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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1986
Stratigraphy and sedimentation of the Upper Cretaceous Montana group of northwestern Montana with detailed bedform analysis of Two Medicine Formation complexly cross-bedded sandstone
Jeffery E. Larson The University of Montana
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Recommended Citation Larson, Jeffery E., "Stratigraphy and sedimentation of the Upper Cretaceous Montana group of northwestern Montana with detailed bedform analysis of Two Medicine Formation complexly cross- bedded sandstone" (1986). Graduate Student Theses, Dissertations, & Professional Papers. 7568. https://scholarworks.umt.edu/etd/7568
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stratigraphy and Sedimentation of the Upper Cretaceous Montana
Group of Northwestern Montana, with Detailed Bedform Analysis of
Two Medicine Formation Complexly Cross-bedded Sandstone
By
Jeffery E. Larson
B.A., St. Cloud State University, 1983 presented In partial fulfillment of the requirements for the degree of
Master of Science
UNIVERSITY OF MONTANA
1986
Approved by:
^ Chairman, Board of Examiners
an, Graduate Scnabl
Date / UMI Number: EP38369
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Stratigraphy and Sedimentation of the Upper Cretaceous Montana Group of Northwestern Montana, with Detailed Bedform Analysis of Two Medicine Formation Complexly Cross-bedded Sandstone
Director: Johnnie N. Moore
The Upper Cretaceous (Campanian) Montana Group comprises a 750 meter thick regressive clastic wedge of transitional marine Telegraph Creek, nearshore Virgelle, and dominantly terrestrial Two Medicine Formations. The Virgelle Formation was deposited along a prograding beach in microtidal conditions with oblique bar and ebb-tidal channel preserved, and longshore sediment transport from south to north. The overlying lignitic shale and sandstone of the Two Medicine Formation was deposited on a marshy coastal plain with distributary channels, crevasse splays, and two transgressive sheet sandstones preserved. The complexly cross-bedded sandstone in the lower 100 meters of the Two Medicine Formation was deposited as a coastal eolian dune system that was then transgressed. The large bedforms in the lower part were wavy-crested and migrated unidirectionally to the southwest. Individual bedforms were less than 10 meters high and largely truncated. Superimposed straight-crested and lunate bedforms had a large bedform normal migration direction. The upper part was deposited by straight-crested and lunate bedforms with no superimposed bedforms and bipolar current directions. Bedform size has decreased vertically, resulting from a decrease in sediment transport or velocity. A shorter relaxation time for upper bedforms may be responsible for preserved secondary azimuth trends.
il TABLE OF CONTENTS
Abstract...... il Table of Contents...... ill List of Figures...... iv Acknowledgements...... v INTRODUCTION...... p.1
CHAPTER ONE: STRATIGRAPHY AND SEDIMENTATION OF THE TELEGRAPH CREEK FORMATION AT SECTION ONE ...... p.8 Descriptions of Stratigraphie Units ...... p.8 Interpretations of Stratigraphie Units...... p. 10
CHAPTER TWO: STRATIGRAPHY AND SEDIMENTATION OF THE VIGELLE FORMATION AT SECTION ONE . . p. 12 Descriptions of Stratigraphie Units . . p.12 Interpretations of Stratigraphie Units. p. 13 Summary of Interpretations...... p.25
CHAPTER THREE: STRATIGRAPHY AND SEDIMENTATION OF THE TWO MEDICINE FORMATION AT SECTION ONE ...... p.26 Descriptions of Stratigraphie Units ...... p.26 Interpretations of Stratigraphie Units...... p.29 Summary of Two Medicine Formation Interpretations p.41
CHAPTER FOUR: STRATIGRAPHY AND SEDIMENTATION OF THE MONTANA GROUP AT SECTION TWO...... p.44 Summary of Interpretations for Section Two. . . . p.50
CHAPTER FIVE: BEDFORM ANALYSIS OF COMPLEXLY CROSS-BEDDED SANDSTONE IN THE TWO MEDICINE FORMATION . . p.51 Descriptions of Stratigraphie Unit . p.54 Descriptions of Stratigraphie Unit 3b . p.60 Descriptions of Stratigraphie Unit 3c . p.64 Interpretations of Stratigraphie Unit 3& p.66 Interpretations of Stratigraphie Unit 3b p.80 Summary of interpretations of 3& and 3b p.85 Interpretations of Stratigraphie Unit 3c p.86
iii REFERENCES...... p.89
APPENDIX...... p.98 One: Legend...... p. 98 Two:Sketches of several sedimentary structures in strat. unit three...... p.99 PLATES...... p. 100 One:Section one outcrops and sedimentary structures • • p.101 Two:Strat* unit 3a sedimentary structures and cross-beds...... p. 102 Three:Sedimentary structures and cross-beds in strat* unit 3a...... p. 103 Four :Sedimentary structures and cross-beds in strat. unit 3&...... p. 104 Five:Cross-bed associations in strat. unit 3 b ...... p. 105 Six:Sedimentary structures in strat* unit 3b and outcrop picture of 3 c ...... p. 106
iv List of Figures
Figure 1: Location map of Two Medicine Formation. . . . p.2 Figure 2: Correlation chart of Upper Cretaceous rocks of Montana ...... P-3 Figure 3: Location map of study area...... p.5 Figure 4: Location map of stratigraphie columns measured p.6 Figure 5: Telegraph Creek Formation at section one. . . p.9 Figure 6: Virgelle Formation at section one ...... p. 13 Figure 7: Typical Virgelle prograding beach sequence. . p. 15 Figure 8: Modern beach profile: subenvironments p. 16 Figure 9: Two Medicine Formation at section one p.27 Figure 10 Major Facies of lower deltaic plain. p.28 Figure 11 Montana Group at section two .... p.44 Figure 12 Two Medicine Formation at section two p.46 Figure 13 Correlation of sections ...... p.49 Figure 14 Complexly cross-bedded unit...... p.52 Figure 15 Outcrop map of complexly cross-bedded sandstone p.53 Figure 16 Rose diagrams at stratigraphie unit 3a p.55 Figure 17 Rose diagrams of superimposed bedforms p.57 Figure 18 Rose diagrams of stratigraphie unit 3b p.59 Figure 19 Rose diagrams of stratigraphie unit 3b p.61 Figure 20 Rose diagrams of stratigraphie units 3b and 3c p.63 Figure 21 Rose diagrams of stratigraphie unit 3c . . . p.65 Figure 22 Distribution of stratification types on dune p.69 Figure 23 Contrasting climbing translatent strata. . . p.71 Figure 24 Wavy-crested bedforms...... p.73 Figure 25 Superimposed dune, and scour pit associations p.75 Figure 26 Lee-eddles on wavy-crested bedform ...... p.76 Figure 27 Reactivation surface formation ...... p.79 Figure 28 Straight-crested and lunate dunes...... p.82 Figure 29 Transgressive sequence at section two. . . . p.87 Figure 30 Depositional environments on modern barrier island p.88 ACKNOWLEDGEMENTS
The research for this thesis was largely funded by grants from the
American Association of Petroleum Geologists, and Sigma Xi Scientific
Research Society, to which I am very grateful. This thesis was largely improved from discussions and encouragement from Dr. Johnnie N. Moore, to whom I am greatly indebted. I would also like to thank Dr. Donald
Winston for valuable insights into sedimentological problems. Dr. David
Rubin for discussions on bedform relationships, and William Cobban for help identifying pelecypods. Thanks also to Dr. Potts for his interest in my project. Also a special thanks to my parents for moral, and periodic financial support.
Vi INTRODUCTION
The Upper Cretaceous (Early Campanian) Two Medicine Formation
(Flg.1) comprises the dominantly terrestrial part of a 750 meter thick clastic wedge of Montana Group rocks (Fig.2) that were deposited as an overall regressive sequence In, and adjacent to, an extensive epicontinental sea that covered much of Montana during the Cretaceous
Period (Vlele and Harris, 1965). The Montana Group In western Montana consists of the Telegraph Creek, Virgelle, and Two Medicine Formations
(Fig.2). Previous authors (Cobban, 1950; Lorenz, 1981; Rice, 1980;
Vlele and Harris, 1965) Infer a transitional marine depositional environment for the silty sandstone of the Telegraph Creek, a strandllne for the dominantly sandstone Virgelle, and a terrestrial environment for the alternating sandstone, shale and coal of the Two Medicine Formation.
In originally defining the Two Medicine Formation, Steblnger
(1914a.) chose as Its type section 600 meters of sandstone, mudstone and lignite located along Two Medicine River In the Blackfoot Indian
Reservation of northwestern Montana (Flg.1), The lower 100 meters Is characteristically thick, laterally continuous sandstone Interbedded with volcanic mudstone and lignite. Recent authors (Cobban, 1950;
Lorenz, 1981; Rice, 1980; Steblnger, 1914a; Vlele and Harris, 1965)
Interpret deposition along a swampy shoreline, wave-dominated delta, or
Interdeltalc prograding shoreline. The upper 500 meters of the Two
Medicine Formation Is characteristically poorly exposed volcanic mudstone and lenticular sandstone that all authors agree are continental fluvial and floodplaln deposits (Cobban, 1950; Lorenz, 1981; WON TA
• 6r#ot FaUw mAuguêfa
Elkhorn Mtn, VÔIconics 50 Km.
