Erg-Margin Deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, Southern Utah

Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah

ttcvrn rr ^i c!^^^^ 1 Institute of General Geology, University of Copenhagen,

ABSTRACT mate. The stacked drying-upward or drying- sequences of the ancient erg-margin deposits and wetting-upward sequences suggest that this (2) to discuss the paleogeography of the erg- The Moenave Formation (Dinosaur Can- long-term climatic change was characterized margin region and relate erg-margin dynamics yon Member) and the closely associated by a number of fluctuations between rela- to climatically controlled variations in vegeta- Wingate Sandstone in southern Utah repre- tively arid and humid conditions. These cli- tion cover, sediment yield, fluvial-transport sent the southwestern erg margin of the matic fluctuations apparently controlled both characteristics, and degree of eolian reworking Lower Jurassic Wingate erg. The erg-margin sediment yield to the basin and the deposi- and deposition. deposits, which extend across a 50- to 100- tional conditions in the erg-margin region. km-wide region, are composed of dune, eo- During arid intervals, ephemeral braided GEOLOGICAL SETTING AND lian sand-sheet, mud-flanked eolian, sabkha, streams transported sandy bed-load into the STRATIGRAPHY ephemeral-stream and lacustrine facies. The erg-margin area, where eolian reworking took succession is divided into a basal erg-margin place. During humid intervals, sheet-floods The erg deposits of the Wingate Sandstone association (8-27 m) dominated by eolian fa- and high-sinuosity rivers transported a more cover at least 110,000 km2 in northern Arizona, cies and an overlying erg-margin association mud-rich sediment load into the basin, where southern Utah, and western Coloradp (Figs. 2, (as much as 90 m thick) composed mainly of eolian activity was minimal. 3; Blakey, 1988; Blakey and others, 1988). The fluvial and lacustrine facies. Wingate Sandstone comprises a complex as- The sediments of the basal association are INTRODUCTION semblage of facies that reflect the initiation, pe- composed of three drying-upward or locally riodic growth, and eventual destruction of a drying-wetting-upward sequences that can Eolian deposits of ancient sand seas have been large Jurassic erg (Clemmensen and Blakey, be traced from the erg margin 100 km slightly described recently (Thompson, 1969; Walker 1989). Throughout much of the extent of that upwind (that is, toward the west-northwest) and Harms, 1972; Kocurek, 1981; Steele, 1983; erg, deposits consist of large-scale, cross-strati- into the outer erg-margin zone. Sequences of Hunter and Rubin, 1983; Loope, 1984), but less fied, fine-grained sandstone formed by migrating the inner erg-margin zone contain dune facies attention has been paid to the deposits of erg dunes and draas (Fig. 3). Along the southwest- in their upper parts, sequences in the inter- margins. Clemmensen and Abrahamsen (1983) ern edge of the erg, small dune, eolian sand- mediate erg-margin zone contain sand-sheet and Clemmensen and J. Hegner (unpub. data) sheet, sabkha, lacustrine, and ephemeral-river facies, and those in the outer erg-margin zone described Permian erg-margin or side-erg depos- deposits sharply increase in abundance (Ed- contain sabkha facies. The basal association its formed between alluvial fans and a narrow wards, 1985). The zone of transition marks an thins markedly toward the west-northwest. elongate sand sea in a fault-bounded basin in area of intertonguing between the Wingate Sediments of the overlying association con- Scotland; Hubert and Mertz (1984) described Sandstone and coeval Moenave Formation. tain up to six drying-upward or drying- rather similar deposits from the Upper Triassic- Thus, along a line of section from northeast to wetting-upward sequences but are appar- Lower Jurassic of Nova Scotia. Porter (1987) southwest, the erg deposits of the Wingate Sand- ently only fully preserved in the outer described the fore-erg sediments of the Lower stone intertongues with the erg-margin deposits erg-margin zone. Individual sequences show Jurassic Aztec Sandstone of the southwest of the Dinosaur Canyon Member (Moenave much lateral variation but characteristically United States. Formation) (Fig. 3). This area of facies change comprises the study area for this paper. contain lower, muddy, ephemeral-stream de- Basal strata of the well-exposed Lower Juras- posits and upper, sandy, ephemeral-stream sic Glen Canyon Group in southern Utah (Fig. Cross-stratification studies document that the deposits; sand-sheet deposits; or incipient- 1) contain examples of marginal eolian, fluvial, eolian dunes in the southern part of the Wingate dune deposits. and lacustrine sediments that formed along the erg system migrated to the southeast (Clem- The general time trend is interpreted as a southwestern edge or lateral margin of the large mensen and Blakey, 1989) and that most of the gradual change toward a more humid cli- Wingate erg. The deposits comprise a facies as- ephemeral streams flowed northerly (Edwards, sociation which has been only rarely described 1985; Olsen, 1989). Coupled with the above from the geologic record. The evolving se- stratigraphic relationships, this suggests that quences are apparently related to climatic cycles. winds blowing across the erg margin reworked *Present address: Geological Survey of Greenland, 0ster Voldgade 10, DK-1350 Copenhagen K, The aim of the paper is twofold: (1) to de- fluvial sand and redeposited it as dunes in the Denmark. scribe and interpret the sedimentary facies and zone of transition.

Geological Society of America Bulletin, v. 101, p. 759-773, 20 figs., June 1989.

759

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SEDIMENTARY FACIES

General

A detailed study of the Wingate-Moenave transition revealed five main sedimentary successions in the ancient erg-margin area. These rocks are assigned to seven major facies deposited during conditions ranging from dry to damp, transitionally dry/wet (ephemerally wetted), to wet. Dry to damp facies include the deposits of dunes and sand sheets, and ephemerally wetted facies consist of mud-flanked dunes or sand sheets, sabkhas, sandy ephemeral streams, and muddy ephemeral streams. The wet facies are composed of lake deposits that show evidence of rare subaerial exposure.

Large-Scale, Cross-Stratified Sandstone (Dune Deposit)

These deposits are composed of reddish- orange, well-sorted, fine- to medium-grained, large-scale, trough-formed to planar cross-stratified sandstone (Fig. 4; see also Fig. 9b below). Cross-stratified sets range from ~20 cm to 2.5 m, but most are less than 1 m thick. The cross- stratification occurs mostly in cosets 2-9 m thick separated by dune-order bounding surfaces (Fig. 4); more rarely they occur as single sets less than 1 m thick. The foresets are composed of sandflow, grainfall, and climbing translatent strata (Hunter, 1977), and they possess maximum dips up to 34 degrees that dip toward the southeast (mean 125°, n = 17). Dune facies commonly grade laterally into sand-sheet facies. At several places in the upper part of the succession, they are intimately associated with ephemeral stream deposits (Fig. 5; see also Fig. 9 below), most commonly the sandy ephemeral stream facies.

