clast supported, quartz conglomerate.
Quartzite clast sizes range from 1-cm to 7-cm and rarely display imbrication. Several of the basal contacts of lithofacies Gp are scoured and less frequently the top contact also scoured. The beds are almost always bounded by planar-tabular cross-bedded conglomerate (Gt) or massive conglomerate (Gm).
Stratification in conglomerates occurs as a result of varying current intensities depositing alternating layers of sand and gravel (Greb and Martino, 2005). These types of conditions are most frequently observed in fluvial successions (Boggs, 2001). After the gravel is deposited, usually as bedload sheets, sand infiltrates into the conglomerate as matrix material. Once deposited, preservation of planar bedded conglomerate is related to the amount of sediment reworking due to flow conditions. The homogeneity of quartz clasts and the sandstone in the matrix suggest a continuous source of sediment and current fluctuations during deposition. 80
Figure 25. A planar-tabular cross-bedded pebble conglomerate (lithofacies Gp) overlain by massively bedded matrix supported conglomerate (lithofacies Gm) from Vallecito Reservoir. At this location gravel bars are common and are characterized by thin sheets of lag gravel. Inset depicts bounding lithofacies. This succession is interpreted as part of braided stream deposits.
81
Therefore, the depositional environment for lithofacies Gl is most likely fluvial, and more specifically a meandering stream deposit.
Lithofacies Gp (Planar-tabular cross-bedded conglomerate)
Planar-tabular cross-bedded conglomerate (lithofacies Gp) was observed at Vallecito
Reservoir where 10 examples were observed in both outcrops of the Ignacio Formation
(Appendix B). Lithofacies Gp consist of poorly sorted, well rounded, planar-tabular cross- bedded, oligomict paraconglomerate (Fig. 25). There was no noticeable different in clast or matrix composition between lithofacies Gm, lithofacies Gp, or lithofacies Gl. Beds of lithofacies
Gp overly lithofacies Gl or Gm and are overlain by lithofacies Gm, lithofacies Gl or lithofacies
Gp. Beds are evenly distributed throughout the outcrops and range from 15-cm to 40-cm thick.
Similar to other conglomerate lithofacies in this study, contacts are sharp on the top and bottom
surface. The exception is in the basal portion of the northern Vallecito Reservoir outcrop where
three thin (<15-cm) lithofacies Gp beds have scoured top and bottom surfaces. The lithofacies
can be normally graded or ungraded throughout the beds. Unidirectional currents were
responsible for the development of cross-bedding. With the exception of one thin bed of
lithofacies Gp at the base of the Vallecito north section were alternating current directions are
preserved in cross-bedding. Generally, there is little paleocurrent variance seen in lithofacies Gp
near Vallecito Reservoir.
Planar-tabular cross-bedded conglomerate (lithofacies Gp) can form in environments
with a high-energy current and high sediment supply (Boggs, 2001). As the sediment is being
transported avalanche face deposits can form which are often preserved as cross-bedding. The
cross-bedding which forms as avalanche faces are deposited on the downstream side of a dune.
Generally, these avalanche face deposits are located adjacent to lithofacies Gm and a channel 82
pool. Lithofacies Gp is most frequently associated with channel-fill deposits (Khadkikar, 1999;
Walker and Plint, 1992, Boggs, 2001). At the southern outcrop at Vallecito Reservoir sandstone
lithofacies Sm and Sp overly a bed of Gt. This most frequently found in areas which the
sandstones were deposited without an erosive contact over the cross-bedded conglomerates
during the waning phase of a flood, or low water stage.
Mudrock Lithofacies
Mudrocks are found in the Ignacio Formation at Coal Bank Pass and Vallecito Reservoir in the form of massive and planar laminated beds. Most examples of mudrocks occur as interbedded heterolithic material and will be discussed in the next section. In the study mudrocks are defined as fine-grained (silt-and clay-sized) siliciclastic sedimentary rocks including shales,
mudstones, and siltstones.
Lithofacies SSm (Massive siltstone)
Lithofacies SSm consists of medium-grained, ferruginous and argillaceous, purple-red
micaceous siltstone (Fig. 27). Massive siltstones average between 2-cm and 10-cm and are found
in the upper portion of the Ignacio Formation at Coal Bank Pass. Often beds of SSm are heavily
bioturbated resulting in a loss of internal depositional structures. The contact relationships of
SSm are most frequently sharp, however examples of scoured and graded were found in the
field. Massive siltstones are often associated with flaser and lenticular bedding, or overlie successions of massive fine-grained sandstone and are overlain by cross-bedded sandstones.
As in lithofacies Sm, massive beds can be a result of disorganization through biotic
activity or by depositional conditions. Destruction of depositional structures by shallow marine
organisms allows for a clear interpretation of the depositional environment. When no evidence
for organisms is found the interpretation becomes more difficult. Massive siltstones can form by 83
Figure 26. Planar bedded oligomict paraconglomerate (lithofacies Gl) overlying massively bedded oligomict paraconglomerate from the southern outcrop at Vallecito Reservoir. Multiple stacked planar bedded conglomerate beds and planar-tabular cross-bedded conglomerates are found within the section and are interpreted as bed load sheets in a meandering stream environment.
84
transport without traction in high concentrated flows such as in debris flows, storm events, or
flood deposits (Walker and Plint, 1992). More commonly, massive siltstones are deposited by
dust fallout in eolian environments if angular-subangular clasts are present. Also, in fluvial
systems, siltstones can be deposited in floodplain or in point bars resulting in lithofacies SSm.
Alteration between mudrocks and sandstones as in the outcrop at Coal Bank Pass suggest an environment of alternating transport and deposition events. This coupled with the presence of neritic organisms allows for a marginal marine/estuarine depositional environment interpretation of massive siltstones in this study.
Lithofacies SSl (Laminated Siltstone)
Lithofacies SSl consists of laminated, medium grained, argillaceous, red-to-grey/green,
siltstone (Fig. 28). Beds are typically thin (5-cm to 15-cm) and limited to the Ignacio Formation.
Examples of lithofacies SSl are found at mile marker 54 of US 550. Here lithofacies SSl is bound
by red planar-laminated red sandstone and planar-tabular cross-bedded red sandstone (Appendix
B). Both the upper and lower contacts are sharp.
Lamination in siltstones can occur due to alternating conditions of current flow and
slackwater (Boggs, 2001). Typical depositional settings which SSl can be found are seasonal
lacustrine deposits, mudflat, and tidal deposits. Also, the alternating lithologies indicates a
strongly tidal effect and allows for the interpretation of tidal rhythmites. Because of the
association with desiccation cracks and sandstone with quartz pebble clasts, the depositional
setting for SSl is a tidal mud flat.
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Mudstone Lithofacies
Lithofacies Mm (Massive mudstone)
Lithofacies Mm is fine-grained, moderate to well-sorted, massive, argillaceous mudstone
(Fig.29 & 30). Massive mudstone is found in moderate frequency in the study area. In the
Ignacio Formation, lithofacies Mm is found as mud drapes within flaser-bedded material such as at Molas Lake, Sultan Creek, and Coal Bank Pass. Lithofacies Mm is also found associated with lenticular bedded heterolithic deposits (lithofacies MSb), wavy-bedded sandstones (lithofacies
Sw), and planar-tabular cross-bedded sandstones (lithofacies Sp). Lithofacies Mm frequently weathers out of the outcrops. Missing section is often evidence of eroded mudstone. The upper contact of lithofacies Mm is often scoured and the bedding contacts are rarely observed.
Mudstones form when there is are calm hydraulic conditions and fine particles are allowed to settle out of suspension. Often these particles form thin (~1-cm) mud drapes over coarse-grained ripples, and are preserved as flaser bedding. Also, mudstones form in overbank deposits during flooding events and are preserved in floodplain and mudflat areas. Often the mud is deposited as floccules which get destroyed during compaction resulting in massive bedding.
The interpreted depositional environment is heavily influenced by the surrounding lithofacies. In the case of mud drapes and flaser bedding, the depositional environment is interpreted to be tidally influenced estuarine, and in the case of thicker (10-cm to 20-cm) deposits of mudstone, a floodplain deposit is more likely.
Sandstone and Mudrock Heterolithic Lithofacies
In the study area, examples of alternating mud and sand lithofacies are found in both the
Ignacio Formation and the Elbert Formation. These beds generally contain evidence for tidally influenced deposition. The presence of these lithofacies allows for depositional environment 86
Figure 27. Massive siltstone in the Ignacio Formation at mile marker 54 of US 550. Poorly exposed examples of lithofacies SSm are often found alternating with lenses of sandstone.
Because of the poor exposures, contact relationships were based on the sandstone lenses and interpolated to the siltstone. At this locality massive siltstone is overlain by medium-grained sandstone containing quartz pebbles and planar laminated sandstone. The bed above lithofacies
SSm (out of photograph range) contains brachiopods as indicated on the inset stratigraphic section (Tape measure = 8.5-cm).