& )
F igure I. Location mop of Two Medicine and correlative Judith River Formations of western Montana. A fterLoreni, (1980). Reference Sequence Northern Lewis Northwest of for Western Interior and Clerk County Sweetgross Arch - 2 —, (This Study) oS •S:s . \ ^ f 5 V WiMow Creek S, ' < ' i. ^ V Hell Creek Willow Creek- St. Mary River XCO Formation i2 Formations St. M ary River Undifferentiated Form ation Beorpow Shale HorsethieT^ Horsethief SS. <0 3 _„,.. Figure 2. Correlation chart of Upper Cretaceous formations, north central Montana,modified ofterViele and Harris, 1965, steblnger, 1914a; and Vlele, 1965). During deposition of the Late Cretaceous Montana Group the major sediment source was an extensive field of volcanic debris (Flg.1) that lined the seaway In northwestern Montana (Klepper et.al., 1971; Lorenz, 1981; Vlele and Harris, 1965). This volcanic field supplied easily erodable quartz and plagloclase-rlch andeslte and rhyodaclte rock fragments that were carried to the nearby coast by streams. Lorenz (1981) has recently suggested that a large foredeep, resulting from loading by Late Cretaceous thrust plates developed along the coastal zone In northwestern Montana. Subsidence of this foredeep then resulted In deposition of a large clastic wedge. Several excellent exposures of the lower Two Medicine Formation crop out 30 kilometers north of Augusta, Montana In the Sun River Canyon area (Fig.3). Within one section along the Plshkun Canal (Fig.4) a well exposed, festoon cross-bedded sandstone Interval (Plate la) has been Identified by Mudge (1965), but no one has studied It In detail. The main purpose of this thesis Is to analyze In detail bedforms of the festoon cross-bedded sandstone, paying special attention to bedform Interactions, mlgratlonal directions, and variation through time. I applied techniques recently developed by Rubin and Hunter (1983) to aid In analysis. The second purpose of this thesis Is a stratigraphie and depositional process analysis of Upper Cretaceous Montana Group strata that are well exposed In several sections within the study area (Figures 3 and 4). Identification and correlation of these adjacent environments greatly aids in the Interpretation of the processes responsible for the % 1 River Teton Choteau STUDY AREA Sun ^ Augusta Great Foils Wolf Creek MONTANA LlHeleno 20 miles Figure 3. Location map of study area. Section 20 Davy High # Ranch P ishkin C anck 29 28 27 N.Fork Sun Section / River TETO N CO. LEW IS ^ AND CLARK 32 CO. 33 3 4 32 Km. to A u g u sta Figure 4. Location of two stratigrophic columns used in this study. deposition of the complexly cross-bedded sandstone. I spent a total of 60 field days on this project. The first 25 days I devoted to a detailed sedimentological analysis of two stratigraphie sections containing Two Medicine Formation and adjacent Montana Group rocks (Fig.4). I separated each column Into different stratigraphie units based on lithologie textures and sedimentary structures, which later allowed stratigraphie unit correlations between sections and depositional process Interpretations. I used the remaining 35 days to conduct a detailed bedform analysis of the complexly cross-bedded sandstone that constitutes the main emphasis of this thesis. CHAPTER ONE: STRATIGRAPHY AND SEDIMENTATION OF THE TELEGRAPH CREEK FORMATION AT SECTION ONE I measured two stratigraphie sections of the lower Two Medicine Formation and underlying Montana Group rocks in the Sun River Canyon area (Fig.2). The Montana Group rocks of section one are well exposed in an anticline along the north bank of the Sun River (Plate la., Fig.4). The lower part of this section is 91 meters of alternating buff sandstone and grey mudstone of the Telegraph Creek Formation (Fig.5). I here divide the Telegraph Creek Formation into two different stratigraphie units by lithologie textures, and sedimentary structures (Fig.5). Stratigraphie unit one is a 30 meter thick, dark grey silty-shale with many grey sandstone interbeds (Fig.5). The silty shale beds are from 50 to 100 centimeters thick, calcareous, locally fissile with abundant wood fragments and strongly bioturbated throughout. The massive-weathering sandstone interbeds are 20 to 50 cm thick, very thinly laminated, slightly calcareous, locally bioturbated, and contain abundant wood fragments. Bioturbation is in the form of simple vertical burrows with density increasing signifigantly on upper bedding planes. Stratigraphie unit two is 61 meters of interbedded, light grey sandstone and dark grey silty-shale (Fig.5). Sandstone beds are horizontally laminated, from 25 to 30 centimeters thick, and locally hummocky cross-stratified. These sandstone beds are very fine-grained, moderately to poorly sorted, and the sand grains are subrounded to subangular. Wood fragments and fragmented pelecypod shells are 8 Sandstone with many silty-shale inter beds, hummocky cross-stratified, locally heavily bioturbated. strat. Unit 2 Alternating silty-shale and fine grained sandstone interbeds, heavily bioturbated. Strat. Unit I 5 meters I 0 Figure 5. Telegraph Creek Formation at section one, 91 meters exposed. Legend see Appendix I concentrated commonly on the bases of the beds. Bioturbation in the form of simple trails are scattered along basal bedding planes. Locally simple, smooth burrows, some up to 2 centimeters in diameter, cut vertically through laminae. Hummocky cross-strata, present commonly near the top of stratigraphie unit two, have sharp lower contacts, average wavelengths of 1.5 meters, amplitudes of 15 centimeters and dips less than 10 degrees (Fig.5). Surfaces are covered with simple worm burrows and oscillation ripples occur several meters from the top. The silty-shale interbeds are from 5 to 15 centimeters thick and decrease in both frequency, and thickness near the top of stratigraphie unit two (Fig.5). These interbeds are grey, calcareous, and completely reworked by bioturbation giving them a mottled appearance. The Telegraph Creek is the lowest formation of the Montana Group in western Montana (Fig.2) as described by Stebinger (1914). Previous authors interpret a transitional environment, below fair weather wave base, for the heavily bioturbated silty-shale, with sandstone interbeds representing storm deposits (Cobban, 1955; Houston and Murphy, 1977; Lorenz, 1981; Rice, 1980; Viele, 1965; Viele and Harris, 1965). The locally hummocky cross-stratified upper sandstone and heavily bioturbated silty-shale of stratigraphie unit two closely resemble sections described by previous authors (Fig.5). I also interpret a transitional marine environmnent, a below fair weather wave base, for these deposits. The term 'hummocky* cross-strata was first introduced by Harms et.al. (1975) for cross-strata that characteristically have erosional lower bounding surfaces, parallel internal lamina, and dips of less than 10 degrees. Most authors agree that hummocky bedding forms 10 below fair weather wave base as the result of storm generated, large scale scale scour and fill, but several schools of thought have developed as to mechanisms of generation (Brenchley et.al., 1979; Hamblin and Walker, 1979; Harms et.al., 1975; Howard and Reineck, 1981). Hayes (1967) working on modern thin blanket sands deposited in normally quiet waters in the Gulf of Mexico, has proposed that storm surge piles water up in shallow coastal areas during hurricanes, that later flows back seaward as ebb entraining large amounts of sand. A second mechanism has been proposed by Walker (1979) and Hamblin and Walker (1981) where storms create nearshore density currents that scour the bottom and later deposit thinly laminated sands in the scour pits after the storm. The presence of local hummocky cross-strata near the top of this section indicates that the sequence was shallowing enough to be effected periodically by storm waves. Lorenz (1981), in Telegraph Creek sections in northern Montana, reports no hummocky cross-strata and therfore interprets deposition below storm wave base. I interpret the interbedded silty-shale to represent normal quiet water transition zone deposition, where burrowers actively churned sediments. The vertical tubes throughout the sandstone interbeds are escape structures left as an organism burrowed upward through the storm deposit. 11 CHAPTER TWO; STRATIGRAPHY AND SEDIMENTATION OF THE VIRGELLE FORMATION AT SECTION ONE The base of the Virgelle Formation is placed above the highest thick mudstone interbed of the underlying Telegraph Creek Formation (Fig.2). I here divide the Virgelle Formation into six stratigraphie units on the basis of lithologie textures and sedimentary structures (Fig.6). Stratigraphie unit one is a 12 meter-thick light grey sandstone with several thin (.5-10 cm) interbeds of dark grey shale. The sandstone is very fine to fine-grained, moderately to well sorted and dominantly thinly laminated (.5 to .75 cm). Hummocky cross-strata with lower erosional surfaces, and internal parallel lamina are the dominant sedimentary structures. Individual hummocks have average wavelengths of 2 meters, amplitudes of 30 centimeters, dips of 3 to 9 degrees, and paleocurrent trends with a dominant mode to the south and subordinate modes to the northwest and southeast (Fig.6). Locally heavy bioturbation, dominantly in the form of simple trails, is present along bedding planes. However, one 20 centimeter thick interbed of very thinly laminated (.25 centimeters), massively weathering sandstone, occuring 10.5 meters from the base, contains abundant U-shaped, vertically stacked spreiten burrows of Dlplocriterion (Fig.6, Plate 1b). Individual burrows have diameters of 2 centimeters and average vertical heights of 6 centimeters. Fine wood fragments and scattered, unidentifiable pelecypod fragments occur throughout. Very thin interbeds of grey, nonorganic, fissile shale near the base pinch and 12 Low angle wedge-planar cross bedded sandstone. T n= 11 S tra t. U n ite t Trough cross-bedded sandstone with S tra t. basal coquina of Ostrea. U n it 5 n= 13 Low angle, wedge-planar cross S tra t. bedded sandstone. U n it 4 n = IO Trough cross-bedded sandstone with S tr a t. U n its jijÇg basal mudchip and shell lag. ^ Low angle wedge-planar cross- bedded sandstone with local trough n = ll cro ss-b e d s. S tra t. U n it 2 n=6 * * • • • * • • • Hummocky cross-stratified sandstone with shale interbeds. S tr a t n= 14 U nit I 3 m ete rs I0 I Figure 6. Virgelle Formation at section one, 61 meters exposed. 13 swell ranging in thickness from .5 to 10 centimeters (Fig.6). Recent authors (Houston and Murphy, 1977; Lorenz, 1981; Rice, 1980; Viele and Harris, 1965) generally agree that the Virgelle Formation represents a prograding shoreline (Fig.7). Rice (1980), working on the Virgelle Sandstone of central Montana, documents a prograding interdeltaic shoreline dominated by shoreface deposits, in which sorting and grain size increase upward, while trace fossils decrease as the result of increasing wave energy (Fig.7). Lorenz (1981) describes similiar sedimentary structures for Virgelle sequences in northwestern Montana and interprets deposition along the wave-dominated prograding shorelines of small deltas. I interpret stratigraphie unit one with dominantly hummocky cross-strata, lower shale interbeds, abundant local heavy bioturbation and interbed of Diplocraterion escape structures as an upper transition zone to lower-middle shoreface environment (Fig.8). In modern environments the upper boundary of the transitional marine facies represents the typical day to day position of wave base (Howard and Reineck, 1981; Reinson, 1979). Reineck and Singh (1982) indicate, where sand and mud are available, the increase in wave energy characteristically results in an increase in grain size and sorting, and a decrease in bioturbation from underlying facies. The presence of hummocky cross-strata in stratigraphie unit one indicates that deposition was below fair weather wave base, and processes were largely controlled by storms. I find no vertical variation in hummocky cross-strata directional modes (Fig.6). Several recent authors (Brenchley et.al., 1979; McGroeder, 1984; Swift, 1983) 14 Coastal Plant "root zone P la in Backshore Foreshore Sandstone, parallel "bedded. ^ Upper Sandstone, tabular and trough Ç S h oreface o cross"bedded, rare Ophiomorpha m "O c o Low er Sandstone with silty-shale inter- S h o re fa c e beds, bioturbated. O ffs h o re Shale and siltstone with sandstone Transition interbeds. I5 meters Figure T. Typical Virgelle prograding beach sequence as seen by Pice (1980) on east side of Sweetgross Arch. 15 DUNES TRANSITION SHOREFACE FORE BACKSHORE ZONE SHORE HWU LWi BEACH BAR BERM LONGSHORE RUNNEL BAR RUNNEL Figure 8. Modern beoch profile showing major subenvironments. A fte r Reineck and Singh (1982). 