The eolian nature of the facies is indicated by the general textural and structural aspects. De- tails of set geometry suggest that the facies was Figure 1. Outcrops of Lower Jurassic erg-margin deposits in the Wingate Sandstone and deposited by small crescentic dunes that mi- overlying Dinosaur Canyon Member of the Moenave Formation, southern Utah, (a) Inner grated toward the southeast and frequently erg-margin deposits at Old Paria. Units A-C are dominated by eolian facies, whereas units formed small transverse draas. The eolian bed- D-E are dominated by fluvial facies. Height of cliff is nearly 100 m. Figure 14 was measured forms represent deposition during dry conditions here, (b) Outer erg-margin deposits at Kanab (units A-I). Height of cliff is -170 m. SS = in the erg margin, and the common lateral shifts Springdale Sandstone Member; NS = Navajo Sandstone. between dune and sand sheet facies indicate that most dune fields were isolated and of restricted size. The intimate association with sandstones of bimodal grain-size distribution. The facies con- (5) gently dipping, curved, wind-formed erosion ephemeral stream origin suggests that many of tains a whole range of sedimentary structures, surfaces frequently overlain by coarse sand the dunes in the study area formed by reworking which are as follows: (1) medium- to high-angle grains (common); (6) irregular or crinkly lami- of these fluvial deposits. cross-stratification with eolian sandflow, grain- nation (Fig. 6) (abundant); (7) bioturbation or fall, and wind-ripple strata (rare); (2) low-angle poorly defined trace fossils (abundant) are tran- Variously Stratified Sandstone or horizontally laminated layers with wind- sitional into the crinkly lamination; and (8) ad- (Eolian Sand-Sheet Deposit) ripple or, more rarely, grainfall strata (Fig. 6) hesion ripple structures (rare). The facies is (abundant); (3) wind ripples of medium- or intimately associated with and transitional to These deposits are composed of reddish- coarse-grained sand (common); (4) eolian de- both dune and sabkha facies. orange, fine- to coarse-grained sandstone with flation lags or horizons with scattered coarse Although the assemblage of sedimentary granule-rich horizons, and commonly have a sand grains or small granules (rare-common); structures clearly indicate that the facies is eo-

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r- v>-ÍTúbá: N ) viCity :•:•:•:•:•! Chinie:- lian, dunes played a small part in its formation. ^ : Sunrise Thus the large-scale, high-angle, cross-stratification represents the migration of relatively rare,

V small and isolated dunes, whereas the low-angle Flagstaff ; ^'-v ;-; or horizontally bedded strata (the wind ripples, A I N ' . deflation lags, and various erosion surfaces) rep- i ^-l, resent eolian deposition and erosion in a Winslow K. relatively dry, low-relief (sand-sheet) environment (Fryberger and others, 1979, 1983; Ko- I curek and Nielsen, 1986). A few of the 25 O —5 0 mij/ o low-angle, cross-stratified sets with indistinct 25 0 50 km/ foresets could be viewed as adhesion ripple, pseudo cross-lamination (compare Kocurek and Figure 2. Index map of part of Colorado Plateau, showing outcrops in study area (inset), Fielder, 1982), but the majority of these low- area of deposition of Wingate Sandstone and coeval Dinosaur Canyon Member of Moenave angle sets have well-developed foresets appar- Formation, and line of cross section A-A'. ently deposited by eolian processes on dry surfaces. The irregular or crinkly lamination probably is polygenetic, although the majority is thought to indicate the influence of vegetation A A' during deposition (compare Kocurek and Niel- ZIONN.P. KANAB PARIA sen, 1986). Finally the bioturbation is probably the combined result of disturbance by plants and animals, whereas the associated trace fossils likely represent the activity of various arthro- pods (Fryberger and others, 1979). The horizons with adhesion ripples indicate damp conditions, either in connection with rainstorms or during periods with a raised ground-water level and sabkha-like depositional conditions.

SUPER Thus, the facies was deposited during condi- SURFACE tions that varied from dry to damp. A sand sheet SAND SHEET environment with some vegetation and the rare WS= WING ATE development of small isolated dunes seems to be FLUVIAL SANDSTONE the best interpretation. -40km DC= Dinosaur Canyon Mbr. Interbedded Cross-Stratified Sandstone and Figure 3. Restored stratigraphic cross section A-A' through the Dinosaur Canyon Member Mudstone (Mud-Flanked Eolian Deposit) of the Moenave Formation (erg-margin deposits) and coeval Wingate Sandstone (erg deposits) from Zion National Park to south-central Utah. Extent and correlation of super bounding These deposits are composed of relatively surfaces (compare Kocurek, 1988) inferred. Note the clear transition from high porosity and thin, reddish-orange, well-sorted, fine- to permeability sandstones (erg) to reduced porosity and permeability deposits at the erg margin. medium-grained sandstone (up to five beds), in-

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tercalated with thin, dark reddish-brown mudstone (Fig. 7). Sandstone beds vary in thickness from 10 cm to 2 m, and mudstone beds are between 10 and 50 cm thick. The sandstone beds show large-scale, high-angle cross-bedding (set size 25 cm to 1 m); low-angle cross-bedding; Figure 4. Dune facies: and horizontal lamination that is composed relatively small, trough- mainly of eolian wind-ripple strata, wind ripples formed sets dipping of coarse sand, irregular to crinkly lamination, toward the southeast (unit and bioturbation. The beds range from sheet- B, Old Paria 2). The like to wedge shaped. In the latter case, upper notch (shadow) conceals contact of the sandstone commonly is erosional a few-centinieters-thick and displays current ripples. The clay-rich mud- mud layer and an erosion stones are laminated to massive and locally con- surface, which separates tain silty or sandy streaks, some of which unit B2 from the overly- contain poorly developed current or wave rip- ing Unit B3, also domi- ples. Desiccation cracks are common. Laterally nated by dune facies this facies shows rapid variations, and it is inti- (see also Fig. 14). Scale = mately associated with muddy ephemeral stream 0.5 m. deposits with rare wind deposits. The texture and sedimentary structure of sandstone beds indicate that they are eolian. Sandstones with high-angle cross-bedding represent small migrating dunes, whereas others are interpreted as sand-sheet deposits. Mudstone horizons are mainly water-laid, although eolian dust and fine sand locally may have been contributed. The present facies apparently represents a low-relief eolian landscape in which the depressions between the dunes and the sand sheets were intermittently flooded by ephemeral streams. Similar mud-flanked dunes, although on a much larger scale, are known from the Algodones Dunes in California and from the margins of the Namib Sand Sea (Fryberger, 1979; Lancaster and Teller, 1988), where silt and mud from ephemeral rivers accumulate in low areas between the dunes.