87
Figure 28. An example of lithofacies SSl, planar-laminated siltstone in the Ignacio Formation from mile marker 54 of US 550. The bed is poorly preserved, however lamination can be seen below the red line and above the scale bar. Below the bed is planar-tabular cross bedded sandstone (lithofacies Sp) and above it is planar-laminated sandstone (lithofacies Sl). When visible, the upper and lower contacts are sharp. The beds are interpreted as forming in a
supratidal mudflat (Tape measure = 8.5-cm).
88
interpretation when other lithofacies in the section are not strongly indicative of the depositional
environment.
Lithofacies MSf (Flaser-bedded heterolithic deposits)
Lithofacies MSf consists of fine-to coarse-grained, moderate to well-sorted, ripple- laminated quartz arenite alternating with fine-grained, ferruginous and argillaceous, purple-red micaceous mudstone (Fig.29). Flaser-bedding is found in moderate frequency in the study area.
In the Ignacio Formation, flaser-bedding occurs regularly at Molas Lake, Sultan Creek, and Coal
Bank Pass. Typically lithofacies MSf is found in associated with lenticular bedded heterolithic deposits (lithofacies MSb), wavy-bedded sandstone (lithofacies Sw), and planar- tabular cross-bedded sandstones (lithofacies Sp). It is common in the basal portion of the Ignacio
Formation at Sultan Creek and becomes less common up section. The same pattern is found at
Molas Lake and Coal Bank Pass. In the McCracken Sandstone Member, lithofacies MSf is found in the lower most beds at Baker’s Bridge, however the sedimentary structures are poorly preserved (Appendix B). The upper contact of lithofacies MSf is often scoured and the bottom contact is rarely scoured.
Flaser bedding is formed when mud is deposited in ripple troughs (and occasionally over the crest of a ripple) during periods of quiescence follow episodes of traction transport and deposition of rippled sand (Boggs, 2001). If the process continues during the next episode of sand transport, the crests will be eroded away and new rippled sand will bury the mud flasers
(Boggs, 2001). More specifically, current velocity increases toward onshore or offshore tides, allowing for the transportation of sand. Then, during the slackwater period at high or low tide, the mud is deposited. The mud is massive because it is deposited as floccules and then the structures is crushed by the overlying sediment. This type of structure forms in strongly tidally 89
Figure 29. Flaser bedded heterolithic deposits in the Ignacio Formation at Molas Lake. The lithofacies consist of rhythmites of ripple laminated sandstone alternating with massive mudstone. Each bed has a scoured base, and is bound by wavy bedded sandstone (lithofacies Sw) and wave-ripple-laminated sandstone (lithofacies Sr) (Pencil = 14-cm).
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Figure 30. Heterolithic lenticular bedded sandstone and mudrock (MSb) from mile marker 54 of
US 550. Sandstone lenses are highlighted in red and continuous mudrock layers can be found in between the sandstone bodies (Hammer = 35-cm).
91
influenced environments and is therefore indicative of an estuarine or tidal flat depositional
setting.
Lithofacies MSb (Lenticular-bedded heterolithic)
Like lithofacies MSf, lithofacies MSb is fine-to coarse-grained, moderate to well-sorted,
ripple-laminated, quartz arenite alternating with fine-grained, ferruginous and argillaceous,
purple-red micaceous mudstone (Fig. 30). It is found exclusively in the Ignacio Formation at
mile marker 54 of US 550 and is absent in the McCracken Sandstone Member. It is found above
wave ripple sandstone (lithofacies Sr), flaser bedded heterolithic sandstone and mudstone
(lithofacies MSf), massive sandstone (lithofacies Sm), and planar laminated sandstone
(lithofacies Sl).
Only two examples were found at mile marker 54 of US 550 and both had scoured upper
and lower contacts. As summarized by Boggs (2001), lenticular bedding forms in a similar
manner to flaser bedding, however a greater portion of mud is preserved resulting in continuous
layers of mudrock. The structure forms as wave action or currents deposit wave rippled sand which is then followed by an episode of slack-water conditions resulting in the deposition of muds. Depositional environments for lithofacies MSf include tidal flats, marine delta-front environments, and in lacustrine delta deposits (Boggs, 2001).
Ichnofacies
The archetypal ichnofacies distribution is controlled by a variety of depositional factors including hydraulic conditions such as velocity and turbidity, the salinity of tidal, coastal, and open waters, the amount of freshwater input and runoff, the morphology of the coastline and the amount of evaporation (Pemberton et al., 1992). For example, in an estuary there is a range of 92
salinity related to the input of freshwater and seawater mixing. Under these conditions, burrowing organisms may advance far up the estuary so long as the salinity matches their physiological constraints (Pemberton et al., 1992). Generally it is accepted that trace fossil abundance and diversity increase seaward and are more prevalent along the margins of estuaries than in deeper channels. Trace fossils of estuarine environments, are often very similar to marine environments, however a distinction can be made between the two using the local lithofacies and lithofacies associations.
Substrate plays a vital role in the types of ichnofacies in the area. Some organisms prefer soft substrates while others may prefer hard, firm, or woody substrates (Pemberton et al., 1992).
For example, Glossifungites assemblages form in firm substrates such as firmgrounds and include trace fossils such as Rhizocorallium and Diplocraterion. The Glossifungites ichnofacies generally are characterized by burrows or surface tracks which avoid obstructions in the substrate (Pemberton et al., 1992). In contrast, the Trypanites ichnofacies is believed to form in weakly lithified marine substrates such as hardgrounds. These fossils tend to be borings perpendicular to the substrate (Pemberton et al., 1992). Trace fossils are common in the study area. Frequently the top portion of the McCracken Sandstone Member demonstrates substantial bioturbation mostly found in certain beds. They are interpreted as firmgrounds and imply low rates of deposition (Appendix B). In the Ignacio Formation trace fossils tend to be escape burrows and occur with much less frequency (Appendix B). Trace fossils were only found in the sandstone portion of both lithostratigraphic units, however it is likely they also exist in the mudrock portion but the beds are too heavily weathered for any to be identified.
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Table 5.Summary of Ichnofacies Ichnogenus Ichnofacies Ichnogenera Lithostratigraphic Code Unit
Dp Glossifungites Diplocraterion McCracken Sandstone Member
Pl Glossifungites Planolites McCracken Sandstone Member
Th Glossifungites Thalassinoides McCracken Sandstone Member
Op Skolithos Ophiomorpha Ignacio Formation
Rz Cruziana Rhizocorallium Ignacio Formation
Ti Cruziana Teichichnus Ignacio Formation
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Ichnogenera
Examples of Glossifungites, Cruziana, Skolithos ichnofacies are found in the study area and are summarized in Table 5. Figures 31 and 32 show examples of ichnofacies assemblages found in the study area. Representatives of Glossifungites ichnofacies in the McCracken
Sandstone Member include the ichnogenera Diplocraterion, Thalassinoides, and Planolites as shown in Figure 31. Representatives of the Skolithos ichnofacies found in the Ignacio Formation
(Fig 32) include the ichnogenera Teichichnus, Diplocraterion, and Ophiomorpha.
Representatives of Cruziana ichnofacies found in the Ignacio Formation include the ichnogenera
Rhizocorallium and Teichichnus.
Ichnogenera Dp (Diplocraterion)
Diplocraterion (ichnogenera Dp) was found in both the Ignacio Formation (Fig. 32) and
McCracken Sandstone Member (Fig. 31) and measure between 4-cm and 12-cm in length and are average 1.2-cm in diameter in width. Diplocraterion is identified by its thin “U” shaped burrow backfilled with concave structures. Ichnogenera Dp is considered part of the
Glossifungites ichnofacies association indicating formation in firm, unlithified substrate such as in a firmground.
Ichnofacies Pl (Planolites)
An example of Planolites (ichnogenera Pl) is found in the McCracken Sandstone
Member at Sultan Creek (Fig. 31). It is characterized by thin (0.5-cm x 5.5-cm) tubular horizontal burrows often overstepping one another. An example of ichnogenera Pl in a thin section of the McCracken Sandstone Member is shown in Figure 33. Here the burrow is backfilled with fine sand. Ichnofacies Pl is found in association with Glossifungites ichnofacies as in Figure 31. 95
Ichnogenera Th (Thalassinoides)
Thin horizontal branching burrows of Thalassinoides were found in the McCracken
Sandstone Member at Sultan Creek (Fig. 31). The fossils have an average length of 6-cm and have an average width of 0.7-cm. They are found on the top surface of bedding and do not extend more than 2-cm into the substrate and most likely represent feeding traces. Ichnogenera
Th is interpreted to be part of the Glossifungites ichnofacies.