16 show that some storm deposits have an early current oriented off or along-shore and a later current oriented shoreward and alongshore. Swift (1983) attributes this to changing wind directions as the storm system approachs a land mass. The net result are storm deposits that have an early unidirectional mode, overlain by multidirectional modes. I argue that storm generated bottom currents in the Upper Cretaceous seaway of this area were dominantly unidirectional, and came from the northeast as indicated by lack of vertical variation. Bioturbation has decreased signifigantly from the underlying Telegraph Creek Formation (Figures 5 and 6). Grain size and sorting increase signifigantly upward, indicating increasing wave turbulence. Diplocraterion escape structures indicate that the shoreface periodically experienced rapid sediment influxes, but that normal conditions were calm enough to allow suspension feeding. In summary the sandstone of stratigraphie unit one represents a lower shoreface very close to fair weather wave base, that was normally fairly quiet, but was periodically inundated by unidirectional storms from the northeast that reworked sands into hummocky cross-strata. Stratigraphie unit two is a 25.5 meter thick light grey, medium-grained, moderately to well sorted sandstone. The dominant sedimentary structures are thin (.5 to .75 cm), laterally continuous lamina, which can be traced 12 meters, they form wedge-planar cross-beds, with dip angles of less than 6 degrees, and unimodial southerly paleo-current trends (Fig.6). Small trough cross-beds, less than 50 centimeters in horizontal width, with dips averaging less than 15 degrees and bipolar current trends to the southwest and northeast, 17 locally truncate underlying wedge-planar sets (Flg.6). I interpret the wedge-planar cross-bedded sandstone of stratigraphie unit two to be a beach foreshore deposit (Figures 6, 7, and 8). In modern environments the foreshore is a high energy zone between mean low and high water levels and is constantly reshaped by breaking waves, resulting deposits are characteristically low angle, laterally continuous, wedge-shaped swash sets (McCubbin, 1982; Relneck and Singh, 1980; Reinson, 1979; Semeniuk et.al., 1982). The unimodial southerly dip of the wedge-sets indicates that during deposition of the Virgelle Formation at section one (Fig.4) a local east-west shoreline trended, with the sea to the south (Reineck and Singh, 1980; Reinson, 1979). In northern Montana Lorenz (1981) also documents local east-west shorelines for specific Virgelle outcrops but states that there is an overall north-south coastal trend, which faced a sea to the east, for the majority of the sections he studied. The fine to medium-grained sandstone of stratigraphie unit two is slightly coarser than those of stratigraphie unit one. This together with absence of fine-grained shale interbeds, low angle wedge-planar cross-beds, and lack of bioturbation supports an interpretation for high energy foreshore deposition. Local small trough cross-beds have bipolar current directions that trend subparallel to the east-west shoreline (Fig.6). In modern nearshore systems several types of environments commonly deposit cross-bedded sands oriented subparallel to the shoreline (Fig.8), including: longshore bars, oblique submarine bars, and beach runnels (Hunter et.al., 1979; McCubbin, 1982; Reineck and Singh, 1980; 18 Reinson, 1979). It Is here impossible to decipher which type of environment deposited these scattered trough cross-beds. Foreshore deposits sharply overlying shoreface and transition zone deposits indicate that the beach prograded southward (relative regression), similiar to that of the Virgelle Sandstone in central Montana (Fig.7; Rice 1980). Section one correlates well with Rice's (1980) but I am unable to divide the shoreface into subunits (Figures 6 and 7). Previous authors working on Upper Cretaceous rocks in northwestern Montana indicate that large amounts of sediment were supplied to coasts via numerous rivers that transported easily eroded rock fragments from nearby volcanic highlands (Klepper et.al., 1971; Lorenz, 1981; Viele and Harris, 1965). Progradation indicates that waves were not powerful enough to remove all the sediment coming into this system. Kuenzi et.al. (1979), working on a modern arc-trench off of Guatemala have documented very rapid deltaic progradation in a high energy, wave dominated system as the result of copious sedimentation rates, but then as sediment supply wanes wave action destroys the deltas and sands are redistributed laterally to prograding beach systems. A similiar set of conditions may have effected Virgelle deposition* I am unable to measure the sandbody geometry of the Virgelle Formation at section one (Fig.4) preventing accurate identification of a deltaic sequence. Stratigraphie unit three is a light grey thinly laminated, fine-to medium-grained, trough cross-bedded sandstone (Fig.6). Individual troughs truncate the underlying wedge-planar sets of stratigraphie unit two, scour each other laterally, range in width from 30 to 100 19 centimeters and decrease slightly in width upward. Trough cross-bed dips range from 5 to 15 degrees, and azimuths dominantly trend mostly to the northeast with a minor mode to the southwest (Fig.6). Many troughs have basal, scattered, unidentifiable shell fragments. A 40 centimeter thick bed of grey, subrounded, pebble-sized shale clasts marks the base of the unit. Draping upper trough surfaces near the top, and comprising the top locally, are interbeds of black, organic-rich, fissile shale that pinch and swell and range in thickness from 10 to 75 centimeters. I interpret the trough cross-bedded sandstone of stratigraphie unit three to be remnant of a beach runnel that, as paleo-currents indicate, subparalleled the east-west shoreline (Fig.8). Ridge and runnel systems commonly form on the foreshores of many modern beach systems that of micro to mesotidal ranges and characteristically have cross-beds oriented in the direction of longshore sediment transport (Dabrlo, 1982; Howard and Reineck, 1981; McCubbin, 1982; Moore et.al., 1984; Orford and Wright, 1978; Reineck and Singh, 1980; Semeniuk et.al., 1982). The lag deposit at the base of this interpreted runnel system may represent a scour product that was deposited by the initial storm (or high tide) that developed the system. The interbedded wavy shales that cap the trough cross-bedded sandstone may represent suspended fine-grained sedimentation during tidal still-stand (Dabrio, 1982; Reineck and Singh, 1980). The organic shale pinches and swells and varies in thickness from 10 to 70 centimeters, this seems to be to thick to result from suspended sediment deposition. One possible explaination may be similiar to a "coffee grounds" model described by Cohen (1970) where organic material is 20 eroded from coastal swamps and carried off-shore during storms, later during another storm peat is washed back on-shore and deposited in topographic lows on the beach. Another possible interpretation is that this wavy shale represents a concentration of compressed fecal pellets. Ridge deposits are often preserved with the underlying runnel in beach sequences with high sedimentation rates (Dabrio, 1982). The lack of ridge preservation in section one may indicate that sedimentation rates slowed and resulting beach progradation temporarily stopped. A second possible mechanism for the lack of beach ridge and berm preservation is erosion by waves generated during periodic, high energy storms (Howard and Scott, 1983). Migration of the ridge and runnel system may also plane off the ridges and only preserve the runnel deposits. Because nearly every modern marine beach has tidal influence and associated runnel system it is impossible to interpret tidal range from this deposit. A second depositional model can also explain the cross-bedded sandstones of stratigraphie unit three that trend subparallel to the paleocoastline. Hunter et.al. (1979) described actively migrating oblique sand bars, that frequently develop at the mouths of rip channels along the Oregon coast. Bars are active during high tide and only slightly modified by normal low tide swash. The net result are periodically submerged bars that migrate obliquely alongshore in response to longshore currents, and deposit cross-beds having current orientations subparallel to the shoreline trend. This interpretation may explain the oblique trend of the cross-beds more adequately than a runnel deposit which would have more shore-parallel cross-bed azimuths. 21 other important information can also be drawn from this deposit. Recent authors (Hunter et.al., 1979; Moore et.al., 1984) indicate that orientations of cross-beds deposited by various coastal environments (runnel systems, longshore bars, and oblique bars) can aid, not only in identifying paleocoastline trends, but also in deciphering the direction of net sediment transport. All three of the above mentioned systems commonly migrate in the general direction of longshore sediment transport. The northeasterly trending trough cross-bed azimuths of stratigraphie unit three indicate that longshore currents, and resulting sediment transport were from southwest to northeast in the area of section one during the Upper Cretaceous. Stratigraphie unit four is a 6 meter thick light grey, fine to medium-grained, moderate to well sorted sandstone. The dominant sedimentary structures are thin, laterally continuous, individual lamina can be traced 15 meters, parallel-laminated wedge-planar cross-beds with dip angles of less than 6 degrees, and paleocurrent orientations that trend unimodially to the south (Fig.6). Scattered trough cross-beds less than 5 centimeters wide truncate underlying wedge-sets. Wood fragments are locally present throughout. The lithologie textures of stratigraphie unit four are very similiar to those of stratigraphie unit two (Fig.6), and again I interpret it to represent a south-facing beach foreshore (Fig.8). This vertical sequence of a runnel system overlain by foreshore deposits can result from several different progressions. One may be that at this time the oblique bar, or runnel system was submergent, and located in the lower portion of the foreshore. Continued high sedimentation rates, 22 and resultant beach progradation would then result in the preservation of the runnel deposits as foreshore sands migrated seaward. As stated earlier ridge and runnel systems migrate upshore, and ultimately welded to the backshore berm. This runnel system may have been migrating upshore, and located high on the foreshore (or in the backshore), In this case a transgression would be needed to place foreshore sediments above the runnel system. Stratigraphie unit five Is a 6 meter thick 15 meter long trough cross-bedded sandstone unit that cuts through part of stratigraphie unit four and is lenticular-shaped In outcrop (Flg.6, Plate Id.). The sandstone Is light grey, fine to medium grained, and moderately sorted. Individual troughs range in width from 30 to 60 centimeters and commonly scour each other laterally. Local small tabular planar cross-beds less than 60 centimeters thick are preserved near the top. All cross-beds have a unimodial southerly paleocurrent orientation (Flg.6). A 15 centimeter thick coquina containing fragments of several types of pelecypods, one of which I have identified as the marine pelecypod Ostrea, is present 2 meters from the base. I Interpret the southerly dipping, trough cross-bedded sandstone of stratigraphie unit five as an ebb tidal channel deposit (Figures 6 and 10), with shell fragments indicating a shallow marine source (Weimer et.al., 1982). The lenticular nature of the deposit within the foreshore indicates that the tidal channel did not migrate laterally, and may have been a long-lived feature, suggesting microtidal deposition (Reinson, 1979). The identification of this tidal channel supports the earlier interpretation that some type of embayment existed along the 23 coast (Fig.10). Lorenz (1980) also documents a trough cross-bedded sandstone overlying foreshore deposits near the top of one Virgelle section. He interprets longshore bar deposition by using the paleocurrent trend subparallel to the underlying foreshore, and trough cross-bed size decrease upward. Stratigraphie unit six is a light grey, thinly laminated, fine to medium-grained, moderately to well sorted sandstone (Fig.6). The dominant sedimentary structures are parallelly laminated, laterally continuous, wedge-planar cross-beds with adjusted dips of less than 5 degrees, and unimodial southerly paleocurrent trends (Fig.6). The lithologie textures stratigraphie unit six greatly resemble those of stratigraphie units two and four (Fig.6), and I again interpret beach foreshore deposition with the sea to the south. 24 Summary of Interpretations The Virgelle Formation in section one represents an overall prograding shorefaoe-foreshore sequence (Fig.7). Progradation indicates that wave and tidal energy were not strong enough to redistribute all the sediments introduced into the system, or that sediment rates varied. Hummocky cross-strata are the dominant sedimentary structure in the upper Telegraph Creek and lower Virgelle Formations in section one (Figures 5 and 6). The bimodial southwesterly paleocurrent orientation of these cross-strata indicate that the storms came unidirectionally from the northeast. The absence of backshore facies suggests that periodic storms were of high enough energy to completely erode those deposits (Heward, 1981; Howard and Scott, 1983; Reineck and Singh, 1980; Rice, 1980), a second possible interpretation is that the foreshore deposits are infact shoreface breaker bar deposits which would have no associated backshore, but the details are not clear. The ebb tidal channel deposit indicates that some type of embayment existed along the coast (Fig.10), and the lenticular-shape suggests it was a long lived feature deposited in microtidal conditions. The paleocurrent orientation of the interpreted oblique bar deposit in this sequence, suggests that along this coastline the longshore sediment transport direction was from the southwest to northeast (Moore et.al., 1984). 