Evaporitic or Mottled Sandstone (Sabkha Deposit)

This facies commonly contains anhydrite nodules and is composed of well-sorted to moderately well-sorted, reddish-orange, very fine- to medium-grained sandstone. Mud occurs as intercalated diffuse centimeter-thick, mud- rich horizons (bioturbation) or more rarely as millimeter-thick, well-preserved clay drapes. Scattered coarse sand grains or granules occur locally. The facies is subdivided into two genetic types. The first type contains common anhydrite nodules (Fig. 8), and physical sedimentary structures such as low-angle to horizontal lamination (wind-ripple strata), adhesion ripple lamination, and irregular to crinkly lamination are typically preserved. Bioturbation is rare. The second type Figure 5. Intimately associated eolian and fluvial (ephemeral stream) facies (unit D, Old contains no anhydrite nodules; strong mottling Paria 2). Es: eolian dune sand; Fs: fluvial sand; Fm: Fluvial mud; M: mixed aeolian and fluvial. due to bioturbation and minor wave ripples, The foreset of the dune deposits dips toward the southeast. Scale = 0.5 m. current ripples, and wavy lamination are pres-

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cate that in many cases these sabkhas eventually were covered with water.

Multistory Sandstones (Sandy Ephemeral- Stream Deposit)

This facies consists of reddish-brown, very- fine- to medium-grained sandstone with thin, local, dark reddish-brown mudstone interbeds (Fig. 9a). Multistory sandstone bodies vary in thickness from 1 to 9 m, and the mudstone interbeds are usually less than 25 cm thick. Sand- stones show small-scale cross-stratification, large-scale trough cross-stratification, low-angle cross-stratification, and horizontal lamination. Wind ripples of medium-grained sand or fine- grained horizontal translatent strata are rare to common in the upper few centimeters of individual stories. The mudstones occur as thin desiccation-cracked massive beds with limited lateral extent (a few meters) but are most common as intraformational clasts. The facies occurs Figure 6. Sand-sheet facies. Intimately associated crinkly lamination (Cr) and low-angle to as sheet-formed bodies characterized by a multi- horizontally laminated wind-ripple strata (Wr) (Unit B, Buckskin Wash). Scale bar divided story internal organization (Olsen, 1989). In- into centimeters. dividual stories are 0.25 cm to ~3 m thick and exhibit almost entirely vertical accretion. Inter- nal erosion surfaces were observed in several ent. The thickness of both subfacies varies from nant sabkhas (Fryberger and others, 1983). The stories. 50 cm to 3 m. The two subfacies are locally strongly bioturbated nature of the second subfa- intimately associated, and both subfacies are cies makes it difficult to make a precise interpre- The sedimentary structures and stratification gradational into the sand-sheet facies. The latter tation. The texture and sorting of the sand in this types indicate a fluvial origin for the bulk of the subfacies also grades into muddy ephemeral- subfacies, however, suggest that part of the sed- sandstones. Multistory sandstone bodies are stream or lake deposits. iment was eolian. The subfacies apparently rep- formed by multichannel systems (for example, Sedimentary structures of the first subfacies resent damp-wet sabkhas, and the water-laid braided streams) and strongly meandering suggest deposition in dry-damp detrital-domi- ripples in the upper parts of these deposits indi- channels (compare Collinson, 1978; Bridge and Diemer, 1983; Friend, 1983). The rare occurrence of lateral accretion bedding and thinness of individual stories excludes a meandering channel origin. More likely they originated as braided-stream deposits. Individual stories were probably laid down in individual braid channels or on braid bars. The scarcity of muddy channel-fill indicates a mud-poor fluvial system. The occurrence of erosion surfaces within individual stories indicates pulsatory or ephemeral flow in the braided streams. Eolian reworking of the sandstones and occurrence of desiccation cracks in interbedded mudstones suggest an ephemeral stream origin (Glennie, 1970). At one locality, a multistory sheet was traced laterally for a few kilometers, revealing varying degrees of eolian reworking of the fluvial sand. In the most extreme case, all fluvial sand has been reworked and deposited as eolian dunes or small draas (Fig. 9b).

Thinly Interbedded Mudstone and Sandstone (Muddy Ephemeral Stream Deposit)

Figure 7. Mud-flanked eolian facies: relatively thin, eolian sandstone bodies intercalated This facies is dominated by thinly interbedded with thin dark mudstones (unit B, Old Paria 1). dark reddish-brown mudstone and very fine- to

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horizons of desiccation cracks indicate episodes of exclusive suspension-load transport. This was presumably the normal type of event, whereas sandstone beds, although volumetrically important, represent exceptional flood episodes of higher magnitude.

Mudstone and Heterolithic Mudstone and Very Fine-Grained Sandstone (Lacustrine Deposit)