Ichnogenera Op (Ophiomorpha)
Ichnogenera Op (Ophiomorpha) is found in the Ignacio Formation at mile marker 54 of
US 550 near Coal Bank Pass. They are characterized by a complex 3D branching structure with a pelleted lining. While rare in other outcrops, the outcrop noted above contains many examples of ichnogenera Op. Based on the penetrating geometry of the Ophiomorpha fossils, it is interpreted that they formed in soft sediment and high energy depositional environment such as a shoreface.
The examples of Ophiomorpha are believed to be of Skolithos ichnofacies.
Ichnogenera Rz (Rhizocorallium)
The U-shapped horizontal burrow structures with spreite of Rhizocorallium was found
only in the McCracken Sandstone Member (Fig 31) and show 7 distinct tubes each measuring 5-
cm to 14-cm in length and 1.4-cm in diameter radiating from a central point. The burrows are
noticeably horizontal with little vertical relief into bedding. They are found in a bed which
contains Glossifungites ichnofacies, however it is believed that they indicate Cruziana
ichnofacies. 96
Figure 31. Field photograph of the bedding surface at the top of the McCracken Sandstone
Member at Sultan Creek. Here examples of Diplocraterion, Rhizocorallium, Thalassinoides, and
Planolites are shown. This assemblage is interpreted as a firmground as no burrows extend vertically into the substrate (Glossifungites ichnofacies). 97
Figure 32. Field photograph of trace fossils from bedding surface the Ignacio Formation at mile
marker 54 of US 550. The photograph shows examples of Diplocraterion, Ophiomorpha, and
Teichichnus. The assemblage is interpreted as part of the Skolithos ichnofacies (Pen = 14-cm).
98
Figure 33. Thin section view of a backfilled burrow of ichnogenus Planolites from the
McCracken Sandstone Member. The burrow is backfilled with fine-grained peloidic material and quartz grains. Burrow outlined by red line. Picture taken at 10x magnification, the entire scale bar is 5-mm.
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Ichnogenera Ti (Teichichnus)
Teichichnus was found in the Ignacio Formation at mile marker 54 of US 550 near Coal
Bank Pass and they are in the form of straight burrows with concave backfill and lobate extensions of sediment (Fig.32). They average 5-cm in length and 0.5-cm in width. The tubes are interpreted as relics of feeding traces. The high amount of stacking and the large amount of sediment displaced indicates and environment with low nutritional value, possibly due to high energy. Teichichnus is interpreted to represent Cruziana ichnofacies found in a shoreface-to- estuarine environment.
Paleocurrent Analysis
A total of 50 unidirectional paleocurrent measurements were collected from cross- bedding and clast imbrications in both the Ignacio Formation and the McCracken Sandstone
Member. An additional 22 paleocurrent measurements were obtained from bidirectional wave ripples. Bidirectional data were interpolated to be consistent with unidirectional data. Figure 34 shows the regional pattern of paleocurrent as preserved in the Ignacio Formation and the
McCracken Sandstone Member. The Ignacio Formation shows unimodal westward paleocurrent directions the (15 unidirectional cross-bedding measurements have a vector mean of 235° and the eight bidirectional wave ripple measurements have a vector mean of 277°). In contrast, the
McCracken Sandstone Member is much more bimodal in nature, with 35 unidirectional cross- bedding measurements having a vector mean of 182° and 14 bidirectional wave ripple measurements having a vector mean of 151°. The bimodal nature of the McCracken Sandstone
Member indicates two distinct flow conditions. The first reflects the effects of coast parallel onshore and offshore wave motion. The second reflects the coast perpendicular longshore drift component. The remaining measurements are a combination of the coast parallel and 100
perpendicular flows along with added hydraulic deviation such as near embayments or migrating bars. This allows for the interpretation that the McCracken Sandstone Member was not confined to a valley with a simple hydraulic influence, but a more complex multi-direction exposure. The
Ignacio Formation on the other hand has a much more wide distribution of paleocurrents. This is representative of the meandering stream depositional environment with the two peaks representing the primairy paleovalley axis. Vector statistics were calculated for both sets of paleocurrent data and are presented in Appendix A.
Provenance Analysis
Provenance analysis was conducted on samples of the Ignacio Formation and the
McCracken Sandstone Member by applying point count data to petrofacies diagrams of
Dickinson (1970). A total of 12 samples of the Ignacio Formation and 12 samples of the
McCracken were plotted on QFL, QmFLt, QpLvLs, and QPK diagrams (Fig. 11 & 35). The QFL mean and standard deviation of the Ignacio Formation is Q= 81.0 + 7.9, F=6.2 + 3.7, L= 12.8 +
5.2 while the QFL mean and the standard deviation for the McCracken Sandstone Member was
Q= 80.1 + 7.8, F=10.1 + 5.4, L= 9.8 + 4.4. On the QFL petrofacies diagram, both units plot in the recycled orogen field and their standard deviations have with approximately 30% overlap.
Similarly, on the QmFLt petrofacies diagram the Ignacio Formation has Qm = 61.8 + 18.9, F =
6.2 + 3.7, Lt = 32.0 + 18.2 while for the McCracken Sandstone Member and Qm= 73.1 + 8.6, F
= 10.0 + 5.4, Lt = 16.8 + 5.8. On the QmFLt diagrams, the Ignacio Formation plots in the quartzose recycled and transitional recycled fields because of relatively larger variation in percentage of lithics than the McCracken Sandstone Member. In contrast, the McCracken
Sandstone Member plots in the quartzose recycled, craton interior, and transitional continental 101
fields. The QmFLt analysis for the Ignacio Formation and the McCracken Sandstone Member
have standard deviations with approximately 30% overlap.
In a QpLvLs analysis the Ignacio Formation has values of Qp= 57.6 + 13.8, Vrf = 0.0 +
0.0, Srf = 42.4 + 13.8, while the McCracken Sandstone Member has values of Qp= 57.4 + 25.4,
Vrf = 0.0 + 0.0, Srf = 16.8 + 25.4. The wide variation and error in the QpLvLs is due to
inconsistencies in the amount of unstable grains in the samples of both formations (Appendix C).
Both units lie primarily in the collision orogen field of the diagram but it’s not likely the
provenance.
Finally in a QmPK analysis the Ignacio Formation had values of Qm= 90.1 + 6.4, P = 4.3
+ 4.2, K = 5.5 + 5.3 and the McCracken Sandstone Member had values of Qm= 87.7 + 6.7, P =
8.0 + 5.5, K = 4.2 + 3.5. Both units plot in the continental block field with an increasing ratio of
volcanic/plutonic components in magmatic arc provenances fields. The standard deviations have
an approximate 50% overlap.
Both the Ignacio Formation and the McCracken Sandstone Member consist of quartz
arenite with few accessory minerals or rock fragments. The provenance analysis shows that the
Ignacio Formation and the McCracken Sandstone Member are very similar in composition and
likely have the same sediment source. According to Earle McBride, the Ignacio Formation and
the McCracken Sandstone Member are almost impossible to tell apart in thin section (Personal
communication, 2011).
Contact Relationships and Paleotopography
As mentioned earlier, there has been a controversy about the contact relationships in the study area. The debate stems from different interpretations about whether the contacts between the Ignacio Formation and the McCracken Sandstone Member are conformable or 102
Figure 34. Average paleocurrent azimuth across the study area per outcrop. Blue arrows indicate the Ignacio Formation and red arrows indicate the McCracken Sandstone Member. The arrows represent the vector mean calculated at each location from cross-bedding and ripple orientation.
All measurements were adjusted for tectonic tilt as illustrated in Figure 10. 103
Figure 35. Dickinson’s (1970) petrofacies diagrams with error polygons for the Ignacio Formation
(solid black) and McCracken Sandstone Member (dashed red). Polygons represent mean and the standard deviation. No error polygongs are in place for the QpLvLs diagram because there were no volcanic rock fragments identified by point counting; therefore, the Lv constituent is not included in identification of provenance fields.
104
unconformable. While other contacts with the Ignacio Formation and the McCracken Sandstone
Member are well understood, examples of each are given as reference in Figures 41 and 42.
Table 6 contains a list of the types of contacts found throughout the area with the type of contact and an example of an outcrop from this study. The nature of the lower contact at each section is particularly important, because this was the base for each stratigraphic section (Appendix B) and it emphasizes the presence of paleotopography.
Ignacio Formation – Precambrian basement
Mostly, the lower-most contact of the Ignacio Formation is either with the unnamed
Precambrian conglomerate in the study area, or else the contact is not visible. However, at Coal
Bank Pass, the Ignacio Formation unconformably overlies the Twilight Gneiss (Fig. 7) with the sedimentary beds onlapping onto the Precambrian paleotopography.
The Ignacio Formation overlies the unnamed Precambrian conglomerate at Sultan Creek
(Fig. 36), Molas Lake, Vallecito Reservoir, and the western margin of Coal Bank Pass. At every locality in the study area, the contact is sharp, and marked by an erosional unconformity.