25 CHAPTER THREE: STRATIGRAPHY AND SEDIMENTATION OF THE TWO MEDICINE FORMATION AT SECTION ONE A partial section of the lower Two Medicine Formation is exposed directly above the Virgelle Formation along the North Fork of the Sun River in section one (Fig.4). I here divide this I6O .3 meter thick sequence into nine different stratigraphie units on the basis of lithologie textures and sedimentary structure to aid in descriptions and interpretations (Fig.9). Previous authors working on the lower Two Medicine Formation of northwestern Montana agree that it represents a continuation of the overall clastic regression into the Cretaceous seaway (Cobban, 1950; Lorenz, 1981; Rice, 1980; Viele and Harris, 1965). Lorenz (1981) in northern Montana and Rice (1980) in central Montana describe blanket sandstone interspersed with mudstone and coal in the lowermost 100 meters of the Two Medicine Formation and Interpret wave-dominated delta and adjacent swampy coast deposition. All authors agree that the thick, fossiliferous, volcanic mudstones with local lenticular sandstones of the middle and upper Two Medicine Formation represent fluvial floodplain deposition that marked the final regression of the Cretaceous seaway in northwestern Montana (Lorenz, 1981; Rice, 1980; Viele and Harris, 1965). Stratigraphie unit one is 43*5 meters of interbedded shale and sandstone (Fig.9). The basal 10 meters is a grey, carbonaceous, noncalcareous, fissile silty-shale with scattered sandstone lenses. The sandstone lenses average 6 meters by 30 cm, are fine-grained, and poorly to moderately sorted, with no visible sedimentary structures. 26 Sfrot. Trough cross-bedded conglomerate. Unit 9 Sandstone with siltstone and wavy- Sfrot. n = 5 bedded shale. Unit 8 Poorly sorted trough cross-bedded sandstone. Tabular planar cross-bedded sand n=l8 stone. Sfrot. Carbonaceous shale with lignite U n its interbed. Low angle, wedge-planar cross-bedded sandstone with loco I small trough cross-beds. n = 9 Strot. Unit 4 n = 7 Carbonaceous shale with sandstone Strot. lenses. Units Wedge-planar cross-bedded sandstone, Strot. with local,small trough cross-beds. Unit 2 n = l3 Carbonaceous shale with lenticular and thinly bedded sandstone. S tro t Unit I 0 meters I 0 Figure 9. Two Medicine Formation of section one; 161.5 m. exposed. Legend see Appendix I. 27 Sharply overlying the grey shale is a 4.5 meter thick, laterally continuous sandstone interbed (Fig.9). The dominant sedimentary structures are large trough cross-beds, up to 2 meters in horizontal length, that truncate each other laterally, and are interbedded with thinly laminated, horizontally bedded sandstones near the top. A mudchip zone, underlying trough cross-beds, makes up the base of this interbed. Individual mud-clasts are grey, non-calcareous, subangular to subrounded and pebble-sized. Wood fragments are scattered throughout, and numerous root casts, several 10 centimeters in vertical length, are present near the top. A 3*5 meter thick interbed of grey, fissile, carbonaceous shale with lenticular sandstones, directly overlies the sandstone interbed. The shales are lithologically very similiar to the previously described shales of stratigraphie unit one (Fig.9). Twenty meters above the base (Fig.9) is a 30 centimeter thick, very thinly laminated (.5 centimeters), horizontally bedded, fine-grained, moderately sorted, sandstone interbed containing a basal coquina of the pelecypod Corbula (written identification William Cobban). Individual shells are whole to slightly fragmented, and unaligned. The upper 23 meters of stratigraphie unit one consists of alternating interbeds of grey, arbonaceous, noncalcareous, fissile, silty-shale, and lignite (Fig.9). Interbed sizes vary from 20 to 150 centimeters and are laterally continuous in outcrop. 28 Rice (1980) and Lorenz (1981) describe lithologie textures similiar to stratigraphie unit one (Fig.9) and interpret deposition in a coastal plain setting, with lenticular sandstone representing fluvial channel deposits, and parallel laminated siltstone ponded, fluvially adjacent, lacustrine or tidal deposits. I interpret the lignitic shale and interbedded sandstone of stratigraphie unit one as the first evidence of dominantly terrestrial lower coastal plain deposition in section one (Figures 9 and 10). The presence of these terrestrial deposits directly overlying the foreshore sandstone of the Virgelle Formation indicates that regression continued during lower Two Medicine deposition. Coleman and Prior (1982) show that lower coastal plains lie in the zone of river-marine interaction to the landward extent of tidal influence; with resulting deposits are then very complex. In modern coastal plains distributary channels are important components, but the largest percentage of environments lie between channels and are: tidal channels, overbank splays, levees, marsh-swamps, and interdistributary ponds. A large percentage coastal plain deposits of stratigraphie unit one are composed of highly carbonaceous silty-shale indicating that the coastal plain was marshy, usually submergent, poorly drained, with plant fragments suggesting heavy vegetation (Coleman and Prior, 1982). Lignite is common in the lowermost beds of many Two Medicine Formation sections (Lorenz, 1981; Rice, 1980; Viele, 1965). Lorenz (1981) sights the lenticular shape and thinness of coal seams, together with absence of identifiable root casts as evidence that coastal swamps were extensive or long-lived along the Upper Cretaceous seaway. He then 29 COASTAL PLAIN DISTRIBUTARY at ^ CHANNELS ^ CREVASSE SPLAY jaiL jUL ùaC ToC WASHOVER FAN ^ 3K TIDAL FLATS' EMBAYMENT FORESHORE EBB TIDAL CHANNEL BREAKER BAR (?) Figure 10. Generalized cross-section of major facies of a lowerower embayed deltaic plain, f-lodified after McCubbin (1982). 30 suggests that coal directly overlying high-energy foreshore sandstone of the Virgelle Formation were deposited as "coffee-grounds" (Cohen, 1970)• The lignites and mudstones in section one are laterally continuous, and thickly bedded, indicating deposition in laterally extensive, long-lived coastal marsh-swamps. The laterally continuous trough cross-bedded sandstone with a basal mudchip lag and uppper root casts and ripples seven meters from the base I interpret as the lowest large-scale fluvial distributary channel that scoured through the marshy coastal plain (Figures 9 and 10). The capping root cast zone and vertical decrease in bedform size indicate that the channel was gradually abandon (Cant, 1982; Cant and Walker, 1978). Distributary channel deposits have also be interpreted for similiar lithologie textures in other Two Medicine sections in northwest Montana (Lorenz, 1981; Rice, 1980). Above the channel deposit grey silty-shale Indicates that fine-grained marsh sedimentation resumed after the channel was abandoned (Fig.10). The silty composition of the shales may be a result of periodic overbank deposition into the normally 'quiet water', channel adjacent swamps-marshes. Twenty meters above the base is a thinly laminated, laterally continuous sandstone interbed with a basal shell coquina of Corbula (Fig.9). I interpret this deposit as a crevasse splay (Fig.10) that breached a distributary channel during a flood and deposited coarse-grained, horizontally bedded sands on top of normally quiet water marsh deposits (Cant, 1982; Coleman and Prior, 1982; Mlall, 1979; Reineck and Singh, 1980). Corbula Is a brackish water pelecypod 31 (written comm. William Cobban) indicating the marsh that the splay covered was at least periodically influenced by marine waters. It may represent an embayment deposit; Coleman and Prior (1982) indicate these 'open water* zones are very common on modern fluvially influenced coastal plains, and are connected to the sea by local inlets. Crevasse splay deposits are very common features in modern interdistributary bays (Coleman and Prior, 1982; Miall, 1979). Stratigraphie unit two is a 13 meter thick, light grey, medium-grained, moderate to well sorted sandstone sharply overlying stratigraphie unit one (Fig.9). The dominant sedimentary structures are thin (.5 to .75 centimeters), parallelly laminated, laterally continuous, wedge-planar cross-beds. Individual wedge-sets dip at angles of less than 6 degrees, and have a unimodial south-southeasterly dip paleocurrent trend (Fig.9). Local small trough cross-beds, less than 50 centimeters wide, truncate underlying wedge-planar laminae. Several logs, one diameter is 70 centimeters, are present near the base, and plant fragments are scattered throughout. A 5 centimeter thick climbing ripple lamina layer occurs near the top of stratigraphie unit two. Wedge planar cross-bedded sandstones of stratigraphie unit two have lithologie textures similiar stratigraphie units in the underlying Virgelle Formation (Fig.6), and I again interpret beach foreshore deposition (Fig.8). The southerly dip of these wedge-sets indicates that the shoreline trended locally east-west shoreline during deposition of stratigraphie unit two. I suggest this "local" anomalous east-west trend (Fig.9) indicates that some type of embayment (or pennisula) 32 existed in the area south of section one (Fig.4). I have documented a similiar east-west shoreline for the underlying Virgelle Formation in section one (Fig.6), which may indicate that this irregularity was a long-lived feature. It is impossible to tell the size of the embayment from this study because of lack of good lateral stratigraphie control. Stratigraphie unit two directly overlies the coastal plain marsh deposits of stratigraphie unit one indicating a relative transgression (rise in sealevel or lowering of baseline by subsidence) occurred followed by progradation. Much controversy still remains concerning the preservation of coastal sand bodies during transgression, and several mechanisms have been developed (McCubbin, 1982). The first was proposed by Kraft (1971) and Swift (1975) where a steep coastal profile is maintained during sealevel rise resulting in net erosion of upper shoreface sediments very similiar to the modern Atlantic coast, the rock record would then show a ravinement (disconformity) with lower shoreface, or transitional sediments overlying continental lagoonal-swamp deposits (Kraft, 1978). A second transgressive model has been proposed by Sanders and Kumar (1975) where "in-place drowning" takes place when coastal sandbodies become submergent as rate of subsidence or sealevel rise is rapid (relatively) and the seafloor slope is at a low angle (McCubbin, 1982). Relatively rapid subsidence may be a result of foredeep loading that Lorenz (1981) indicates was occurring during the Late Cretaceous along the coastline in western Montana as a result of downwarping by advancing Laramide thrust sheets (Fig.1). The sharp contact, lack of large scale erosion of underlying sediments, and preservation of foreshore deposits supports an "in-place drowning" model 33 as described above. Several logs within the foreshore deposits may have been flooded into the sea by rivers and driven onshore by waves. Stratigraphie unit three is an 8 meter thick, partially covered carbonaceous green-black shale, with one 4 meter by 50 centimeter sandstone lense near the base (Fig.9). The shale is locally silty, fissile, noncalcareous, and contains abundant visible plant material. The sandstone lense is thinly laminated (.75 centimeters), horizontally bedded, moderately sorted, and fine-grained. Stratigraphie unit three is lithologically very similiar to stratigraphie unit one (Fig.9) and I again interpret deposition in a coastal plain marsh environment (Fig.10). Stratigraphie unit four is a 53-5 meter thick, light grey, medium-grained, sorted to well sorted sandstone sharply overlying stratigraphie unit three (Fig.9). The lithologies and sedimentary structures here are very similiar to those in stratigraphie unit two (Fig.9), where again the dominant sedimentary structures are thin (.5 to .75 centimeters), parallelly laminated, laterally continuous, wedge-planar cross-beds. Individual wedge-sets dip at angles of less than 8 degrees, and dip south-southwesterly (Fig.9). Local small trough cross-beds, less than 60 centimeters in horizontal width, truncate underlying wedge sets and have southeasterly dip. Twelve meters from the base is a 1.5 meter thick, very thinly laminated (.25 centimeters), maroon silty-shale interbed containing climbing ripples, soft sediment deformation, and flute casts. Individual flutes are sinuous and symmetrical. Seven meters from the top of stratigraphie unit four is a 34 3 meter thick, maroon, very thinly laminated, silty-sandstone. The interbed is heavily bioturbated, locally displays climbing ripples, mudcracks, calcareous sandstone concretions, and abundant wood fragments. The burrows are simple, randomly oriented, and up to 2 centimeters in diameter. The low angle, wedge-planar cross-beds of stratigraphie unit four are very similiar to those of stratigraphie unit two, and I again interpret a deposition as a beach foreshore, with the vertical position over coastal plain marsh deposits indicating that another transgression has occurred (Fig.10). The lack of a basal lag deposit, sharp lower contact and the preservation of high energy foreshore sediments again suggests that "in-place drowning", as described by Sanders and Kumar (1975) may again be the mechanism of preservation. Near the top of stratigraphie unit four (Fig.9) is a thinly laminated, locally heavily bioturbated, mudcracked, silty-shale with silty-sand interbeds, that may represent tidal flat or lacustrine deposits, both of which have similiar lithologie textures (Fouch and Dean, 1982; Reineck and Singh, 1980; Weimer et.al., 1982). Rice (1980) has described similiar lithologie textures for lower Two Medicine equivalent deposits in central Montana, and interprets tidal flat deposition behind sand bars. Reineck and Singh (1980) indicate that tidal flats develop along gently dipping coasts with marked tidal rhythms and lack of strong wave action. I suggest that these tidal flats formed in an embayment along the coast. Klein and Ryer (1978) indicate that for unusually broad continental shelfs, like the interior Cretaceous seaway, tides of exceptional amplitude could have occurred. 