This is the finest-grained facies and consists of dark reddish-brown mudstone, siltstone, and heterolithic mudstone and very fine-grained sandstone in beds 0.15-1.2 m thick (Fig. 11). Mudstone always forms 50% or more of the facies. Silt- or sand-sized material is characterized by small-scale cross-stratification, both bidirectional and unidirectional; wavy bedding and horizontal lamination; and it displays strong sheetlike bedding geometry. Coarsening-upward sequences 15 to 30 cm thick were in places observed conformably overlying wavy bedding to Figure 8. Sabkha facies. Irregular lamination and displacive anhydrite nodules (Unit D, Johnson Canyon). small-scale cross-stratified (unidirectional) sandstone. Desiccation cracks are common at the top of these as well as other small-scale sequences. The rocks also contain rare anhydrite nodules, fine-grained red sandstone. Mudstone beds vary sandflow, and grainfall strata) and occasionally stromatolites, oolites, and vertebrate fossils, from millimeters to a few decimeters in thick- coarse sand wind ripples in their upper portions. mainly nonmarine fish fragments (Harsberger ness. Sandstone beds vary from a few centime- In the outer erg-margin zone, the facies is closely and others, 1957). The facies is closely asso- ters to a maximum of 1 m, most beds ranging associated with the semipermanent lake facies. ciated with, and transitional to, the muddy between 10 and 25 cm. The sand/mud ratio The assemblage of sedimentary structures in ephemeral-stream facies and occasionally is varies considerably (0.1 to 10) from one locality the thinly interbedded mudstones and sand- transitional to the sabkha facies. to another and between stratigraphic levels. The stones clearly indicates that the facies is mainly most common bedding type is stacked vertically fluvial and formed by ephemeral streams. The The sediments were deposited under sub- accreted sandstone sheets separated by mud- dominance of thin, vertically accreted sandstone aqueous, low-energy conditions. They formed stone (Fig. 10). The sandstones are dominated sheets bears similarities with ancient supposed during the wettest times and in the wettest places by small-scale cross-stratification, typically ex- sheetflood deposits (Leeder, 1973; Steel and Aa- of the erg margin. The sheet geometry, rare inter- hibiting climbing ripple stratification. Horizontal sheim, 1978; Hubert and Hyde, 1982; Due, nal desiccation cracks, and anhydrite nodules, lamination, low-angle trough cross-stratification 1983; Tunbridge, 1981, 1984). The flows were along with the occurrence of stromatolites, oo- and massive bedding form less than 5% of the probably true sheetfloods and only to a small lites, and nonmarine fish fragments point to a sequence. The sandstone beds primarily fine extent of overbank origin (Olsen, 1989). The lake or playa origin. The bidirectional cross- upward, commonly in association with changes dominance of small-scale cross-stratification and stratification resembles type M2 and SI of de in structures, for example, from horizontal lami- paucity of horizontal lamination (upper plane Raaf and others (1977) and is interpreted as nation to small-scale cross-stratification. The bed) indicate low-energy floods compared to wave generated. The thin coarsening-upward mudstones are massive or horizontally lami- many modern and ancient higher energy sequences probably represent mouth bars nated, commonly sand-streaked, and display ephemeral streams (McKee and others, 1967; formed by shallow ephemeral streams flowing desiccation cracks. Tunbridge, 1981, 1984; Stear, 1985). into the lakes (Elliott, 1974). The close associa- This main type of uniform bedding of stacked The ribbon- to narrow sheet-farmed sand- tion to the muddy ephemeral-stream facies indi- decimeter-thick sheets is cut by thicker (0.5 to stone bodies were formed by solitary sinuous cates a delicate balance between shallow lakes 3.5 m) ribbon-shaped or narrow, sheet-formed, channels which migrated laterally by point-bar and muddy flood plains. This association is a single-story, sandstone bodies. The sand bodies deposition (Olsen, 1989). Each bedset lying be- close analogue to the Lower Jurassic East Berlin are usually composed of several bedsets partly tween the inclined erosion surfaces (lateral ac- Formation of the Hartford Basin as described by separated by thin mud-partings and succeeding cretion surfaces) was probably deposited during Demicco and Kordesch (1986). erosion surfaces. Lateral accretion is common. a single major flood event. The eolian deposits The sandstone bodies are dominated by small- in the upper portion of sandstone bodies may SEDIMENTARY SEQUENCES scale cross-stratification. Low-angle cross-strati- reflect the deposition of eolian sands as sand fication and large-scale trough cross-stratifica- drifts or small dunes in dry channels. General tion also occur. In the inner erg-margin zone, The facies in general is mud-rich (10%-90%) these sandstone bodies frequently exhibit eolian compared to the sandy ephemeral-stream facies. The Wingate Sandstone and overlying Dino- high-angle cross-bedding (translatent strata, The decimeter-thick mudstone beds with several saur Canyon Member of the Moenave Forma-

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margin deposits. In a lateral sense, both litho- stratigraphic units represent loosely defined inner, intermediate, and outer erg-margin zones (Fig. 12). The sedimentary features of these zones gradually changed with time because of an over-all climatic change toward more humid conditions. At the same time, the central-erg and adjacent erg-margin zones probably retreated stepwise toward the northeast. The strata are arranged in cyclic sequences with vertically repeated packages of sandstone or sandstone and mudstone 2-30 m thick. De- spite complex lateral changes, local detailed sections were correlated across the region using continuous, excellent, intervening exposures. The result of the correlation is our establishment of units A-I (Fig. 12). Each unit is composed of a lateral series of drying-upward or, less commonly, drying-wetting-upward local sequences, in which sedimentary features differ across the region (Fig. 12). Deposits of each unit are more or less synchronous, and related sequences are the local sedimentary record of regional climatic fluctuations, specifically alternating humid and arid periods. The basal erg-margin deposits that comprise units A-C form predominantly drying-upward sequences. Locally unit B contains three subdivisions. The overlying erg-margin succession comprises units D-I. Unit D locally is composed of four subdivisions. The transition between the basal eolian-dominated succession and the overlying fluvial-dominated succession is frequently sharp and easily recognized in the field. Parts of the uppermost units are locally removed due to erosion preceding deposition of the overlying fluvial Springdale Sandstone Member. The most complete pre-Springdale succession is preserved at Kanab (Fig. 12).

Drying-Upward Sequences

Nineteen of the recognized twenty-seven fully preserved local sequences are classified as drying-upward. They contain from two to four facies and all differ slightly (Fig. 12). Thick- nesses vary from 2 to 14 m. Examples of some of the most complete sequences show the fol- Figure 9. Upper part of unit H at Kanab. (a) Sandy ephemeral-stream fades (Fs): a multisto- lowing facies development. (1) At Lees Ferry, ry sandstone body (8 m thick). Each story, defined by erosional bounding surfaces, reflects sequence A is interpreted to contain basal deposition in a braid channel or on a braid bar. (b) Dune facies (Es): large-scale, semi-planar, muddy-stream deposits overlain by sabkha, eo- cross-stratified sandstone. One bounding surface (lower left) is draped by up to 0.5-m-thick lian sand sheet, and uppermost eolian-dune sed- water-laid mudstone. Four meters of the 6-m-thick eolian sequence is exposed. These eolian iments. (2) At Old Paria, sequence C contains sediments are laterally equivalent to Figure 9a within 1,600 m and suggest complete eolian basal wet sabkha deposits overlain by damp reworking of the sandy ephemeral-stream deposits. sand sheet and topped by dry sand-sheet sediments. (3) At Kanab, sequence B has basal lake tion of south-central Utah are herein interpreted Ferry (unit A), it represents small eolian dunes deposits, overlain by damp-wet sabkha, detrital- as erg-margin or side-erg deposits composed of or draas with associated sand sheets in the transi- dominant sabkha, and uppermost dry-sand- eolian and associated lacustrine and ephemeral- tion zone between the central erg and the erg- sheet sediments. (4) Finally at Kanab, sequences stream facies. In general, where the Wingate margin zone. The Dinosaur Canyon Member F and G both display basal muddy-stream de- Sandstone is present, as at Old Paria and at Lees consists of both eolian and water-laid erg- posits overlain by sandy-stream deposits.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/6/759/3380481/i0016-7606-101-6-759.pdf by guest on 26 September 2021 Figure 10. Muddy ephemeral-stream facies: thinly interbedded very fine-grained tabular sandstones and mudstones. Cross-lamination dominates the sandstones. Notice the abundance of desiccation cracks. Measure is divided into centimeters and is 10 cm long.