Bedding structures with the two lithostratigraphic units are disconformable at slight angular discordance. However, the outcrops do not show the effects of onlap.
McCracken Sandstone Member – crystalline basement
The McCracken Sandstone Member is in contact with crystalline basement rocks at
Baker’s Bridge, and the railroad outcrop at Shalona Lake. Here the McCracken Sandstone
Member onlaps paleo-topographic highs of the Baker’s Bridge Granite as seen in Figure 37. At
Baker’s Bridge the contact is noticeably sharp with no paleosols separating the units. At the railroad outcrop however, the McCracken Sandstone Member also onlaps the Baker’s Bridge 105
Figure 36. The contact between the Ignacio Formation and the unnamed conglomerate at Sultan Creek.
Here the bedding of the Ignacio Formation shows low discordance angle of (~5°) to the bedding of the unnamed conglomerate for over 30-m until the contact becomes covered by soil and vegetation.
106
Figure 37. The Baker’s Bridge Granite and McCracken Sandstone Member contact at the Baker’s Bridge
Outcrop. The contact is unconformable and is characterized by the onlapping of the McCracken
Sandstone Member onto paleotopography on the granite body. 107
Figure 38. An example of the unconformable contact between the McCracken Sandstone
Member and the unnamed Precambrian conglomerate unit from the railroad outcrop at Shalona
Lake. The irregular shape to the contact reflects the roughness of the cobbles at the top of the conglomerate bed. The beds dip uniformly to the south west.
108
Granite, but the granite is significantly weathered to grus making it more difficult to describe the sharpness of the contact.
McCracken Sandstone Member – unnamed conglomerate
The McCracken Sandstone Member directly overlies the unnamed conglomerate at the railroad outcrop at Shalona Lake in the study area. Here the Baker’s Bridge Granite is blanketed by the unnamed conglomerate, which is unconformably overlain by the McCracken
Sandstone Member. The conglomerate bedding is ~5º to the bedding of the McCracken
Sandstone Member with both units dipping to the southwest (Fig. 38). In Figure 38 an example of the contact can be seen. It is irregular and reflects structure and internal organization of the underlying cobbles.
Ignacio Formation – McCracken Sandstone Member
Contacts between the Ignacio Formation and the McCracken Sandstone Member can be seen at Sultan Creek, Molas Lake, Coal Bank Pass, and mile marker 54 of US 550. Figure
39 shows an outcrop view of the contact at Molas Lake. The more friable McCracken
Sandstone Member overlies the blocky beds of the Ignacio Formation. Here the contact is gradational as seen in Figure 40. While not immediately apparent in outcrop view, Figure 40 shows a sample taken from the contact as well as a photomicrograph taken from a thin section made from the hand specimen pictured. It is possible to see intermixing of quartz clasts along with uniform cementation across the contact. Quartz grains of similar size and angularity are found in both portions of the sample indicating a similar source material. The portion of mud, however shifts dramatically as the transition between the two units. The mud content drops significantly from the Ignacio Formation to the McCracken Sandstone Member indicating a change in depositional environment. 109
Figure 39. A field photograph of the contact between the Ignacio Formation and the
McCracken Sandstone Member. At this location, the contact is sharp. However, there is no evidence for weathering (paleosols) in the upper part of the Ignacio Formation, nor any incision into the lower unit. The McCracken Sandstone Member overlies the Ignacio
Formation and is a 0.5-m thick cross-bedded sandstone. To the left of this photograph the
McCracken thins out over the Ignacio Formation to a thickness of 15-cm. The red line represents the contact (Hammer = 35-cm).
110
Figure 40. The Ignacio Formation and McCracken Sandstone Member contact at Sultan Creek.
The left photo shows a slabbed hand specimen of sample 10JTM23 from the contact in Figure
39. The contact is sharp with some mixing of sedimentary material from both units. The right photomicrograph is taken from a thin-section from sample 10JTM23 showing the mixing of coarse quartz clasts of the McCracken Sandstone Member with fine quartz and mud of the
Ignacio Formation. The red line represents the contact between the two formations. The thin section is taken at 2.5 x magnification.
111
Evidence for an unconformity between Ignacio Formation and the McCracken Sandstone
Member is missing. For example, there is no evidence of a weathered surface on the upper
portion of the Ignacio Formation. Instead, the contact is characterized by, unweathered sediment
and a lack of paleosols. Second, there is no evidence for incision into the top of the Ignacio
Formation. If the units are separated by an unconformity, either due to a depositional hiatus or erosion, a surface of erosion should be present. The lack of paleosols, although not necessarily present at all unconformities, coupled with the lack of an erosional surface suggests there is no unconformity between the units. Finally, there is no apparent diagenetic difference across the contact. As discussed previously, the two units have a very similar composition and cementation history. This suggests that the two units have similar burial and diagenetic histories and most likely formed contiguously.
Ignacio Formation – Upper Member
The contact between the Ignacio Formation and the Upper Member is only found at
Vallecito Reservoir in the study area. The basal contact for the Upper Member is typically poorly
exposed due to the high erodability of the shales. Because of the lack of a physical identifiable
contact and the presence of shales above the thin succession of conglomerate lithofacies as
discussed previously, the absence of the McCracken Sandstone Member is the most notable
feature at the locality.
McCracken Sandstone Member – Upper Member
The McCracken Sandstone Member is in regular contact with the Upper Member in the
study area. Examples of the contact exist at Sultan Creek, Molas Lake, Coal Bank Pass, as well
as the various outcrops in the Rockwood/Baker’s Bridge region. Previous studies (Baars and
Knight, 1957; Baars and See, 1968; Cole and Moore 1996) have gone into great detail regarding 112
the contact so it was not discussed here. Moreover the conformable relationship between the two
units as described by Cole and Moore was verified.
Upper Member – Ouray Limestone
The Upper Member to Ouray Limestone succession is understood to be conformable as noted by Knight and Baars (1957) and was not extensively evaluated in this study. The units are found in contact pervasively across the area and at every outcrop studied the units were found in contact with one another.
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Table 6. Summary of lithostratigraphic contacts
Lithostratigraphic units Type of contact Example Outcrop
Ignacio Fm. – Crystalline basement Unconformable/Sharp Coal Bank Pass
Ignacio Fm. – unnamed Precambrian Unconformable/Sharp Sultan Creek conglomerate
McCracken SS Mbr.- Crystalline basement Unconformable/Sharp Baker’s Bridge
McCracken SS Mbr.- Unnamed conglomerate Unconformable/Sharp Rockwood RR
Ignacio Fm. – McCracken SS Mbr. Conformable/Gradational Molas Lake
Ignacio Fm. – Upper Mbr. Conformable/Sharp Vallecito Reservoir
McCracken SS Mbr. – Upper Mbr. Conformable/Sharp Electra Lake
Upper Mbr. – Ouray LS Conformable/Sharp Sultan Creek
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DISCUSSION
Valley Incision
The primary cause of incision to form a paleovalley system in the study area was most likely the result of, regional drop in relative sea level. During this episode of sea level fall, the relative base level fell at a similar rate, resulting in an increase in the potential energy of the fluvial systems in the area. This resulted in incision into the Precambrian basement rocks of the region. Though the exact event responsible for the incision is not known, it is possible that it was a result of several factors including an increase in the discharge rate through climate change, tectonics, eustatic changes, or another mechanism which promoted a greater volume of water channelization, or a decrease in sediment supply due to similar means.
Questions raised regarding the depositional model for the Ignacio-Elbert Formations can be answered using new tools and methods. These tools include sequence stratigraphy and the understanding of incised valley sequences. In order to identify the formations as the basal succession of an incised valley fill, clear evidence for a paleovalley must be presented. As previously mentioned, the Ignacio Formation has troubled previous workers with its inconsistent lateral continuity as well as the diversity in depositional environments which it represents. This study has found that when present, the Ignacio Formation is always associated with the Elbert
Formation and most often, the McCracken Sandstone Member. However, In areas where the
Ignacio Formation is absent, the Elbert Formation unconformably overlies the Precambrian bedrock surface. Frequently both of these contacts are near areas of paleohighs on the
Precambrian bedrock surface. For example, at Baker’s Bridge a meter-scale outcrop of the
Baker’s Bridge Granite is overlain by the McCracken Sandstone Member. The knob of granite
represents paleotopography interpreted as a relict of a paleo-shoreline, such as a tombolo. The 115
tombolo would represent a feature within the incised valley that was subsequently infilled by
sediments during a later transgression.
Timing of Valley Incision
There is little control on the timing of incision of the paleovalley due to the omission of
early Paleozoic strata in the area. The Devonian rocks of this study represent the lowest
Paleozoic rocks in the study area, thus the underlying unconformity represents nearly a billion
year hiatus in the geological record. During this hiatus, it is plausible that there were several
eustatic changes yet sediment was not preserved. This may be because sea-level fluctuations
were not enough for sediment preservation in low-lying areas, or tectonic and eustatic
fluctuations acted together to increase the effects of erosion. Regardless, most valley incision
takes place during lowstands (Dalrymple et al., 2006a and b) so it is accepted as the mechanism
for valley incision in this study thought a specific timing is not described.