35 Lorenz (1981), however, finds few well developed tidal flat structures and therefore Interprets a microtidal range for Upper Cretaceous nearshore deposits of the Montana Group of northwestern Montana. Mudcracks Indicate that this tidal flat was periodically emergent and dessleated. Stratigraphie unit five Is a 12 meter thick layer of Interbedded lignite, and grey-green carbonaceous shale (Fig.9). The Interbeds are of varying thickness, but all are composed of noncalcareous, fissile shale, with a large percentage of organic matter. The llgnltlc shale of stratigraphie unit five are very similiar to the upper part of stratigraphie unit one, and stratigraphie unit three (Fig.9), and I again Interpret deposition on a coastal plain marsh environment (Fig.10). The thickness and the aneoroblc character of the shale Indicates that the area was submergent, possibly for long periods of time. Stratigraphie unit six Is a 2 meter thick, light grey, thinly laminated, well sorted, fine-grained, sandstone. The dominant sedimentary structures are small, less than 30 centimeters In vertical height, thinly laminated (.5 centimeters), tabular planar cross-beds with an overall northeasterly paleocurrent trend. Local soft sediment deformation, and climbing ripples are visible near the top, where cross-beds grade Into thinly laminated plane beds. The base Is marked by a subrounded, pebble to sand sized, grey, mud clast zone. 36 Coleman and Prior (1982) indicate that coastal plain swamps are dominantly quiet water zones but a variety of other lithologies can result from fluvial and marine influence. The lack of reactivation surfaces indicates that this sandstone was deposited by a single event. I interpret the landwardly dipping tabular planar cross-beds of stratigraphie unit six as a high energy, marine generated washover fan deposit (Reinson, 1979), indicating that some of the marsh deposits were close enough to the strand to be influenced by marine processes. This is a thick washover fan deposit by modern standards suggesting that a large storm was resonsible (McCubbin, 1982; Reineck and Singh, 1980). The basal rip-up clast zone was probably transported from the open ocean and is also a good identifying characteristic of washover fan deposits (Reinson, 1979). Stratigraphie unit seven is a 10 meter thick, light grey, well indurated, locally iron-stained calcareous sandstone sharply overlying stratigraphie unit six (Fig.9). The sandstone is thinly laminated, moderately to poorly sorted, and very fine to medium-grained, with sorting and grain size varying upward. Trough cross-beds up to 5 meters in horizontal width with overall southwesterly paleocurrent orientations, and adjusted dips of 10 to 15 degrees, are the major sedimentary structure (Fig.9). One thin bed of grey, subangular to subrounded, pebble-sized mud clasts is located near the base. Wood fragments in the form of wavy stringers, and actual logs are common throughout. 37 I Interpret the southwesterly trending trough cross-bedded sandstone of stratigraphie unit seven to again represent fluvial distributary channel deposition (Cant, 1982; Walker and Cant, I960; Fig.10). The rapid variations in grain size and sorting indicate that channel velocities varied, possibly in response to seasonal conditions. Stratigraphie unit eight is 16.5 meters of interbedded sandstone and silty-shale (Fig.9). The base is marked by a 10 centimeter grey, subrounded to subangular, pebble-sized, shale mud clast zone. The lower 8 meters is composed of alternating wavy beds (Reineck and Singh, 1980) of silty-shale and sandstone. The sandstone is green, medium grained, and moderately to poorly sorted. The dominant sedimentary structures are small trough cross-beds less than 10 centimeters in horizontal width. Current ripples and sand dikes occur near the top of several sandstone interbeds. The wavy, silty-shale interbeds are scattered throughout, vary in thickness from 25 to 50 centimeters, are heavily bioturbated, and locally mudcracked (Fig.9). I interpret the interbedded sandstones and silty-shales of stratigraphie unit eight to again represent tidal flat deposition. Reineck and Singh (1980) indicate that wavy bedding requires conditions where both sand and mud are deposited and preserved. Wavy bedding in these deposits indicate that flow conditions rhythmically alternated between current activity and still-stand phases. During still-stand the tidal flat was heavily bioturbated and subject to desiccation. During high tide currents were strong enough to generate small megaripples and ripples. 38 stratigraphie unit nine is a 3 meter thick trough cross-bedded, andesitic conglomerate (Fig.9). Trough cross-beds are large, up to 5 meters in horizontal width, scour each other laterally at low angles, and have a dominant paleocurrent trend to the south-southwest (Flg.9). Grain sizes varies greatly verically with fairly well sorted sand-sized interbeds alternating with sand to pebble-sized conglomerates. The trough cross-bedded conglomerate in stratigraphie unit nine as first evidence of a braided river system in section one. Cant and Walker (1978) and Cant (1982) state that when compared to meandering systems, braided rivers have higher slopes, coarser bedload, and more easily eroded banks often controlled by rapidly flucuating discharge. The trough cross-beds here are up to 5 meters across and truncate each other laterally. Mudge (1972a) also describes this conglomerate and indicates that the clast composition greatly resembles the lithology of the Big Skunk Formation. Viele and Harris (1965) originally named the Big Skunk Formation for a thick volcanoclastic sequence of rocks that begins 130 meters from the base of the Two Medicine Formation along the Dearborn river 20 kilometers to the southeast of section one (Fig.2). Previous authors (Klepper, et.al., 1971; Lorenz, 1981; Viele and Harris, 1965) report that volcanic activity increased signifigantly during the Upper Cretaceous adjacent to the coastline in northwestern Montana (Fig.2). Volcanism provided a source of easily erodiable rock fragments supplied to the nearshore system by numerous fluvial channels. This conglomerate is first evidence of a great increase in grain size, and lack of sorting in the lower Two Medicine Formation indicating first influence of a local volcanic source on sedimentation and stream 39 gradients in the area of section one (Fig.4). This local volcanism could then have greatly increase the rate of regression. Stratigraphie unit nine is the last unit preserved in section one, and may well represent first evidence of middle Two Medicine fluvial sedimentation at section one. 40 Summary of Two Medicine Formation Interpretations The lower 100 meters of the Two Medicine Formation at section one (Fig.9) contains a thick sequence of lignitic, silty-shale that I interpret as coastal plain marsh deposits. Lenticular and thinly bedded sandstones scattered throughout the section (Fig.9), are fluvial distributary channel deposits that locally dissected the fine-grained marsh deposits of the coastal plain. The direct vertical location of the coastal plain deposits over the strandline sandstones of the Virgelle Formation indicate that the overall Upper Cretaceous regression has continued during Two Medicine deposition. The regression is interrupted by two transgressions that are recorded by dominantly foreshore sandstone deposits, and appear th have resulted from "in-place drowning" of the shoreline as proposed by Sanders and Kumar (1975). These transgressions may have resulted from basinal subsidence pulses, and/or a relative sealevel rise. Marine processes influenced sedimentation periodically in the Two Medicine Formation of section one (Fig.9). Presence of the pelecypod Corbula indicates that marine waters were periodically introduced creating brackish water conditions in strand adjacent marshes (Fig.10). The washover fan deposit indicates that periodic storms were strong enough to breach the shoreline and deposit sands on the adjacent * quiet water' marsh. 41 Tidal deposits in section one possibly indicate a microtidal range (0 to 2 meters). The protective feature that permitted tidal deposition may have been an embayment that Coleman and Prior (1982) indicate are often connected to the sea in modern environments, or a large subtidal region which would have greatly decreases wave energy. The conglomerate at the top of section one (Fig.9) indicates that volcanic sources began to emerge on or adjacent to the coastal plain resulting in increasing gradients and sediment supply to generate braided streams. This may represent the beginning of the final regression in the area of section one. 42 CHAPTER FOUR: STRATIGRAPHY AND SEDIMENTATION OF THE MONTANA GROUP AT SECTION TWO Section two is a partial section of the Montana Group exposed along the Pishkun canal two kilometers north of section one (Fig.4). Stratigraphie control of this section is limited by the poor exposure of the lower-most Two Medicine Formation and underlying Montana Group strata (Fig.11). Ten meters of the uppermost Virgelle Formation are exposed as the basal stratigraphie unit of section two (Fig.11). The sandstone is light grey, medium-grained, and moderate to well sorted. The dominant sedimentary structures are thin (.5 to .75 centimeters), parallelly laminated, laterally continuous, wedge-planar cross-beds. Individual wedge-sets dip at angles of less than 8 degrees, and dip southwesterly (Fig.11). The wedge-planar cross-bedded sandstone of the Virgelle Formation in section two has lithologie textures identical to several I have described in the Virgelle for section one (Fig.6), and I again interpret this as a beach foreshore deposit. The southerly low-angle dip indicates that the "local" east-west shoreline trended for at least two kilometers from section one to section two (Fig.4). A partial section of the lower Two Medicine Formation is exposed in section two (Fig.11). I divided this section into four stratigraphie units on the basis of lithologie textures and sedimentary structures (Fig.11). 43 strata U n it4 _ Low angle, wedge-plcncr cross-bedded sandstone. Strat. A compoundly cross-bedded sandstone, Unjt 3 many high angle dips. Strat.'“ Unit 2 Very fine-grained, thinly laminated sandstone. Covered Section 8 meters IO Interbedded shales and lignites, Strat. with scattered sandstone lenses. Unit I Wedge-planar cross-bedded sandstone Virgelle with local trough cross-beds. n=9 Figure 11. Virgelle and Two Medicine Formations at section two. Legend see Appendix one. 44 stratigraphie unit one is 7 meters of Interbedded lignite, grey silty-shale and sandstone (Fig.11). The lower part is a grey, fissile, noncalareous, highly carbonaceous silty-shale with scattered sandstone lenses, and thin silty-sand interbeds. The lenticular sandstones and thin sandstone interbeds are poorly exposed and contain no visible laminae or sedimentary structures. The upper part is composed of grey-green, fissile, highly carbonaceous shale, interbedded with well developed lignite (Fig.11). The lithologie textures of stratigraphie unit one closely resemble several other stratigraphie units in section one (Fig.9), and again represents coastal plain marsh-swamp deposition with local dlsection by distributary channels (Fig.10). Thick sequences of dominantly fine-grained deposits in the lower parts of both sections indicates that the coastal plain was extensive, and the carbonaceous character suggests that it was heavily vegetated and dominantly submergent (Rice, 1980). Stratigraphie unit two is a 3 meter thick dark grey, massive weathering, locally iron-stained, very thinly laminated (<.25 cm), silty sandstone (Figures 11 and 12). The sandstone is moderate to well sorted and very fine-grained. Plant material is abundant throughout, and bioturbation in the form of simple tube trails together with numerous small mounds and depressions are visible along bedding planes. Subparallel grooves with lengths of 10 to 15 centimeters, and depths below bedding planes of about 1 centimeter, and what appear to be ripple cross-bedding occur locally (Fig.12). Near the top is a lenticular, 20 centimeters by 4.5 meters, lignite deposit. 45 A low angle, wedge planar n= 16 cross-bedded sandstone. strat. U nit4 A trough and tabular planar cross-bedded sandstone. n= 230 Strat. Units A large scale, tabular planar cross-bedded sandstone with many small trough cross -beds n = 32 A very thinly laminated, very fine-grained sandstone, with local heavy bioturbation. Strat. Unit 2 I m eter • • • • I O Figure 12. Upper three strat. units of the Two Medicine Formation at section two. Legend see Appendix one. 46 The very thinly laminated, well sorted, fine-grained burrowed sandstone of stratigraphie unit two indicates deposition in a high energy9 upper flow regime environment. On modern coasts foreshores, washovers, and sand flats have these energy conditions (McCubbin, 1982; Reineck and Singh, 1980; Reinson, 1979). The horizontal lamination of the sandstone precludes foreshore deposition. I have previously identified a washover deposit in section one (Fig.9), with similiar lithologie textures (McCubbin, 1982; Reinson, 1979). The presence of bioturbation along bedding planes appears, however, to indicate that periods of high energy alternated with still-stands that permitted active burrowing. A third possible interpretation is tidal sandflat deposition. Reineck (1975) originally proposed the terms sandflat, mixed flats, and mudflats to divide the various tidal facies between low and high tide, with each representing different energy conditions. Sandflats are then closest to low tide and the highest energy deposits. Weimer et.al. (1982) indicates that laminated sand is often a characteristic of sand flats, and that bioturbation in this area should be lower that other tidal zones as a result of higher energy levels. The striations on some bedding surfaces could then be some type of groove or tool cast. The lenticular-shaped lignite deposit near the top of stratigraphie unit two may represent "coffee grounds" detrital peats as described by Lorenz (1981) and Cohen (1970),that have no root casts, lenticular nature, and rest on upper flow regime beach facies. 