Figure 11. Semipermanent lake facies: (a) heterolith composed of horizontally laminated and cross-laminated siltstones and mudstones and centimeter-thick, massive, very fine-grained sandstones. The lake deposits are underlain and overlain by sheet-flood deposits exhibiting desiccation cracks. Pencil, 12 cm long, for scale, (b) Heterolith composed of mudstones and sandstones with wave-generated structures. Measure is 10 cm long.

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ward top is always the thinnest part. In most cases, the wetting-upward part fines upward and displays a vertical facies sequence toward wetter depositional conditions. The following facies sequences illustrate the following trends. (1) At Old Paria 2 in sequence C, basal sand-sheet deposits are overlain by dune and sand-sheet sediments and are finally topped by sabkha sediments (Fig. 14). (2) Near Johnson Canyon, sequence C contains basal lake sediments overlain by sand-sheet and uppermost sabkha sediments. (3) Near Johnson Canyon, sequence D displays muddy ephemeral stream deposits overlain by sand-sheet, sabkha, and uppermost muddy ephemeral-stream deposits. (4) At Kanab, sequence H contains basal muddy ephemeral- stream, dune, and uppermost sandy ephemeral- stream sediments (Fig. 16). In some cases, the wetting-upward part of the sequence is less obvious and indicated only by a slight shift in sedimentary features within the same facies (for example, fining-upward top of sandy fluvial deposits, slight wave-reworking of sabkha or sand-sheet facies or increasing biotur- Figure 12. Simplified sedimentary sequences between Lees Ferry and Kanab. Based on the bation of sand-sheet facies). Such sequences are occurrence of drying-upward or drying-wetting-upward sequences, a number of sedimentary gradational into the asymmetric drying-upward units (A-I) have been recognized. These units can be correlated throughout the region and sequences. Thus there seems to exist a contin- define a basal erg-margin association (A-C) dominated by eolian facies and an overlying erg uum of sequences, the two end-members of margin association (D-I) dominated by fluvial facies. which are asymmetric drying-upward sequences (most common) and the more symmetric drying-wetting-upward sequences. Locally subdivisions are recognized within terpret in climatic terms. Frequent occurrence of units B and D. These are commonly of drying- eolian facies in the uppermost sandy portion of Lateral Relationships upward nature. In unit B, three such subdivi- fluvial sequences suggests that they too are sions occur at Lees Ferry (1-4.5 m thick), at drying-upward sequences. Thus, the basal The individual sedimentary units are consid- most localities around Old Paria (Figs. 13, 14, muddy fluvial deposits were laid down during ered to be thin chronostratigraphic units; they 15), and probably also at Buckskin Wash and less ephemeral conditions than were the overly- display marked lateral variation on both a sub- near Johnson Canyon. In unit B, the subdivi- ing sandy-stream deposits. The highly ephemeral regional and local scale (Figs. 13, 14, 15, 17). sions commonly display basal muddy-stream nature of the uppermost deposits are particularly From the study of individual units, several deposits overlain by mud-flanked dune deposits well portrayed by unit H at Kanab where an obvious patterns emerge (see Fig. 17). (1) Eolian (B2 and B3, Lees Ferry) or muddy-stream de- 8-m-thick sequence of fluvial sand is completely dune and sand-sheet facies decline in importance posits overlain by dune deposits (B2, Old Paria 3, reworked by wind within a distance of 1,600 m toward the west (that is, away from the main erg Fig. 15). (Fig. 9). body); fluvial and associated lacustrine facies in- In unit D, four subdivisions are recognized at Strictly speaking, the drying-upward se- crease in importance in the same direction. Old Paria and probably also at Lees Ferry. quences dominated by eolian facies in their (2) Dune facies are the dominant eolian depos- These subdivisions characteristically contain upper sandy parts only reflect changes in surface its between Lees Ferry and Old Paria (inner muddy-stream deposits overlain by sandy- conditions (wet, ephemeral wet, or dry) at the erg-margin zone). (3) Sand-sheet facies are stream deposits (Dl, D2, and D3, Old Paria 1, depositional site. The pure fluvial coarsening- the dominant eolian deposits at Buckskin Wash Fig. 13), or muddy ephemeral-stream deposits upward sequences also seem to reflect changes and near Johnson Canyon (intermediate zone). overlain by sand-sheet or dune sediments (D4, in the over-all sediment yield to the basin, and in (4) Sabkha facies mark the erg margin at Lees Ferry and Old Paria 1, Fig. 13). this way they possibly indicate climatic changes Kanab (outer zone). (5) Fluvial facies are All units are interpreted as representing depo- in the distant source-areas. It is therefore sug- preserved only locally in the basal erg-margin sition in an increasingly dry environment. This is gested that the formation of local drying-upward succession; they are widespread in the overlying most obvious for the sequences or subdivisions sequences were intimately linked with climatic erg-margin succession. (6) Lacustrine deposits that are topped by eolian facies. For example, fluctuations that influenced not only the erg are common in the intermediate and outer zone, deposition of lake, muddy stream, or muddy margin but also the source-areas. but they are lacking in the inner zone. sabkha facies (wet or alternating dry/wet depo- Lateral variations in the composition of facies sitional conditions) was gradually replaced by Drying-Wetting-Upward Sequences sequences clearly reflect changes in the paleo- deposition of sand sheets or eolian dunes (dry geography of the relatively wide erg-margin re- depositional conditions). The pure fluvial Eight local sequences are classified as drying- gion, and they show a crude zonation away coarsening-upward sequences are difficult to in- wetting-upward sequences. The wetting-up- from the central erg during deposition of the dry

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OLD PARIA 1 m SANDY CO m 'D, 25- MUDDY O SANDY £ m 50- D2 < MUDDY < o

GC SANDY Si <

20- CO MUDDY Di o

SAND 45— SHEET T.T-'.rr-.rrv^Try.A SABKHA

MUDDY 40- EPHEM. STREAM

DUNES

MUDDY EPHEM. STREAM 35- SAND SHEET

MUDDY EPHEM. 30 — STREAM

27— w^wg

Figure 13. Detailed sedimentary log (Old Paria 1) at Cockscomb, ~5 km S of Old Paria site. Water-laid sandstones are stippled.