Geographic Setting
As discussed by Zaitlin et al. (1994a) there are two distinct types of geographic settings
which incised valleys occur: piedmont and coastal plain. Piedmont systems are said to be those
which have headwaters in mountainous areas and cross multiple kick points each reducing the
gradient of the system. They have longer fluvial reach than coastal plain systems and are
associated with underlying structural features. A coastal plain system on the other hand, is
confined to low-gradient areas which do not cross kickpoints. The sediment contained in a
piedmont system is coarse-grained, immature, fluvially-derived sediment as opposed to the
mature, finer-grained, coastal plain sediments. The incised valley in this case represents a
mixture of both piedmont and coastal plain systems.
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Provenance Analysis
As seen in Figure 35, both the Ignacio Formation and the McCracken Sandstone Member
plot in the “recycled orogen” or “cratonic interior” petrofacies fields. This is consistent with the
understood regional depositional context of the area. Both units also contain a similar ratio of
volcanic/plutonic rock fragments. Overlapping standard deviation fields in the petrofacies
diagrams suggests a similar source rock for both the Ignacio Formation and the McCracken
Sandstone Member. Deviation in composition including the amount of recycled sedimentary
rock fragments is explained by latent maturity of the formation sediment. The sediments most
likely formed from the shedding of sediments of a similar source rock. Furthermore, the presence
of quartzite rock fragments in both units suggests that the source rock was in part Precambrian
metasedimentary rocks including the Twilight Gneiss and the Uncompahgre Formation.
In contrast, the conglomerates of the Ignacio Formation are mineralogically distinct from
the Precambrian conglomerates in the area. The Precambrian conglomerates are >95% quartzite
clasts derived from the Uncompahgre Quartzite, with minor vein quartz, schist, greenstone, and
banded iron formation (Evans, 2007). The deposits are organized into clast supported boulder-
cobble conglomerates usually several meters thick, with clasts up to 1.4 m in diameter. In these
Precambrian conglomerates, there are relatively few interbedded sandstones which show line
grain contacts and embayed grain contacts. In contrast, the conglomerate in the Ignacio
Formation consists of mostly thin stringers of vein quartz, of pebble to granule size, with interbedded, thick sandstones showing cross-bedding and hummocky stratification. The
sandstones have point grain contacts indicating a much different depositional history than the
Precambrian conglomerates. Evans (2007) also notes that similar thin conglomerates are also in
the found in the McCracken Sandstone Member and in the Upper Member. 117
It is interpreted that the Ignacio Formation has a similar sediment source as the Elbert
Formation indicating a genetic relationship between the two units. This relationship is not seen with other sedimentary rocks in the area, including the distinctly different Precambrian conglomerate.
Paleocurrent Analysis
The paleocurrent data shows an overall northwest-trending paleocurrent direction within the Ignacio Formation and a north-to-northwest trending paleocurrent in the McCracken
Sandstone Member. The unidirectional nature of the current indicators in the Ignacio Formation is indicative of the fluvial component and therefore the longitudinal extent of the paleovalley.
Though that data is unimodal, it is also is highly dispersed. The dispersion is due to the nature of the meandering stream environment. The bidirectional nature of the McCracken Sandstone
Member is a reflection of tidal influences in the sediments of shoreline normal, onshore-offshore transport by waves, and shoreline parallel, longshore drift transport. In this study the north-to- south portion of the paleocurrent data represents the longshore drift component and bar migration.
It is possible to estimate the geographic setting of the paleovalley by examining the paleocurrent data. The primary flow direction, that is fluvial influenced portions, is from the southeast flowing toward the northwest. The continent mass was therefore set in the east- southeast. The longshore drift component was north-to-south indicating a north-to-south trending shoreline at the mouth of the incised valley.
Morphology
As noted by Zaitlin et al. (1994a), the morphology of an incised valley system is a function of the erodability of the material into which the valley is eroding, the depositional 118
gradient, the time available for incision, and the pre-existing structural grain. The Precambrian
basement is the material eroded for the valley which would be considered relatively hard to
incise. The presence of grus on the Precambrian granite at various outcrops suggests substantial
subaerial exposure and nearly total disaggregation of the granite material. Conjointly, the
immense time gap between the initial incision and infilling of the valley could allow for large
(km-scale) channels to form. The structural fabric of the fault blocked basement rocks may have served as areas of weakness where the bedrock was easily incised.
The length and depth of the channel is also a reflection of the erodability of the material and the time allotted for incision. Figure 41 shows a longitudinal cross section of outcrops across
the study area. Tie lines drawn at the boundaries between depositional environments highlight
the wedge shaped nature of the fill structure, and the lateral shape of the valley which could be
inferred from the upper and lower boundaries of the Figure. The bottom boundary is a major
unconformity between Precambrian basement which acts as the sequence boundary for the
development of the incised valley. The upper portion represents the maximum flooding surface
which caps the depositional sequence. Figure 42 shows a cross section across the valley outlining
the overall shape and highlighting the deep incised channel which contains fluvial deposits. The
section is drawn from the northeast to the southwest across the study area. It is inferred that the
section is part of the proximal or nearshore area of the incised valley. Evidence presented here
suggests a single, deep, main incised channel extending over kilometers in length. The channel
was most likely flanked by subsidiary channels but the small amounts of outcrops, and the lack
of seismic or core data makes identification of these channels difficult. The main channel
contains some fluvial deposits whereas the subsidiary channels likely contain only shoreface and
over bank deposits. 119
Figure 41. Longitudinal transect across the incised valley with interpreted fill and depositional… 120
Figure 41 (cont.) …environment classification. Orange represents fluvial deposits, yellow represents estuarine deposits, gray represents shoreface/shallow marine deposits, and green represents condensed sections. Solid tie lines represent the interpreted top of the depositional environment. The dashed line is an approximation of the location of the contact between the marine and estuarine sediments at Vallecito Reservoir. Question marks indicate areas which the outcrop was covered or to poorly preserved to measure. The unit representing the base of each section is labeled at the bottom. Transect drawn over approximately 60-km. Horizontal spacing is not to scale. 121
Figure 42. Cross-section view of the incised valley and interpreted fill and depositional environment across the study area. Orange represents fluvial deposits, yellow represents… 122
Figure 42 (cont.) …estuarine deposits, gray represents shoreface/shallow marine deposits, and
green represents condensed sections. Solid tie lines represent the interpreted top of the
depositional environment. The dashed line between the Electra Lake outcrop and the Coal Bank
Pass outcrop represents an omission of estuarine strata likely due to pinching out against a
paleotopographic high or non-deposition. Notice the deep channel infilled with basal fluvial deposits transitioning into estuarine, shoreface/shallow marine, and finally condensed sections.
The presence of estuarine lithofacies at the base of Baker’s Bridge may indicate a subsidiary channel in the area. Figure not drawn to scale, cross-section represents ~50-km.
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Valley Fill
The incised valley was infilled during a regional transgression as sediments backfilled the previously incised valley. Areas which have estuarine deposits directly overlying the basement rocks such as Baker’s Bridge (Figure 41, Appendix B) support valley filling during this transgression. Because the valley fill is the only measureable component of the incised valley fill, most of the interpretations are based on the preserved fill. These interpretations allow for the classification of the valley as either simple (filled during one lowstand-transgressive-highstand sequence) or compound (filled during two or more cycles). They also allow for the classification of depositional environments, which are discussed below.
Lithology and Lithofacies
Explaining the relationship between lithology and depositional environments preserved in the study area was a particular point of interest in this study. This was accomplished by examining lithofacies preserved in the stratigraphic sections and relating them to a depositional environment by making associations between packages of lithofacies. The process involved detailed observation of unique depositional features including the presence of fossils, evidence of wave action, and evidence of fluvial influence. Once the depositional environment was established, the lithofacies could be placed into a sequence stratigraphic framework.
Four general types of sediment deposition were interpreted based on lithofacies assemblages including: fluvial, estuarine, shallow marine/shoreface, and deep marine/condensed section. Each of the fore mentioned environments were based on sub-environments within the section including gravel bars, bayhead deltas, and tempestites.
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Fluvial Deposits
Fluvial deposits, found exclusively in the Ignacio Formation, are composed of mudrocks,
sandstones, and conglomerates as shown in Table 2. They are dominated by planar-tabular cross-
bedded conglomerates, and sandstones, representing fluvial bars, with massive and laminated
mudrocks representing overbank, point bar, or flood deposits. Figures 41 and 42 show the
structural relationship between the fluvial deposits and the surrounding rocks. Fluvial deposits
are found in the proximal portions of the valley, then progressively thin and pinch out westward toward the shoreline. The fluvial lithofacies transition westeard into bayhead delta and prograding shoreline deposits.