47 Sharply overlying stratigraphie unit two is a 10 meter thick complexly cross-bedded sandstone of stratigraphie unit three that is the main emphasis of this study. I have devoted an entire section to the detailed description and bedform analysis of this sandstone and so will leave further discussion until then. Above the complexly cross-bedded unit, is a 3.5 meter thick, light grey, bench forming sandstone here designated as stratigraphie unit four (Fig.12). The dominant sedimentary structures are thinly laminated (.5-.75 cm), low angle wedge-planar cross-beds. The adjusted dips average about 11 degrees, ang the dip is the the southwest. The low angle, wedge-planar cross-bedded sandstone with a unimodial southwesterly paleocurrent trend I here interpret as a beach foreshore deposit (Fig.8). The low-angle dip indicates that a northwest-southeast shoreline trended (Fig.11). The lithologie textures of stratigraphie unit four are very similiar to all previously interpreted foreshore deposits of both the Virgelle, and lower Two Medicine Formations, but shoreline trend varies slightly (Figures 6 and 9) indicating that the previously interpreted embayment had * local* shoreline variations. Because of only partial exposure of the Two Medicine Formation at section two (Fig.11) stratigraphie unit correlation with section one is difficult. I have, however, documented two transgressive beaches in section one and then can correlate (Fig.13) this foreshore deposit with one of them and also interpret transgressive deposition. The location of this foreshore deposit above the complexly cross-bedded sandstone is of key importance for later interpretation of depositional processes. 48 2 Km 8 meters » Foreshore deposit Figure 13. Correlation of Two Medicine Formation sections 49 Summary of Interpretations for Section Two The lithologie textures of the stratigraphie units in section two (Fig.11) are very similiar to those of section one and I interpret deposition in the same environments for both. The lithologie textures of stratigraphie unit three are, however, very different from any in section one. Foreshore deposits directly overlying the complexly cross-bedded sandstone indicate deposition very near the strandline, but a more detailed bedform analysis is needed to decipher the processes of deposition. In chapter five I will discuss the complexly cross-bedded sandstone of stratigraphie unit three in detail paying special attention to bedform morphologies, migration directions and variations through time. I will also use stratigraphie correlation to aid in interpreting the environment the complexly cross-bedded sandstone was deposited in. 50 CHAPTER FIVE: BEDFORM ANALYSIS OF COMPLEXLY CROSS-BEDDED SANDSTONE IN THE TWO MEDICINE FORMATION Descriptions of Stratigraphie Unit Three I have divided the complexly cross-bedded sandstone of stratigraphie unit three into three seperate units, labelled 3a, 3b, and 3c, based on cross-bed size and morphology (Fig.14), Cross-bed associations are described using a morphological classification, where simple cross-beds are one type only (i.e.tabular planar), compound are composed of two simple cross-beds of the same type (one bounded by the other), and complex are one type of cross-bed with two or more different types of superimposed cross-beds (Breed and Grow, 1977). When describing specific sedimentary structures, I have labelled the structure using a letter followed by a number and a directional coordinate. For example the tabular planar cross-bedded set in Plate 2d is listed I, .5, SE, where the letter (and associated number in several) identifies an individual pedestal-shaped 'hoodoo* (Fig.15; Plate 2a), the number indicates the height (in meters) above the base of the hoodoo the feature being described, and the directional reading is the side of the exposure the feature is located. Outcrop A1 (Fig.15; Plate 2a) is the westernmost hoodoo at section two (Fig.4); the rest of the hoodoos referenced in the text can then be located by using Figure 15 and Plate 3a as guides. Hopefully this system will allow later workers to return to section two (Fig.4) and rexamine the features discussed. 51 T Low angle cross-bedded sandstone . * * * • X with scattered high angle tabular S tra t. planar cross-beds Unit 3c Simply cross-bedded tabular planar and trough cross-bedded sandstone Strat. Unit 3b Complexly cross-bedded tabular planar sandstone S trat. Unit 3a .5 meter 1 I O Figure 14. Stratigraphie unit tnree subdivided by cross-bed morphologies. 52 03 02 A3 A4a U 1 Jl J2 U) Scale: 0 7(Tieters 0 PISHKIN CANAL Figure 15. Outcrops at section two, labelledfor idenlificolion ond location. stratigraphie unit 3a is a 2 meter thick compound, and complexly cross-bedded sandstone (Fig.14; Plate 2b and c) comprising the base of stratigraphie unit three and directly overlying stratigraphie unit two (Fig. 10, 11, and 14). The sandstone throughout stratigraphie unit three is fine to medium-grained (.25 to .4 mm), sorted to well-sorted, and grains are subangular to subrounded. The dominant large scale cross-bed type is tabular planar sets with well developed tangential toes (see plate 2d). Lamina are characteristically thin (.75 centimeters) and parallel in most preserved cross-beds with no visible grading. In the toes of many preserved sets, individual lamina become more distinct, and visibly thicken (Plate 2d). Within numerous tabular planar cross-bed sets (A1, 1.0, 1.4, NW; A2, .75, SW; A4, 1.1, NW) wavy zones several lamina thick are bounded above and below by tabular planar lamina (Appendix 2a). Individual laminae are laterally continuous, and several (A1, .75, 1.2, 1.6, NW; A4, .6, 1.1, 1.4, NE) are traceable for 30 meters (Plate 2b). Cross-bed directions range up to 20 degrees within individual laminae over entire exposed traces (Appendix 2b). Set thicknesses (bounding surface to bounding surface), of the tabular planar cross-beds, in stratigraphie unit 3a range from 60 centimeters (see plate 2d) to 125 centimeters (Plate 2c). I have identified twenty-nine tabular planar cross-bedded sets in stratigraphie unit 3a. Adjusted dips of these sets range from 2 to 28 degrees, with an average of 11.5 degrees. Fifteen sets are simply cross-bedded. Cross-beds trend unimodially to the southwest, with a subordinate mode to the north-northeast (Fig.l6a). 54 3 Superimposed cross-bed 3 Principle cross-bed Figure 16. (a) Simply cross-bedded sets in strat. unit 3a. (b-d) Azimuth trends of principle and superimposed cross-beds in strat, unit 3a. 55 The majority of cross-beds in stratigraphie unit 3a are compound and complexly cross-bedded (Plate 2b and c). Rubin and Hunter (1983) have recently devised new techniques for analysing compound and complex cross-bed assemblages, some of which I apply in this study. There are several examples of compound cross-bedding (Plate 2b) in stratigraphie unit 3a, with the majority being complex sets (Plate 2b). I measured the strike direction of the bounding surface of the smaller tabular planar set (Pig.l6b, solid mode), then the line of intersection between the bounding surface of the smaller tabular planar set and the cross-bed trend of this smaller set (Fig.l6b, hatched mode). Trough cross-beds, exposed in plan view, directly overlying individual tabular planar lamina are the dominant type of complex cross-bed association (Plate 2c, 3a and b). Widths of troughs range from from 30 to 120 centimeters (Plate 3a and b), with 50 centimeters the most common. Many of the complex cross-bed associations in stratigraphie unit 3a consist of only one trough cross-bed overlying a tabular planar set (Fig.16 c, d, and 17a). Axes of trough cross-beds for this group consistently trend nearly normal to the tabular planar cross-bed trend. Another group of trough cross-beds trend in the same quadrant as the tabular planar set they are superimposed on (Fig.lTd and I8a). On several individual tabular planar lamina whole groups of trough cross-beds are preserved, (Fig.17c) with bimodial trends to the southeast and northwest over the southwest trending tabular planar cross-bed (Fig.17c). Several trough cross-bedded sets are exposed only in vertical section, and bounded above and below by tabular planar cross-beds. For these sets I have used Rubin and Hunter's (1983) 56 9 0 n=2 n=2 a b VZ//\ = Superimposed cross-bed = Principle cross-bed Figure 17. (a-d) Principle and superimposed cross-bed relationships at strat. uni 3a. 57 technique of meaauring and plotting trough axes (Fig.17b, arrow for Interpretation section). Near the top of stratigraphie unit 3a several cosets containing trough cross-beds truncate each other laterally and are bounded above and below by tabular planar cross-beds (Plate 3c, arrows). Trough axes trend near normal to tabular planar cross-bed azimuths (Fig.18b). Within several tabular planar cross-bed sets In stratigraphie unit 3a (A2, .5, SW; A3, .75, SE) local wavy bounding surfaces are traceable for short distances with tabular planar lamina above and below (Plate 3d). Cross-bed trends are consistent above, below, or on either side of these Internal bounding surfaces. Additional sedimentary features occur In stratigraphie unit 3a (Plate 4a-d). Plant fragments and logs are preserved locally (A1, .75, NW; A1, 1.2, SE; A2, 1.7, SW). Cross-bed lamina wrap around the log with tabular planar lamina above and below (Plate 4a). Thin zones of thinly laminated, carbonaceous, often contorted shales locally seperate bounding surfaces in stratigraphie unit 3a (Plate 4b; A1, 1.3, SW; A2, 1.5, NE) and are tracable between hoodoos 10 meters apart. Ball and pillow structures are visible In one fine-grained zone (Plate 4d). Several vertical burrows occur within cross-bed sets in stratigraphie unit 3a (Plate 5c; A6, 1.2, SE). Burrows are simple tubes with diameters of 1 centimeter and are 1 to 20 centimeters high. Several packages of deformed lamina are 5 to 8 lamina thick, have lateral traces of 3 to 5 centimeters, are bounded by tabular planar cross-beds, and vertical thicknesses of 10 centimeters (Plate 4d; A6, 1.6, SE). 58 n=G6 c d Y///\ Superimposed cross-bed = Principle cross-bed Figure 18. (a-c) Principle and superimposed crossbed relationships at strat. unit 3a. (c-d) Vertical relationships of cross bed azimuths at strat. unit 3b. 59 stratigraphie unit 3b is a 6.5 meter thick unit composed of low angle, simply cross-bedded trough and tabular planar sets (Fig.13, plate 5a-d). Lamina are characteristically .75 centimeters thick and ungraded (Plate 5a, b, and d), but within some sets (Plate 5c) lamina are up to 1.5 centimeters thick. Lamina in some sets can be traced laterally up to 6 meters (Plate 5a), and have a crescentic shape in outcrop. The majority of the sets have lateral traces of less than 2 meters (Plate 5a-c). Tabular planar and low angle trough cross-beds are interspersed throughout stratigraphie unit 3b. I have arbitrarily divided the unit into three groups of equal vertical thickness and plotted cross-bed azimuth trends for each (Fig.l8c, d, 19a). The lowest component, 2 to 4.16 meters from the base, has a dominant cross-bed mode to the southeast with two subordinate modes to the southwest and northeast (Fig.iSc). Cross-beds in the zone 4.2 to 6.3 meters from the base have a dominant unimodial trend to the south-southeast with scattered modes to the southwest and southeast, and a bipolar component to the northwest (Fig.l8d). The upper 6.3 to 8.5 meter zone has cross-bed azimuths with dominant modes to the southwest and southeast, and minor modes to the west and northeast (Fig.19a). Cross-beds in the lower part of stratigraphie unit 3b truncate each other laterally (Plate 5a; I, 2.3, NW) and have a unimodial azimuth trend to the southwest (Fig.19b). Plate 5a (H, 1-2.5, SE) illustrates a characteristic outcrop; trough cross-beds are very low angle appoaching tabular planar, axes are commonly preserved, and cross-beds trend strongly to the southeast with a mode to the southwest and a minor mode 60 n=3 n=IO n=2 c = Cross-bed - Individual cross-bed azimuth Figure 19. (a-c) Cross-bed azimuth relationships of sets in strat. unit 3b. (d) Azimuth trends of two adjacent tabular planar sets. 61 to the northwest (Fig.19c). Near the top of stratigraphie unit 3b (Plate 5d, A1, 4.2, SW) bimodial cross-bed sets trend southeast and northwest with minor modes to the north-northeast and southwest (Fig.20a). Locally, tabular planar cosets are preserved that directly overly each other (see plate 5c) and have very similiar azimuth trends to the southeast (Fig.lÇd). The thickness of the lower set is 40 centimeters, and the upper 25 centimeters. In one set basal lamina in two adjacent troughs are laterally continuous, and azimuth trends vary by 200 degrees (Appendix 2c; Fig.20b). Several cosets in