facies. Such a zonation was less obvious during fore appear to be controlled by external factors is restricted to the basal sequence (unit A at Lees deposition of the wet facies. Succession around (that is, climate). Ferry and Old Paria); (3) thickness of fluvial Old Paria indicates that paleogeographical vari- facies within superimposed sequences increases ation was large during deposition of the eolian Time Trends upward; and (4) the proportion and thickness of facies. Thus small dune fields graded into eolian muddy fluvial and associated lacustrine deposits sand sheets within short distances (see Figs. 13, The studied successions consist of several in superimposed sequences commonly increase 14,15). In spite of the control governed by local stacked drying-upward or locally drying- upward as at Kanab. paleogeography on the facies pattern, drying- wetting-upward sequences (Fig. 12). At most This general time trend is terminated by the upward and associated drying-wetting-upward localities, basal sequences (units A-C) are dom- deposition of perennial sandy fluvial deposits in sequences are recognized throughout the erg- inated by eolian facies; overlying beds (units the overlying Springdale Sandstone Member. margin region. This is strongly suggestive of a D-I) are dominated by fluvial facies, or stated A general change toward a more humid cli- climatic allocyclic control on sedimentation otherwise: (1) the proportion and total thickness mate and a stepwise retreat of the erg toward the instead of tectonic control. In other words, of eolian facies within superimposed sequences northeast occurred over time. This climatic regional drying-upward or drying-wettening- decrease, in most cases, upward; (2) the occur- change affected (1) the source-region and there- upward sequences cross facies belts and there- rence of relatively thick units of dune sandstone fore the fluvial sediment yield to the basin (in-

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OLD PARIA 2

Ü * uj y £ < z ° LU O O z 2 DC Û. CO

SAND SHEET Z MUDDY 2 CO or I— m SANDY < o CE UJ X

Llip^r7 " -n^i^'s'o^, '— b 1 Kt AM (j

Figure 14. Detailed sedimentary log (Old Paria 2) at the north side of Paria River at Old Paria site.

creased proportion of mud) and (2) the local During arid intervals, the side-erg plain was scale, have been similar to the Great Sand depositional conditions (decreasing tendency to characterized by eolian processes. Dunes or Dunes in southern Colorado, where small iso- form eolian facies). Clearly the climatic change mud-flanked dunes were the dominant eolian lated dune fields separated by sand sheets gradu- was interrupted by fluctuations. facies in the inner zone closest to the erg; sand ally merge downwind into the main dune field sheets were dominant in the intermediate zone, (Andrews, 1981). DISCUSSION and sabkhas were dominant in the outer zone During the early evolution of the erg margin, (Fig. 17). The observed pattern seems to indi- the arid climatic intervals were most pro- Paleogeography cate that eolian erosion and transport dominated nounced, and eolian facies dominated. With in the outer and intermediate zones, while eolian time the over-all climate became more humid, The Wingate erg in northeastern Arizona and deposition became increasingly more important the arid intervals became weaker, and fluvial adjacent parts of southern Utah and Colorado in the inner zone. At most places, there was facies gradually replaced the eolian facies. was bounded to the southwest by a wide erg- probably a gradual transition from isolated dune The southern part of the Wingate erg was margin system (Figs. 18, 19). The paleogeog- fields in the inner zone into the main erg body. probably mainly source-biased, and sandy flu- raphy of the erg-margin region changed repeat- The transition between the inner erg-margin vial sediments in the erg margin apparently edly in response to climatic fluctuations. zone and the main erg body may, apart from formed an important source for the eolian sand.

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OLD PARIA 3 m MUDDY D EPHEM CO STREAM CO LU LU

SAND SHEET z < w £ STREAM M SAND CL SHEET CO

SABKHA m 45- SABKHA

SAND SHEET B3 CO DUNES B o> 40- Z < B? O er MUDDY SANDY D EPHEM. < EPHEM. STREAM STREAM CO O B, 35-

LU DUNE_ £ M.EPH. Z STREAM LU "DUNEl" O 30- M. EPH. STREAM DUNJL MUDDY EPHEM. 27- STREAM ÜlVÚ.V's'o'oUi u °> ^ ' SO

Figure 15. Detailed sedimentary log (Old Paria 3) at the south side of Paria River at Old Paria site. Distance between Old Paria 2 and Old Paria 3 is -300 m.

Resultant paleowinds from northwest (Peterson, and paleogeographical interpretation. Our anal- and the major interruption of eolian sedimenta- 1988; Clemmensen and Blakey, 1989) blew the yses of his data show several, perhaps as many tion by regional super surfaces seems best ex- fluvial sand into the adjacent part of the erg as seven or eight, fluvial-eolian (wet-dry) cycles. plained by climatic fluctuations. Our analyses of (Fig. 18). Clemmensen and Blakey (1989) documented published and unpublished regional data show a The paleoenvironmental picture of the whole cyclic changes in the Wingate erg complex near minimum of four, and more likely seven or erg margin changed completely during the Many Farms, Arizona. Work in progress by Na- eight, region-wide cycles. Given the relatively humid intervals, when fluvial and muddy lake tion and Blakey demonstrates a regional cyclic- short period of Wingate time (Peterson and sediments dominated, commonly changing from ity in the Wingate erg deposit; of southern Pipiringos, 1979), the periodicity of these cycles muddy streams near the erg body to lakes and Utah. Using super bounding surfaces, they have corresponds well with climatic controls (see wet sabkhas toward the west (Fig. 17). correlated several individual eirg complexes below). Considerably more information on the Win- across much of the region. A number of wet-dry gate erg deposits and related depositional units is cycles are apparent. These studies all show a Erg Margin Dynamics rapidly emerging. Edwards (1985) provided de- prominent cyclicity within the Wingate Sand- tailed description of the Wingate-Dinosaur stone and related units (primarily Dinosaur The local sequential accumulation of the erg- Canyon transition in the Tuba City, Arizona, Canyon Member of Moenave Formation) that margin deposits was partly controlled by paleo- area and gave comprehensive sedimentological is apparently basin wide. Such basin-wide events geographical position (inner, intermediate, and

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KANAB BUCKSKIN WASH KANAB JOHNSON CANYON I OLD PARIA LEES FERRY SANDY EPHEM. STREAM

MUDDY EPHEM. STREAM

SANDY CHINLE FM EPHEM. STREAM Figure 17. Simplified cross section of the erg-margin deposits showing sequences of events over time and space. All cycles are made equally thick for simplicity. Cycle boundaries may be gradational or erosional. It is unclear if the base of the Springdale Sandstone Member is an erosional regional unconformity. Alternatively, the Springdale Sandstone Member may be coeval with unit I. MUDDY EPHEM. STREAM

Figure 16. Detailed sedimentological log at Kanab (units G and H).