Fluvial facies present vary greatly in character depending on the grain size of the sediment supplied, the fluvial gradient, and other factors. Deposits that are relatively coarse grained to conglomeratic may reflect higher energy conditions in steeper gradient systems.
Sandy meandering stream deposits are tend to be more abundant in the basal portion of valley fills due to the abundant source of sand at the basal contact. In this case, reworking of the granitic or gneiss basement rocks would supply sufficient sand to promote deposition of such deposits. If the fluvial deposits would continue to develop during later stages of a transgression it is more likely that the trend would shift to more distal meandering or anastomosing deposits.
Estuarine Deposits
Estuarine deposits contained within the Ignacio Formation reflect the regional deepening and continental flooding event which drowned the paleovalley. The estuarine deposits consist chiefly of cross-stratified sandstones, mudstones, and heterolithic deposits containing packages of fluvially, tidally, and wave influenced sedimentary structures (Table 4). The deposits are interpreted as meandering channel, floodplain, interdistributary bay, and bayhead delta 125
depositional environments. The architecture of the estuarine deposits is wedged shaped across
the study area, with thicker successions in the back valley. The thicker packages of estuarine
deposits in the back valley may reflect the progressive drowning of the valley and may indicate
rapid coastal transgression.
Lingulid brachiopods and mixed mudflat deposits found at the US 550 mile marker 54
outcrop allow for the implication of brackish water in a tidally influenced environment with a
shallow-marine connection. The Diplocraterion, Ophiomorpha, and Teichichnus ichnogenera
represent typically brackish-water conditions as part of an impoverished Skolithos ichnofacies
(Rossetti, 2006). Wave dominated estuarine deposits include those found in the upper portion of
Sultan Creek and Molas Lake where fluvially dominated estuarine facies transition into bayhead
delta and central basin facies.
Shallow Marine/Shoreface Deposits
Shallow marine and shoreface deposits in the study are found in the McCracken
Sandstone Member. They are composed of cross-stratified (lithofacies Sx), planar-laminated
(lithofacies Sl), and massive sandstones (lithofacies Sm). The entirely sandy nature of the
shallow marine facies indicates a continuous reworking of the sediment in a relative high-energy
setting above storm-wave base. The large amount of cross-stratification suggests sustained flow
in the upper shoreface with continuous reworking by longshore currents. The bidirectional nature
of the paleocurrent data (Appendix 1) supports this conclusion. Also, the many tempestite
deposits contained within the outcrops suggests the area experienced high frequency storm
events. The high-energy, storm-dominated shoreface deposits are characterized by alternating packages of swaley/hummocky-stratified quartz arenite with horizons of Ophiomorpha. In
general, Ophiomorpha has been used as an indicator of marine conditions (salinity, turbidity, 126
substrate). The depositional environments interpreted in these units include delta, barrier, strandplain and shoreface. Much of the strata displays reworking above storm weather wave base and progressive deepening transitioning into tempestite deposits indicating storm wave base.
Open Marine/Condensed Sections
Lithofacies of open marine depositional sequences are found in the McCracken
Sandstone Member and the Upper Member of the Elbert Formation. They are composed of planar-laminated sandstones and mudstones (lithofacies Sl, Ml), planar-tabular cross-bedded sandstones (lithofacies Sp), and massive sandstones and mudstones (lithofacies Sm, Mm) representing shoreface below storm weather wave base and lagoon depositional environments.
The top beds of the McCracken Sandstone Member contain firmgrounds at Sultan Creek and
Molas Lake. These horizons are characterized by heavily bioturbated surfaces containing
Glossifungites ichnofacies (Fig. 31). Firmgrounds represent areas of low sediment supply and may represent the upper limit of the depositional sequence.
Within the Upper Member, thick deposits of shale containing halite hopper crystals indicate a transition to lower-energy highly saline environment typical of lagoonal sequences.
The unit is widespread across the study area, and unlike the Ignacio Formation, was not contained to the paleovalley. The most significant factor that influenced the distribution of the
Upper Member may have been the extent of the marine invasion onto the protocontinent.
Depositional Model
Characterization of the incised valley requires information on the mechanism for incision and the organization of the valley fill. In this study, subaerial exposure of the Precambrian basement allowed for fluvial incision into the bedrock surface. During an approximately one billion hiatus, the basement rock was exhumed, and deep incision into the surface resulted in a 127
varying paleotopography. This surface then became the lower sequence boundary overlain by
Paleozoic sedimentary rocks (line in Figure 43).
The valley was then in-filled by sedimentary rocks representing progressively deepening environments until the valley was overfilled and the shoreline transgressed on-land. Figure 43 shows a hypothetical cross-section of the outcrops in the study area displaying the model for the incised valley fill. The depositional environment is an interpretation based on lithofacies in the section. The Figure is not to scale and each outcrop does not necessarily represent a transect across the paleovalley. For example, the US 550 mile marker 54 outcrop could represent a more landward region where as the Sultan Creek and Molas Lake may represent more seaward sections. The transgressive surface of erosion (blue line) is interpreted based on the extent of flooding in which the braided fluvial deposits (pre-transgression, perhaps lowstand) are overlain by meandering fluvial and bayhead delta deposits. The maximum flooding surface (dashed green) is located at firmgrounds. The capping rocks are the lagoonal shales of the Upper Member and capping carbonate platform rocks of the Ouray Limestone.
This model is more likely than the Baars fault model as his model would create an intraformational unconformity within the Elbert Formation There is no unconformity within the
Elbert Formation nor the Ignacio Formation. In order for the fault model to explain the distribution of sedimentary units in the study area, several fault blocks would have to move up and down with little structural control.
The age of the Ignacio Formation-Ouray Limestone is a Late Devonian, Famennian sequence. It is known that the Ouray Limestone is mid-Famennian in age and the Elbert
Formation is genetically related to the Ouray Limestone. Fish fossils in the Ignacio Formation 128
Figure 43. A depositional model for the incised valley and subsequent backfill in a sequence stratigraphic framework. The fill architecture and depositional environment interpretations are made based on lithofacies analysis of the outcrops in the study. Example outcrops are marked with black dashed lines to indicate the possible positioning within the incised valley sequence.
The Ignacio Formation is restricted to the channel and the Elbert Formation overfills the channel on-lapping onto paleotopographic highs and relics of the paleoshoreline.
129
and the Elbert Formation are considered to be either from Frasnian or Famennian. The Devonian contains multiple eustatic sea level changes including a sea level drop at the Frasnian-Famennian boundary (Haq and Schutter, (2008). This drop would result in an unconformity within the
Ignacio Formation-Ouray Limestone sequence if they were different ages. There was no sequence boundary found within the Ignacio Formation or the Elbert Formation, therefore the
Ignacio Formation should be considered to be Famennian in age.
130
SUMMARY AND CONCLUSIONS
The Ignacio Formation, Elbert Formation, and Ouray Limestone are the lowest
succession of Paleozoic strata in southwest Colorado. Collectively, they include lithofacies
which are interpreted to represent a transgressive sequence of sedimentary rocks transitioning
from basal fluvial deposits into estuarine deposits into prograding shoreline deposits into
lagoonal and shallow marine deposits and finally into shelf and reef carbonates. Debate on the
relationship of the two formations has been discussed in several important papers (Cross et al.,
1905a &b; Barnes, 1954; and Rhodes and Fisher, 1957). Their arguments hinged on the presence
of body fossils of organisms. Since that time, better understanding of depositional models and
sedimentological principles has allowed for reexamination with heavy emphasis on the
architecture, contact relationships, and lithofacies of sedimentary formations. The
lithostratigraphic units of the study have been fit into a sequence stratigraphic framework which includes a model for incision of the paleovalley and describes the subsequent incised valley fill.
This study examined the petrology, lithofacies, ichnofacies, paleocurrent, and stratigraphy of the Ignacio Formation and the McCracken Sandstone Member. A total of 24 thin sections were point counted and described with 12 being from the Ignacio Formation and 12 being from the McCracken Sandstone Member. The point counts were used to determine provenance, cementation history, and grain contacts in both units. It was determined that both units are very similar petrographically, and both units have experienced similar cementation and diagenesis. Lithofacies analysis determined that the Ignacio Formation is composed of both fluvial and estuarine deposits and the McCracken Sandstone Member is composed of shoreface deposits. The ichnofacies analysis found that both units experienced significant biotic activity with most of the bioturbation in the Ignacio Formation being at the upper portion of the section 131
and the McCracken Sandstone Member having activity throughout the unit. This and the presence of tidal sedimentary structures allowed for the interpretation that the upper portion of the Ignacio Formation is brackish, and presumably estuarine, whereas the McCracken Sandstone
Member is mostly high-energy shoreface depositional environments. Finally, the paleocurrent analysis found that the Ignacio Formation experienced relatively unilateral flow reflecting the fluvial nature of the unit, whereas paleocurrents in the McCracken Sandstone Member are bimodal, indicating shoreline normal, onshore-offshore waves, and shoreline parallel, longshore drift components.