stratigraphie unit 3b have tabular planar cross-bed sets separated by a well developed bounding surface, but overlying foresets then truncate the bounding surface resulting in the development of one set (Plate 6a, lower arrows; J2, 3.1, SW; C, lower arrows; J2, 3*1, SW; C, .5, SE). The cross-bed azimuths of these two sets are to the southeast and vary by only 14 degrees, with (a) in the figure representing the overlying set (Fig.20c). Within this same outcrop near the top (Plate 6a, upper arrow) a tabular planar cross-bed set has * normal' concave up cross-beds but near the upper bounding surface the lamina locally become concave down. Several interesting sedimentary features occur in stratigraphie unit 3b. Asymmetrical, straight-crested ripples with indexes of 11 occur on a bounding surface .5 meters from the top (plate 6b; K, .5, SW). Also one bounding surface is bioturbated (Plate 6c; A4a, 3.5, SE), burrows are circular in plan view and have concentrations of mica on rims. 62 n=2 n=l4 d = Cross-bed ^ = Individual cross-bed azimuth Figure 20. (a) Cross-bed azimuth relationship at strat. unit 3b. (b-c) Two cross-bed azimuths, (d) Azimuth trends of low angle cross-beds at strat. unit 3c. 63 stratigraphie unit 3c is the upper 1.5 meters of stratigraphie unit three9 and sedimentary features differ eonsiderably from underlying eross-beds of stratigraphie units 3a and b (Plate 6d). Lamina are from 2 to 6 eentimeters thiek and erumble in outcrop. The dominant sedimentary structures are horizontal lamina and low angle, locally wedge-planar eross-beds (< 8 degrees). The low angle cross-beds trend to the southwest with a mode to the southeast and several to the northeast. Locally, however, steeply dipping tabular planar cross-beds occur with azimuth trends bimodially to the southeast and northwest, with minor modes to the southwest (fig.21a). Cross-bed dips are 16 to 24 degrees with an average of 22 degrees. Stratigraphie unit four directly overlies stratigraphie unit 3c (Fig.10 and 11). For a complete discussion see chapter four. 64 n = 12 I = Cross-bed Figure 21. Azimuth trends of high angle, tabular planar cross-beds at strat. unit 3c. 65 Interpretations for Complexly Cross-bedded Sandstone The complexly cross-bedded sandstone of stratigraphie unit three clearly records an ancient dunefield. Recent work indicates that a wide variety of modern environments with adequate sand supply and relatively high current velocities have actively migrating dunefields (Dalrymple et.al., 1975; Flemming, 1978, 1981; Harms et.al., 1982; McCave, 1971; Rubin and Hunter, 1983; Rubin and McCulloch, 1980; Swift, 1975). Hunter (1981) shows how similiar eolian large scale cross-beds are to subaqueous cross-beds, and that small scale stratification types are the best way to distinguish eolian from subaqueous sedimentary structures in the rock record. In this section I will use detailed bedform analysis to discuss bedform morphologies, migration directions, superimposed dunes, and dunefield variations through time, and also analysis of small scale stratification to interpret processes of deposition* Features in stratigraphie unit three (Fig.14) possess structures attributed to both subaqueous and eolian processes. Analysis of the sedimentary structures leads me to conclude that the complexly cross-bedded sandstone of stratigraphie unit three was deposited by eolian processes. Sediment transport and resulting dune development that exist in water and air fundamentally differ, resulting in different types of small scale stratification that appear to be reliable indicators of depositional environment (Hunter, 1977a, 1977b, 1981, 1985; Kocurek and Dott, 1981). Hunter (1977) shows that sediments moving over dunes deposit three main types of strata: 1) grainfall strata, generated by 66 seperatlon of flow at the dune-crest and subsequent suspension deposition of previously saltating grains on the calm lee-slope, 2) sandflow strata* generated from avalanching of sand down an oversteepened slipface, and 3) climbing translatent strata generated from the migration of ripples under conditions of net deposition. Grainfall lamina occur in both eolian and subaqueous dune slipfaces, and are characteristically thin, laterally tabular, and ungraded (Hunter, 1977). Kocurek and Dott (1981) indicate that grainfall strata deposited in water can not be distinguished from those deposited by wind. Sandflow cross-strata commonly form on subaqueous and eolian dunes and are identified by near angle-of-repose dip angles, great thickness ( commonly > 1 centimeter), sharp contacts, and lateral pinchouts (Hunter, 1977a, 1977b, 1981, 1985; Kocurek and Dott, 1981). Recent work by Hunter (1985) on subaqueous dunes with slipfaces of less that 2 meters indicates that foresets are composed almost entirely of sandflow cross-strata that may result from the nearly continuous avalanching of ripples over dune crests, suggesting a reliable criteria for distinguishing processes responsible for small dune cross-bedding. Subaqueous and eolian sandflow cross-strata deposited on slipfaces less than 2 meters high also differ in that subaqueous tend to be several times wider than slipface height, whereas eolian are narrower have well defined lateral pinch-outs and are commonly interbedded with grainfall deposits (Hunter, 1981). Climbing translatent strata, where identifiable, are the most reliable indicator of processes of deposition (Hunter, 1977a, 1977b, 1981, 1985; Kocurek and Dott, 1981). Subcritically climbing eolian strata are characterized by low dip 67 angles» thinness, slmiliarity in thickness, sharp contacts and lack of well defined foresets because angle of climb is low (Hunter, 1985). A large percentage of lamina in stratigraphie unit 3a are thinly laminated, have dips from 13 to 28 degrees, and pinch out laterally (plate 2c), suggesting sandflow deposition (Hunter, 1977, 1981). The processes that deposited these sandflow strata are indiscriminant. In modern dunes Kocurek and Dott (1981) have attempted to use individual sandflow thickness to determine minimum dune slipface height, with limited success. By using their technique, I can infer a minimum height of one meter for bedforms in stratigraphie unit 3a. Locally well developed lobate sandflow structures are bounded above and below by parallel, laterally tabular lamina that I interpret as grainfall deposits (Plate 4d). This relationship indicates that grainfall and sandflow processes alternated during slipface migration, or that deposits were zoned on the slipface (Fig.22) and that truncation by ensueing dune migration eroded the majority of high angle sandflow strata. Preservation of low angle grainfall deposits indicates that dunes were small enough for grainfall sands to reach basal aprons. Recent work by Hunter (1981) suggests that this relationship occurs in dunes less than 10 meters high. Thinly laminated, parallel, tabular planar lamina with lateral traces of 5 to 10 meters commonly are interspersed with interpreted sandflow deposits in stratigraphie unit 3a (Plate 2c), and were probably formed by grainfall deposition (Hunter, 1977a, 1977b, 1981 ,1985: Kocurek and Dott, 1981). In plate 3d a topset is preserved indicating that dune height of this set was about one meter, total preservation of 68 I I Groinfoll Deposits II I II Grainflow Deposits 13^ Wind Ripple Deposits Figure 22. Distribution of stratification types on a simple crescentic dune. After Hunter (1977a). 69 a small dune lee face permits analysis of strata types in reference to modern small dunes and interpretation of the processes of deposition (Hunter y 1985). Well developed tangetial toes that can be traced directly to foreset lamina indicate deposition by grainfall, and argues strongly for eolian deposition (Hunter, 1985). Within the low angle toesets of many tabular planar sets are 'packages* of even, very distinct, one centimeter thick lamina I interpret as eolian climbing translatent strata (Fig.23; Plate 2d, arrow). Structures formed by subcritically climbing wind ripples are common in both modern and ancient eolian sands (Hunter, 1981) and commonly are preserved on basal dune aprons, as seen in stratigraphie unit 3a. Several low dipping, thinly laminated tabular planar cross-beds have large lateral traces contridicting the interpretation of deposition by relatively small bedforms. I suggest that these deposits are planebed lamination which Hunter (1981) indicates are formed by traction load by high winds, and look very similiar to grainfall strata. The low dip angle of these strata suggests an original hummocky appearance. Cross-bed thicknesses (bounding surface to bounding surface) of the majority of tabular planar sets range from 60 (Plate 2d) to 150 centimeters (Plate 2c) and probably only represents a small percentage of the original dune height (Hunter, 1981; Kocurek and Dott, 1981). Kocurek and Dott (1981) indicate that the preserved thickness of migrating dunes is controlled by migration rate and sand supply. The dominance of shallow dips for the majority of the foresets in stratigraphie unit 3a suggests that much of the original high-angle 70 (A) Figure 23. Contrasting climbing translatent strata formed by migrating wind ripples (A), and water ripples (B). (A) are tnin, uniform, with few visible foresets and low amplitudes. (B) are thicker, have abundant foresets and nigh amplitudes. After Kocurek and Dott (1981). 71 upper slipface have been beveled off from erosion by of uppercurrent dunes. Azimuth variations laterally along individual foresets in unit 3a (Appendix 2b) indicate that the bedform crestlines were wavy (Fig.24). Wavy-crested bedforms are common features in flume experiments (Costello and Southard, 1981), modern marine environments (Dalrymple et.al., 1978, Flemming, 1980; Rubin and NcCulloch, 1980), and in ancient sandstone sequences interpreted to be eolian (Rubin and Hunter, 1983). Fifteen of the wavy-crested bedforms in stratigraphie unit 3& (Fig.14) show no evidence of superimposition by smaller bedforms, and have a strong unimodial azimuth trend to the southwest (Fig.l6a). The unimodial trend and vertical stacking of the tabular planar cosets suggests that the wavy-crested bedforms migrated in trains to the southwest for extended periods of time (Hunter, 1981). The majority of the sets in stratigraphie unit 3a are, however, complexly cross-bedded with several examples of compound cross-bedding (Breed and Grow, 1977; Plate 2b, c, and 4a, b). Rubin and Hunter (1984) indicate that compound differ from simple cross-beds In that some type of erosion occurs on the lee slopes of the larger forms, resulting in internal bounding surfaces. Several processes can produce lee slope erosion: 1) flow changes (shifting winds, reversing currents) that create reactivation surfaces, 2) or local erosion generated by small dunes that migrate across larger dune lee faces. The compound and complex cross-bedding is the result of the migration of small bedforms on the slipfaces of larger bedforms. Many authors have documented superimposed bedform migration on lee sides of larger bedforms in eolian 72 V, 1 Figure 24. Wavy-crested dunes and the cross-beds they deposit After Harms et.al. (1982). 73 and subaqueous sandstones by identifying complex cross-bed associations (Brookfield, 1977; Kocurek, 1981; Rubin and Hunter, 1983). Rubin and McCulloch (1980) suggest that in equilibium flows superimposition of bedforms is the rule rather than the exception. Many superimposed bedforms In stratigraphie unit 3a have cross-bed azimuth trends nearly perpendicular to the underlying larger wavy-crested bedform (Fig.l6b-d, 17a, 25a). This trend indicates large normal components of sand transport existed, and that the overall bedform assemblage migrated laterally (obliquely) in a direction between that of the large and smaller bedform azimuth trends (Fig.25a). Rubin and Hunter (1985) have identified similiar migration trends for superimposed dunes in the eolian Entrada and Navajo sandstones. Identifying these dune parallel components of transport, indicates that the dunes were not perfectly transverse to the current, but possibly more oblique. Several different mechanisms may have been responsible for the migration of small bedforms on the lee-slopes of larger bedforms in stratigraphie unit 3a. As bedforms faces vary from transverse to more oblique to dominant current direction, lee slope currents in the form of heliocoidal eddies greatly increase and become dominant, possibly resulting in superimposed lee-dune migration (Fig.26; Blatt et.al., 1980; Hunter, 1981; Kocurek and Dott, 1980; Rubin and Hunter, 1985). Rubin and Hunter (1985) indicate that a second mechanism may be two directions of current movement, here being to the southwest with several periodic smaller components to the west and east (Fig.l6b-d, 17a). Superimposed bedform migration in both east and west large bedform 74 a Figure 25, (a) Laterally migrating superimposed dune and resulting cross-bed association, (b) Dune and associated scour pits. After Rubin and Hunter (1983). 75 Y V Figure 26. Schematic maps showing relation of eddy winds to lee- slope orientation based on modern dune studies. Hatchured line represents lee-slope. U= direction of free wind, V= direction of lee-eddy wind near ground, X= overall dune migration, Y= direction of lee-slope migration at point. Modified after Hunter (1981). 