outer zones) and partly by climatic conditions, which affected deposition in the basin and the environment in the source region. The source region of the fluvial streams, which contributed most of the sediment to the erg margin, was situated toward the south in the Mogollon highlands in central Arizona (Figs. 18, 19). Fluvial sediment yield to a basin is controlled by four main groups of factors (Knighton, 1984): precipitation and runoff characteristics, soil resistance, basin topography, and the nature of the plant cover. The variation of sediment yield with climatic change has been described by Langbein and Schumm (1958). From their studies, it is evident that sediment yield reaches a maximum at a precipitation of about 300 mm Figure 18. Hypothetical paleogeography of Wingate erg system per year, and desert shrub is replaced by grass- during arid intervals with maximum eolian sedimentation. Larger ar- land. Increased precipitation results in increased rows show main inflow of sand into erg system, smaller arrows show plant cover, thereby reducing sediment yield to secondary influx of sand from inactive upwind fluvial system (dashed the basin. lines). More continuous lines with arrows show ephemeral wadi sys- In Early Jurassic time, grassland was absent, tems. Erg expansion to southwest is related to strong influx of sand but during humid intervals, the Mogollon high- from northwest as well as locally derived sand from inactive or wan- lands and to a lesser degree the erg margin was ing fluvial system to west.

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Figure 19. Hypothetical paleogeography of Wingate erg system during humid intervals with minimum eolian sedimentation. Dashed arrow shows waning influx of sand from the northwest. Solid and dashed lines show perennial to ephemeral streams that decreased in flow toward the north, away from their source. (Note that no up- stream deposits are preserved in the rock record.) Open stippled areas represent decaying erg conditions within the Wingate erg, and heavier stippled areas show separate, smaller, and still-active erg centers (loca- tions generalized). It is during these "humid intervals" that super bounding surfaces or Stokes surfaces would likely form. Subsequent decay of river systems would form fresh eolian source areas and contribute to erg expansion.

the basin, where eolian reworking of the fluvial sand was favored on the poorly vegetated erg- margin plain. Resultant winds from the northwest blew the fluvial sand into small dunes or draas, which during migration were added to the main erg body toward the northeast. In this way, the central erg expanded southwestward (Fig. 20). During more humid intervals, sheet-floods and high-sinuosity rivers transported a fine- grained, mud-rich load into the basin, where eolian activity was retarded by vegetation and possibly covered by vegetation, including trees. was reduced significantly, and the rivers trans- scarcity, of sand. Lack of fluvial sand for wind- During the present study, a tree trunk was found ported mainly mud into the basin. These sug- reworking caused starvation of the central erg, in fluvial erg-margin deposits. According to J. B. gested relationships between precipitation (cli- which accordingly gradually retreated (or at Thornes (1986, personal commun.), any plant mate) and sediment characteristics are supported least stabilized) eastward during humid intervals cover of 25% is sufficient to significantly reduce by observations in the Colorado River of Texas, (Fig. 20). The stacking of drying-upward or erosion and thereby sediment yield. which carries only a coarse load during arid in- drying-wetting-upward sequences suggests repeated climatic fluctuations and corresponding Sediment yield to the basin thus was probably tervals and finer material rich in silt and clay periods of erg expansion and erg contraction or governed mainly by the degree of Early Jurassic during humid periods (Baker and Penteado- stabilization, although the general trend was one plant cover in the source regions. During rela- Orellana, 1977, 1978). of slow southeastward erg migration. tively low precipitation rates, the highlands were Fluctuations between relatively arid and more less vegetated, erosion was active, and much humid conditions would therefore control both The dominance of drying-upward or asym- sandy bedload was transported into the basin. sediment yield and depositional conditions in metric drying-wetting-upward sequences may During more humid intervals, it is likely that the the basin. During arid intervals, ephemeral indicate that the amount of sediment yield highlands were covered by vegetation, erosion braided streams transported sandy bedload into transported into the basin was much larger during arid intervals and at the beginning of the humid intervals until the plant cover became 39= sufficiently established (compare Knox, 1972). In this way, shifts from the sandy top of the sequence into the muddy base of the next sequence should be characterized by only thin development of a transitional wetting-upward top.

3?0 Figure 20. Sketch depicting balooning Wingate erg. During erg maxima (arid intervals), sand fed from the northwest causes erg to prograde southeastward. Additional sand derived from desiccated rivers would accrete onto the southwestern margin, thus accounting for balooning at right angles to predominant winds. During erg minima (humid intervals), erg contracts due to expansion of, and reworking by, fluvial systems as well as a decrease in sand supply. Expansion of fluvial systems was in accord with north-northwest palaeoflow. Solid- head arrows: directions of fluvial paleocurrents and fluvial expansion. -35° Open-head arrows: direction of eolian expansion. Dashed arrows: paleowind directions. Sutured line shows "average" boundary between the two systems.