The Ignacio Formation is composed primarily of sandstone and conglomerate with minor mudstones and mudrocks. In the eastern portion of the study area, the Ignacio is an oligomict paraconglomerate interpreted to represent coarse-grained meandering stream deposits. These deposits represent the landward most rocks studied in the succession. Moving toward the paleoshoreline to the west, the Ignacio Formation becomes much sandier and the depositional environment shifts from braided stream into meandering stream and eventually estuarine deposits. The Ignacio Formation is not found at every outcrop in the study area. In some places, the McCracken Sandstone Member is present on top of Precambrian bedrock and the Ignacio
Formation is omitted. This is particularly true in outcrops at the southwest portion of the study area. Near Baker’s Bridge and Shalona Lake, the Ignacio Formation is not present. At Electra
Lake outcrops may be present, but they were submerged when the former Ignacio Lake was redeveloped and dammed to form Electra Lake. Moving toward the northwest, the Ignacio
Formation reappears in outcrops and is mostly estuarine with minor fluvial lithofacies in the bottom of the formation. This is reflecting the landward shift in marine deposits as the paleoshoreline shifted onto land during a regional transgression. The limited range of the Ignacio 132
Formation, along with the lithofacies present in the unit suggests that the Ignacio Formation was limited to the incised valley and did not extend outside the margins of the incision.
The McCracken Sandstone Member is a quartz arenite with minor mudstones in the study area. It is more widespread than the Ignacio Formation as it extends across the study area and into the Paradox Basin. It is composed of shallow marine and shoreline lithofacies which transition into the lagoonal and shallow marine rocks of the Upper Member. Unlike the Ignacio
Formation, the McCracken Sandstone Member was not restricted to the paleovalley accounting for is wider range and distribution.
Locally, the top of the McCracken Sandstone Member contains evidence for firmgrounds that are highly bioturbated. This horizon may represent the maximum flooding surface and the maximum extent of the basal depositional sequence including the Ignacio Formation in the study area. This surface was then followed by a second pulse of marine transgression which overlapped the Upper Member onto the McCracken Sandstone Member as the shoreline moved more landward. At most outcrops the Upper Member overlies the McCracken Sandstone
Member with varying thickness and follows a logical lithofacies transition from shoreface deposits to shallow marine deposits. The capping Ouray Limestone completes the transgressive sequence and the top of the formation is the next sequence boundary in the region.
133
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148
APPENDIX A: PALEOCURRENT DATA 149
Table A1: Summary of paleocurrent data; cross-bedding measurements in the Ignacio Formation Midpoi Class nt sin θ cos θ n n sin θ n cos θ (n sin θ)2 (n cos θ)2
000-030 015 0.259 0.966 0 0 0 0 0 030-060 045 0.707 0.707 1 0.707 0.707 0.499849 0.499849 060-090 075 0.966 0.259 0 0 0 0 0 090-120 105 0.966 -0.259 0 0 0 0 0 120-150 135 0.707 -0.707 1 0.707 -0.707 0.499849 0.499849 150-180 165 0.259 -0.966 2 0.518 -1.932 0.268324 3.732624 180-210 195 -0.259 -0.966 1 -0.259 -0.966 0.067081 0.933156 210-240 225 -0.707 -0.707 4 -2.828 -2.828 7.997584 7.997584 240-270 255 -0.966 -0.259 2 -1.932 -0.518 3.732624 0.268324 270-300 285 -0.966 0.259 2 -1.932 0.518 3.732624 0.268324 300-330 315 -0.707 0.707 1 -0.707 0.707 0.499849 0.499849 330-360 345 -0.259 0.966 1 -0.259 0.966 0.067081 0.933156
Summation of Column Values: 15 -5.985 9.015 17.364865 15.632715
Vector Statistics: Vector Mean: θ = tan-1 (Σn sin θ /Σ n cosθ) -1.27818 Vector Magnitude: R = (1/n) [Σ (n sin θ) 2 + Σ (n cosθ)2]0.5 0.3829568 Circular Standard Deviation: S = (180o / π) [2(1-1.0015R)]0.5 89.9722 Circular Variance: S2 8094.9902 Test of Significance: p = e [exp-(nR)2/n] 0.78838
Note: p < 0.05 means can reject the null hypothesis of uniform distribution
150
Table A2: Summary of paleocurrent data; wave ripple measurements in the Ignacio Formation
n cos Class Midpoint sin θ cos θ n n sin θ θ (n sin θ)2 (n cos θ)2 000-030 015 0.259 0.966 0 0 0 0 0 030-060 045 0.707 0.707 0 0 0 0 0 060-090 075 0.966 0.259 0 0 0 0 0 090-120 105 0.966 -0.259 0 0 0 0 0 120-150 135 0.707 -0.707 0 0 0 0 0 150-180 165 0.259 -0.966 0 0 0 0 0 180-210 195 -0.259 -0.966 1 -0.259 -0.966 0.067081 0.933156 210-240 225 -0.707 -0.707 2 -1.414 -1.414 1.999396 1.999396 240-270 255 -0.966 -0.259 0 0 0 0 0 270-300 285 -0.966 0.259 3 -2.898 0.777 8.398404 0.603729 300-330 315 -0.707 0.707 1 -0.707 0.707 0.499849 0.499849 330-360 345 -0.259 0.966 1 -0.259 0.966 0.067081 0.933156 Summation of Column Values: 8 -5.537 2.463 11.03181 4.969286
Vector Statistics: Vector Mean: θ = tan-1 (Σn sin θ /Σ n cosθ) 0.80416 Vector Magnitude: R = (1/n) [Σ (n sin θ) 2 + Σ (n cosθ)2]0.5 0.50002 Circular Standard Deviation: S = (180o / π) [2(1-1.0015R)]0.5 80.9663 Circular Variance: S2 6555.53887 Test of Significance: p = e [exp-(nR)2/n] 0.69323
Note: p < 0.05 means can reject the null hypothesis of uniform distribution
151
Table A3: Summary of paleocurrent data; cross-bedding measurements in the McCracken Sandstone Member.