76 normal directions argues for a heliocoidal eddy mechanism (Fig.26). Several large wavy-crested bedforms contain complex groups of smaller superimposed bedforms with widely varying cross-bed azimuths separated by only a few foreset lamina (Fig. 17c). The two superimposed dunes trend 100 to 130 degrees to the larger dune (Fig. 17c) and are preserved very low on the slipface suggesting that the dunes migrated at oblique angles up the slipface as a result of heliocoidal lee eddies (Hunter, 1981; Kocurek and Dott, 1981; Rubin and Hunter, 1985). Locally near the top of stratigraphie unit 3a (Plate 3c, arrows) several sets of cross-strata contain local trough cross-beds that scour each other laterally and are bounded above and below by tabular planar foresets. This type of cross-strata was deposited by bedforms with laterally migrating scour pits and intervening spurs (Fig.25b, personal comm. David Rubin). The trough above the upper arrow in plate 3c is totally preserved while preceding ones are partially scoured Indicating it is the last scour pit in a train and was preserved as migatlon ceased (Rubin and Hunter, 1983). Structures deposited by migrating scour pits occur locally in several other dunes but all can only be traced short distances, indicating that migration of scour pits was no continuous. The axis of the preserved scour pit trough is 90 degrees off the trend of the large bedform (Fig.iSb), indicating migration in a direction normal to the base of the large bedform (Fig.25b); the resulting overall bedform assemblage migration direction was then at an angle between the two azimuths (Rubin and Hunter, 1983). Scour pits on wavy-crested bedforms have been identified in flume experiments (Costello and Southard, 1981), modern tidal environments (Dalrymple 77 et.al., 1978), and in ancient eolian sequences (Rubin and Hunter, 1983, 1984). The local vertical variation In bedform morphology from wavy-crested bedforms with superimposed bedforms to wavy-crested bedforms with well developed scour pits Indicates that wind velocities periodically Increased (Rubin and McCulloch, 1980; Rubin, 1984). In modern coastal duneflelds seasonal winds commonly reach speeds of 9 to 13 m/sec and storm winds 22 m/sec. I suggest that this local change In bedform morphology was the result of high velocity storms that blew In the same direction as normal bedform migration (Hunter et.al., 1983). Hunter et.al. (1983) Indicate that dune migration Is dominantly controlled by storm winds on the Oregon coast. In several tabular planar sets well developed Internal bounding surfaces are visible (Plate 3d). I Interpret these as reactivation surfaces, which McCabe and Jones (1977) describe as "an Inclined surface within a cross-bed set that separates adjacent foresets with similiar orientations, and truncates lower foreset lamina". Reactivation surfaces commonly form In modern environments by flow reversals (Blatt et.al., 1980), changes In stage (McCabe and Jones, 1977), and by the random erostonal behavior of Interacting bedforms (Allen, 1973; Fig.27). The wavlness of the reactivation surfaces suggests that they were formed by flow reversals, where originally the lamina were hardened possibly by moisture, then a storm Incised the slipface causing differential erosion, with avalanching resumed afterward. Wood fragments and actual logs are locally scattered throughout stratigraphie unit 3a (Plate 4a). Lower dune leeface morphology visibly changes locally In response to this obstruction (Plate 4a). Eolian dune systems 78 C R E S T ROUNDED FLOW Figure 27. (a) Reactivation surface formed when crest of leading bedform is influenced by upcurrent bedform lee eddies. After Allen (1973). (b) Formation of reactivation surface by flow reversals. After Blatt et.al. (1980). 79 along modern coasts are often partly vegetated, indicating that this bedform migrated over a fallen tree, and possibly suggesting that the coast was locally heavily vegetated. Zones of thinly laminated, carbonaceous, shales occur locally on bounding surfaces (Plate 4b). Kocurek (1981) indicates that eolian cross-strata are puncuated by bounding surfaces, the most extensive of which are overlain by horizontal interdune deposits. Ahlbrandt and Fryberger (1981) and Kocurek (1981) indicate that the primary sedimentation features in interdune deposits are often modified by loading from migrating dunes, and that carbonaceous character, and thin laminations indicate vegetation and standing water. Several examples of simple vertical burrows exist in stratigraphie unit 3a (Plate 4c). Local bioturbation is also very common in modern and interpreted ancient eolian deposits and is characteristically simple like the type in stratigraphie unit 3a Ahlbrandt et.al. (1978) have indentified a large suite of organisms that commonly inhabit in eolian dunes. Stratigraphie unit 3b is a 6.5 meter thick unit composed of low angle trough and tabular planar cross-bedded sandstones, with no superimposed bedforms (Fig.13; plates 5a-d). Lamina throughout (Fig.14) are dominatly thinly laminated, laterally tabular, and parallel (Plate 5a, b, c), indicating they are grainfall deposits (Hunter, 1977a, 1977b, 1981; Kocurek and Dott, 1981). The dominance of grainfall preservation suggests that orginal dunes were relatively small (Hunter, 1981; Kocurek and Dott, 1981). This decrease in bedform size from stratigraphie unit 3a may be the result of a decrease in sediment transport rate or velocity (Rubin and 80 McCulloch, 1980). Plan view morphologies indicate that tabular planar cross-beds were deposited by straight-crested bedforms, and trough cross-strata by lunate shaped bedforms (Fig.28). Previous authors indicate that straight-crested and lunate dunes often exist in equilibrium in many modern environments (Dalrymple et.al., 1978; Reineck and Singh, 1980; Rubin and Mcculloch, 1980). Lateral lamina traces indicate that no bedforms have preserved crestlines longer than 6 meters, suggesting short lateral traces, or that overlying bedforms scoured out crestlines. Bedforms morphologies have changed from the wavy-crested bedforms with local well developed scour pits in stratigraphie unit to dominantly straight-crested to slightly wavy-crested dunes in stratigraphie unit 3b, suggesting that a decrease in velocity has occurred through time (Costello and Southard, 1981; Rubin and McCulloch, 1980). One possible explaination for this is that storm winds were responsible for the majority of coastal dune migration in stratigraphie unit 3a, and have subsequently decreased in intensity in 3b (Hunter et.al., 1983). Azimuth analysis of the three vertical components (Fig.l8c, d, 19a) indicates that a dominant mode to the southeast existed throughout, differing from the dominant mode to the southeast in stratigraphie unit 3a (Fig.l6a-c, 17a,). This variation in wind direction and subsequent bedform migration may be related to changes in storm wind directions. 81 (a) Cross-bedding produced by migration of straight- Figure 28. crested dunes, (b) Cross-bedding produced by migration of lunate dunes. After Reineck and Singh (1980). 82 A large mode of azimuths are clustered around the dominant southeast mode (Fig. I8d, 19a). This 'scatter' could result from local migration direction variations resulting from dune to dune Interactions (Allen, 1973). A strong bipolar mode to the northwest exists throughout the unit (Flg.lSd, 19a, c, 20a). This may Indicate that seasonal winds Influenced bedform migration part of the year and storm winds the other part of the year (Hunter, et.al*, 1983). This seasonal variation may also have been present during deposition of stratigraphie unit 3a, but bedforms where larger and contained more sand volume resulting In a larger relaxation time where secondary, seasonal wind deposits might not then be preserved (Allen and Friend, 1976; Rubin and McCulloch, 1980). This equilibrium relationship Is, however. Impossible to know from the rock record In unit 3& (Rubin and McCulloch, 1980). A secondary mode also exists to the west-southwest (Flg.lSc, 19c, 20a). This component correlates well with several superimposed bedform and scour pit azimuths in stratigraphie unit 3& (Fig. 16c, 17b, 18a, b). This may Indicate that a secondary current direction was resonslble for some superimposed bedform migration Instead of lee-eddles resulting from large bedform wavy-crestlines* In plate 5c an excellent example of tabular planar sets deposited by straight-crested bedforms that are subcritically climbing. The trends of the two bedforms are almost Identical (Fig.20c) and they dip at 14 and 16 degrees. These bedforms migrated right up the back of each other. 83 other interesting cross-bedding relationships also exist in stratigraphie unit 3b (Fig.15, Appendix 2c). Two trough cross-beds roll over and join (have dips of 5 and 6 degrees), this indicates that some type of double crescentic bedform deposited this set. Several examples of variable bedform migration speeds are also visible in stratigraphie unit 3b (Plate 6a, lower arrows;) dips are 8 to 6 degrees. Overtaking of one bedform by another can be explained by the fact that bedform height increases with flow velocity so that at a constant depth big dunes are generated by flows having high sedimentation rates, and then will migrate faster than little dunes (Rubin and McCulloch, 1980), indicating that upper bedform was larger and subsequentally overtook the lower. Several interesting sedimentary features also occur in stratigraphie unit 3b (Plate 6b and c). Ripples occur very near the top of the unit and have a asymmetrical, straight-crested shape similiar to wave ripples, with indexes of 11 strongly suggesting they were deposited in the subaquoeus environment of an interdune. Bioturbation (Plate 6c) is also locally very extensive in upper areas of stratigraphie unit 3b. This bioturbation occurs on bounding surfaces, and is very different from that described in stratigraphie unit 3a (Plate 4c), and looks subaqueous in origin. I interpret this as interdune bioturbation. 84 Summary of Interpretations of Stratigraphie Units 3a and 3b I interpret deposition by eolian processes for the complexly cross-bedded sandstone of stratigraphie units 3a and 3b. I sight as evidence the rarity of sandflow deposits on small bedforms, the occurence of carbonaceous interdune deposits along bound surfaces exclusively, the presence of only very local simple trace fossils, the lack of mud drapes and stratigraphie relationships. The dominantly tabular planar compound and complexly cross-bedded sandstone of stratigraphie unit 3& was deposited by wavy-crested bedforms less than 10 meters high that were largely truncated. Superimposed bedforms were generated by large bedform normal heliocoidal eddies. The wind direction was dominantly from the northeast and periodically increased in velocity resulting in development of lee-face scour pits and spurs. Bedform migration may have been largely controlled by storm winds, with Increasing intensity generating scour pitted bedforms (Hunter et.al., 1983). The simply cross-bedded tabular planar and trough cross-bedded sandstone of stratigraphie unit 3b was deposited by straight-crested and lunate bedforms. The dominance of grainfall deposits indicates that bedform size has decreased through time possibly as the result of velocity or sediment transport decrease related to storm wind Intensity. Azimuth directions of cross-beds indicate that winds were variable during deposition of stratigraphie unit 3b. The dominant opposing wind directions may have been present throughout deposition of stratigraphie unit three, but the larger bedform size in stratigraphie unit 3a would 85 result in a larger relaxation time, and winds that blew between storms might not have been strong enough to modify bedforms. Stratigraphie units 3a and 3b were then preserved during transgression which resulted from **in-place drowning" (Figures 29 and 30). I intrepret the low angle locally wedge-planar cross-beds, with a dominant azimuth trend to the southwest, of stratigraphie unit 3c as beach foreshore deposits Indicating a northwest-southeast shoreline trended (Fig.20d; Reineck and Singh, 1980). The high angle tabular planar cross-beds with seaward and landward dipping modes are then backshore-berm deposits, that Reineck and Singh (1980) indicate are made up of complex bedding structures that fill up beach runnels resulting in cross-bedding that dips up to 25 degrees in land and seaward directions. These backshore and foreshore deposits directly overly the eolian dunes indicating that transgression has occurred (Figures 29 and 30). I interpret an "in-place drowning" mechanism (Kumar and Sanders, 1975) for the preservation of the eolian coastal dune system. Rapid subsidence may have resulted from sediment loading of the foredeep that existed along the Cretaceous shoreline in western Montana (Lorenz, 1981). 86 29. Transgress,' ve , 87 EMBAYMENT (?) SAND FLATS DUNE FIELD BACKSHORE f o r e s h o r e t Figure 30. Deposit!onal environments on a modern barrier island Modified after Dickinson et.al. 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Zarillo, G.A., 1982, Stability of bedforms in a tidal environment: Marine Geology, v.48, p.337-351. 97 APPENDIX ONE: Legend Rock Type ; Sandstone S iltsto n e Shale Lignite Covered Sedimentary Structures : Wedge-planar cross-beds Trough cross-beds Hummocky cross-strata Lenticular sandstone Wavy-bedded shale Shell coquina Log Burrows Mud chips Mud cracks Climbing ripples %\%% Diplocerterion spreiten Plant fragments >>» Grooves 98 APPENDIX TWO: Sketches 5 cm i 45 cm 0 lOm b 2.5 m 99 PLATE ONE m / y ! & «/* f? f.' 100 PLATE TWO *4? a: ,;vr IH'.^ »■ • ,rî4 PLATE THREE jQ ^ ^ » . ; 'f: %, ! .'V'.' 03 m m ■ n 102 PLATE FOUR - ■a 103 PLATE FIVE e à ' r m * . \ ■■tCi>-.V .il.:,' r a \ ’ tt~ 104 PLATE SIX * * y V ■ '■ 'ii;''i''Mr i ! » 1 .:.lf I ! ï % \ _ \ Àc\ ,(t ' ' '/ T ' , ! ! '):) [ i # : u