n cos Class Midpoint sin θ cos θ n n sin θ θ (n sin θ)2 (n cos θ)2 000-030 015 0.259 0.966 2 0.518 1.932 0.268324 3.732624 030-060 045 0.707 0.707 5 3.535 3.535 12.496225 12.496225 060-090 075 0.966 0.259 6 5.796 1.554 33.593616 2.414916 090-120 105 0.966 -0.259 0 0 0 0 0 120-150 135 0.707 -0.707 1 0.707 -0.707 0.499849 0.499849 150-180 165 0.259 -0.966 1 0.259 -0.966 0.067081 0.933156 180-210 195 -0.259 -0.966 1 -0.259 -0.966 0.067081 0.933156 210-240 225 -0.707 -0.707 6 -4.242 -4.242 17.994564 17.994564 240-270 255 -0.966 -0.259 5 -4.83 -1.295 23.3289 1.677025 270-300 285 -0.966 0.259 3 -2.898 0.777 8.398404 0.603729 300-330 315 -0.707 0.707 2 -1.414 1.414 1.999396 1.999396 330-360 345 -0.259 0.966 3 -0.777 2.898 0.603729 8.398404 Summation of Column Values: 35 -3.605 31.395 99.317169 51.683044
Vector Statistics: Vector Mean: θ = tan-1 (Σn sin θ /Σ n cosθ) -8.67043 Vector Magnitude: R = (1/n) [Σ (n sin θ) 2 + Σ (n cosθ)2]0.5 0.3510918 Circular Standard Deviation: S = (180o / π) [2(1-1.0015R)]0.5 92.2716 Circular Variance: S2 8514.04392 Test of Significance: p = e [exp-(nR)2/n] 1.46193
Note: p < 0.05 means can reject the null hypothesis of uniform distribution
152
Table A4: Summary of paleocurrent data; wave ripple measurements in the McCracken Sandstone Member
n cos Class Midpoint sin θ cos θ n n sin θ θ (n sin θ)2 (n cos θ)2 000-030 015 0.259 0.966 1 0.259 0.966 0.067081 0.933156 030-060 045 0.707 0.707 2 1.414 1.414 1.999396 1.999396 060-090 075 0.966 0.259 1 0.966 0.259 0.933156 0.067081 090-120 105 0.966 -0.259 0 0 0 0 0 120-150 135 0.707 -0.707 2 1.414 -1.414 1.999396 1.999396 150-180 165 0.259 -0.966 0 0 0 0 0 180-210 195 -0.259 -0.966 1 -0.259 -0.966 0.067081 0.933156 210-240 225 -0.707 -0.707 3 -2.121 -2.121 4.498641 4.498641 240-270 255 -0.966 -0.259 0 0 0 0 0 270-300 285 -0.966 0.259 1 -0.966 0.259 0.933156 0.067081 300-330 315 -0.707 0.707 3 -2.121 2.121 4.498641 4.498641 330-360 345 -0.259 0.966 0 0 0 0 0 Summation of Column Values: 14 -1.414 12.586 14.99655 14.996548
Vector Statistics: Vector Mean: θ = tan-1 (Σn sin θ /Σ n cosθ) -8.86351 Vector Magnitude: R = (1/n) [Σ (n sin θ) 2 + Σ (n cosθ)2]0.5 0.39119 Circular Standard Deviation: S = (180o / π) [2(1-1.0015R)]0.5 89.3688 Circular Variance: S2 7986.77691 Test of Significance: p = e [exp-(nR)2/n] 0.76193
Note: p < 0.05 means can reject the null hypothesis of uniform distribution
153
APPENDIX B: STRATIGRAPHIC SECTIONS
154
Figure B1: Columnar section from Baker’s Bridge containing the basal 10 m of the McCracken Sandstone Member. 155
Figure B1 cont. 156
Figure B2. Columnar section from SR 250 near Rockwood displaying the facies succession of the McCracken Sandstone Member. 157
Figure B2 continued 158
Figure B3. Stratigraphic section of the McCracken Sandstone Member at the rail road outcrop near Rockwood. 159
Figure B4. Stratigraphic section of the Ignacio Formation at Electra Lake. 160
Figure B5. Stratigraphic section at Sultan Creek including the Ignacio Formation and the McCracken Sandstone Member 161
Figure B5 cont. 162
Figure B6. Stratigraphic section at Molas Lake displaying the Ignacio Formation and the McCracken Sandstone Member. 163
Figure B7. Stratigraphic section of the Ignacio Formation at the northern Vallecito Reservoir outcrop. 164
Figure B8. Stratigraphic section of the Ignacio Formation at the northern Vallecito Reservoir outcrop. 165
Figure B9. Stratigraphic section of the Coal Bank Pass outcrop. 166
Figure B10. Stratigraphic section of the Coal Bank Pass mile marker 54 outcrop. 167
Figure B10 cont. Stratigraphic section of the Coal Bank Pass mile marker 54outcrop. 168
APPENDIX C: POINT COUNT DATA
169
Table C1. Raw Modal Point Counts of the Ignacio Formation
Samples 7 8 9 19 22 23 26 29 31 34 35 36
QZ Monocrystaline 189 204 205 86 114 87 116 164 76 102 98 58 QZ Polycrystalline 14 18 27 13 19 8 26 14 88 79 62 92 QZ Cryptocrystalline 2 - - - 8 ------Potassium Feldspar 5 4 5 1 7 3 13 - 9 2 14 12 Plagioclase Feldspar 9 3 8 6 3 6 25 6 1 4 - 2 Lit. Volcanic ------Lit. Plutonic ------Lit. Foliated Metam. - - - 3 9 - 16 7 2 - 2 - Lit. Non-Foliated Metam. ------Lit. Sedimentary 14 14 12 15 21 18 28 10 32 29 38 29 Accessory Minerals 5 5 9 9 9 15 16 18 12 16 12 9 Cement 34 23 38 15 37 21 21 31 34 32 22 43 Unknowns - 11 4 14 5 12 2 - - - 2 - Porosity 26 34 31 2 1 2 12 38 36 21 39 29 Carbonate 3 - 15 144 83 129 28 12 24 17 11 7 Raw Total 301 316 354 308 316 301 303 300 314 302 300 281
Note: See Appendix B for stratigraphic position of the samples. Abbreviations: QZ-Quartz; Lit- Lithic 170
Table C2. Raw Modal Point Counts of the McCracken Sandstone Member
Samples 1 2 4 12 A 12 B 14 16 17 18 23 24 30
QZ Monocrystaline 179 162 176 179 158 207 196 192 154 151 48 209 QZ Polycrystalline 15 36 33 17 13 9 9 11 2 7 5 32 QZ Cryptocrystalline - - 3 3 - - 1 - 4 - - - Potassium Feldspar 4 7 32 11 12 10 4 6 2 6 2 5 Plagioclase Feldspar 13 14 12 25 28 27 12 3 5 16 12 1 Lit. Volcanic ------Lit. Plutonic ------Lit. Foliated Metam. 6 32 24 21 18 20 6 2 6 - 3 - Lit. Non-Foliated Metam. ------Lit. Sedimentary 6 10 3 7 8 4 11 17 28 13 9 6 Accessory Minerals 3 4 18 7 14 9 4 15 17 7 6 2 Cement 26 16 43 19 28 15 12 24 58 32 21 16 Unknowns - 1 3 - 4 2 6 3 17 9 11 - Porosity 2 3 5 2 6 2 24 11 14 12 4 12 Carbonate 55 15 17 12 13 5 - 26 7 48 196 4 Raw Total 310 302 369 303 302 324 301 310 314 301 317 287
Note: See Appendix B for stratigraphic position of the samples. Abbreviations: QZ-Quartz; Lit- Lithic 171
Table C3. Recalculated Point Counts of All Samples
QFL% QFL% QFL% QmFpL% QmFpFk% QmFpFk% QmFpFk% QmFpL% QmFpL% Q F L Qm Fp L Qm Fp Fk
7 Ignacio Fm 88.0 6.0 6.0 89.2 6.6 4.2 93.1 4.4 2.5 8 Ignacio Fm 91.4 2.9 5.8 92.3 6.3 1.4 96.7 1.4 1.9 9 Ignacio Fm 90.3 5.1 4.7 91.1 5.3 3.6 94.0 3.7 2.3 19 Ignacio Fm 79.8 5.6 14.5 78.2 16.4 5.5 92.5 6.5 1.1 22 Ignacio Fm 77.9 5.5 16.6 77.6 20.4 2.0 91.9 2.4 5.6 23 Ignacio Fm 77.9 7.4 14.8 78.4 16.2 5.4 90.6 6.3 3.1 26 Ignacio Fm 63.4 17.0 19.6 62.7 23.8 13.5 75.3 16.2 8.4 29 Ignacio Fm 88.6 3.0 8.5 87.7 9.1 3.2 96.5 3.5 0.0 31 Ignacio Fm 78.8 4.8 16.3 68.5 30.6 0.9 88.4 1.2 10.5 34 Ignacio Fm 83.8 2.8 13.4 75.6 21.5 3.0 94.4 3.7 1.9 35 Ignacio Fm 74.8 6.5 18.7 71.0 29.0 0.0 87.5 0.0 12.5 36 Ignacio Fm 77.7 7.3 15.0 65.2 32.6 2.2 80.6 2.8 16.7 Mean 80.1 10.1 9.8 78.1 18.2 3.7 90.1 4.4 5.5 1st D 7.9 5.4 4.4 10.2 9.8 3.5 6.4 4.2 5.3
172
Table C3. Continued 1 McCracken SS 87.0 7.6 5.4 87.7 5.9 6.4 91.3 6.6 2.0 2 McCracken SS 75.9 8.0 16.1 74.3 19.3 6.4 88.5 7.7 3.8 4 McCracken SS 74.9 15.5 9.5 81.9 12.6 5.6 80.0 5.5 14.5 12A McCracken SS 75.7 13.7 10.6 77.2 12.1 10.8 83.3 11.6 5.1 12B McCracken SS 72.2 16.9 11.0 74.5 12.3 13.2 79.8 14.1 6.1 14 McCracken SS 78.0 13.4 8.7 80.2 9.3 10.5 84.8 11.1 4.1 16 McCracken SS 86.2 6.7 7.1 87.1 7.6 5.3 92.5 5.7 1.9 17 McCracken SS 87.9 3.9 8.2 89.7 8.9 1.4 95.5 1.5 3.0 18 McCracken SS 79.6 3.5 16.9 79.8 17.6 2.6 95.7 3.1 1.2 23 McCracken SS 81.9 11.4 6.7 83.9 7.2 8.9 87.3 9.2 3.5 24 McCracken SS 67.1 17.7 15.2 66.7 16.7 16.7 77.4 19.4 3.2 30 McCracken SS 95.3 2.4 2.4 96.8 2.8 0.5 97.2 0.5 2.3 Mean 81.0 12.8 6.2 81.6 11.0 7.3 87.8 8.0 4.2 1st D 7.9 5.2 3.8 8.1 5.0 4.9 6.9 5.5 3.5
Note: See Appendix B for stratigraphic position of the samples. Abbreviations: Q-Total Quartz; Qm-Monocrystalline Quartz; F-Total Feldspar; Fp-Plagioclase; Fk-Potassium; L-Total Lithics; 1st D-First Derivative