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of Sedimentary Petrology, v. 49, p. 733- 746. Considerable erosion, both by deflation tervals when dunes or small draas commonly Fryberger, S. G., Al-Sari, A. M., and Clisham, T. J., 1983, Eolian dune, (wind) and degradation (water) likely took were replaced upwind by sand sheets and interdune, sand sheet, and siliciclastic sabkha sediments of an offshore prograding sand sea, Dhahran area, Saudi Arabia: American Associa- place within, and especially between, cycles. sabkhas. tion of Petroleum Geologists Bulletin, v. 63, p. 280-312. Glennie, K. W., 1970, Desert sedimentary environments. Developments in Very likely, some cycles now preserved as 5. The sequences are thought to reflect long- sedimentology, Volume 14: Amsterdam, The Netherlands, Elsevier, drying-upward sequences were once drying- term, Milankovitch-type climatic fluctuations 222 p. Harsberger, J. W., Repening, C. A., and Irwin, J. H., 1957, Stratigraphy of the wetting-upward sequences with the wet tops that determined fluvial sediment yield to the ba- uppermost Triassic and the Jurassic rocks of the Navajo Country (Colo- rado Plateau): U.S. Geological Survey Professional paper 291, 74 p. removed by erosion consequent to preservation sin, depositional conditions in the erg margin, Hubert, J. F„ and Hyde, M. G., 1982, Sheet-flow deposits of graded beds and and deposition of the next cycle. The correlations and erg dynamics. The erg prograded southwest- mudstones on an alluvial sandflat-playa system: Upper Triassic Blorni- dan redbeds, St. Mary's Bay, Nova Scotia: Sedimentology, v. 24, shown in Figure 12 suggest that possibly some ward during arid intervals, but it retreated east- p. 361-387, Hubert, J. F., and Mertz, K. A., Jr., 1984, Eolian sandstones in Upper Triassic- wet caps of sequences were removed in the sec- ward during humid intervals. Lower Jurassic red beds of the Fundy basin, Nova Scotia: Journal of tions east of Johnson Canyon. This pattern 6. On a larger scale, the succession is divided Sedimentary Petrology, v. 54, p. 798-810. Hunter, R. E„ 1977, Basic types of stratification in small eolian dunes: Sedimen- could also reflect a tendency toward eolian ero- into a basal erg-margin association dominated tology, v. 24, p. 361-387. Hunter, R. E., and Rubin, D. M., 1983, Interpreting cyclic cross-bedding, with sion rather than deposition during transitions to by eolian facies and an overlying association an example from the Navajo Sandstone, in Brookfield, M. E„ and humid conditions in the eastern part of the erg dominated by fluvial and lake facies. Ahlbrandt, T. S., eds., Eolian sediments and processes: Developments in Sedimentology No. 38, p. 429 454. margin. 7. The over-all sequential upbuilding of the Knighton, D., 1984, Fluvial forms and processes: Baltimore, Maryland. Ed- ward Arnold, 218 p. it is suggested that the climatic shifts between erg-margin deposits probably reflects a long- Knox, J. C., 1972, Valley alluviation in south-western Wisconsin: Association arid and humid were caused by long-term varia- term change toward a more humid climate, of American Geographers Annals, v. 62, p. 401 410. Kocurek, G„ 1981, Erg reconstruction: The Entrada Sandstone (Jurassic) of tions in the summer position of the Early Juras- coupled with a stepwise migration of the erg- northern Utah and Colorado: Palaeogeography, Palaeoclimatology, Pa- laeoecology, v 36, p. 125-153. sic Inter-Tropical Convergence Zone and margin region toward the northeast. 1988, First-order and super bounding surfaces in eolian sequences- thereby the influence of monsoonal wind. It is Bounding surfaces revisited: Sedimentary Geology, v. 56. p. 193-206. Kocurek, G., and Fielder, G., 1982, Adhesion structures: Journal of Sedi- considered likely that these climatic shifts were ACKNOWLEDGMENTS mentary Petrology, v. 52, p. 1229-1241. Kocurek, G., and Nielsen, J., 1986, Conditions favorable for the formation of controlled by Milankovitch-type changes in warm-climate aeolian sand sheets: Sedimentology, v. 33, p. 795-816. solar radiation at low latitudes (Rossignol- Lancaster, N., and Teller, J. T„ 1988. interdune deposits of the Namib Sand We wish to thank D. Baars and F. Peterson Sea: Sedimentary Geology, v. 55, p. 91-107. Strick, 1985). In a recent paper, Olsen (1986) for constructive reviews of the paper. The field Langbein, W. E., and Schumm, S. A., 1958, Yield of sediment in relation to mean annual precipitation: American Geophysical Union Transactions, convincingly related lacustrine cycles in the Late work of Clemmensen and Olsen was funded by v. 39, p. 1076-1084. Triassic-Early Jurassic Newark Supergroup to Leeder, M., 1973, Sedimentology and palaeogeography of the Upper Old Red a grant from the Danish Ministry of Energy. Sandstone in the Scottish Border Basin: Scottish Journal of Geology, orbital forcing of climate in the ice-free early Blakey's field work was funded by a grant from v. 9, p. 117-144. Loope, D. B„ 1984, Eolian origin of upper Paleozoic sandstones, southeastern Mesozoic. Olsen (1986) recognized cycles of the Organized Research Committee, Northern Utah: Journal of Sedimentary Petrology, v. 54, p. 563-580. about 25,000,41,000, 100,000, and 400,000 yr, McKee, E. D., Crosby, E. J., and Berryhill, H. L., 1967, Flood deposits, Bijou Arizona University. Creek, Colorado: Journal of Sedimentary Petrology, v. 37, p. 829 851. with the latter cycles being very obvious in the Olsen, H., 1989, Sandstone-body structures and ephemeral stream processes in the Dinosaur Canyon Member, Moenave Formation, Lower Jurassic. REFERENCES CITED field. Having 9 main erg-margin cycles at Kanab Utah: Sedimentary Geology, v. 61 (in press), Olsen, P. E., 1986, A 40-million-year lake record of early Mesozoic climatic with a total thickness of 102 m and knowing Andrews, S., 1981, Sedimentology of Great Sand Dunes, Colorado, in Ethridge, forcing: Science, v. 234, p. 842-848. F. G., and Flores, R. M., eds.. Recent and ancient nonmarine deposi- Peterson, F., 1988, Pennsylvanian to Jurassic eolian transportation systems in that deposition of the complete Moenave For- tional environments: Models for exploration: Society of Economic Pa- the western United States: Sedimentary Geology, v. 56, p. 207 260. leontologists and Mineralogists Special Publication No. 31, p. 279-291. mation (155 m at Kanab) approximately cor- Peterson, F., and Pipiringos, G. N., 1979, Stratigraphic relations of the Navajo Baker, V. R., and Penteado-Orellana, M. M., 1977, Adjustment to Quaternary Sandstone to Middle Jurassic formations, southern Utah and northern responds to the Sinemurian or ~6 m.y., the climatic change by the Colorado River in central Texas: Journal of Arizona: U.S. Geological Survey Professional Paper 1035-B, 43 p. Geology, v. 85, p. 395-422. erg-margin cycles seem to record 400,000-yr ec- Porter, M. L., 1987, Sedimentology of an ancient erg margin: the Lower Juras- 1978, Fluvial sedimentation conditioned by Quaternary climatic change sic Aztec Sandstone, southern Nevada and southern California: Sedi- in central Texas: Journal of Sedimentary Petrology, v. 48, p. 433-451. centricity cycles. mentology, v. 34, p. 661-680. Blakey, R. C., 1988, Basin tectonism and erg response: Sedimentary Geology, Raaf, J.F.M. de, Boersma, J. R., and Gelder, A. van, J 977, Wave-generated v. 56, p. 127-151. structures and sequences from a shallow marine succession, Lower Car- Blakey, R. C., Petersen, F., and Kocurek, G., 1988, Synthesis of late Paleozoic boniferous, County Cork, Ireland: Sedimentology, v. 24, p. 451 483. and Mesozoic eolian deposits of the Western Interior of the United CONCLUSIONS Rossignol-Strick, M„ 1985, Mediterranean Quaternary sapropels, an imme- States: Sedimentary Geology, v. 56, p. 3-125. diate response of the African monsoon to variation of insolation: Pa- Bridge, J. S., and Diemer, J. A., 1983, Quantitative interpretation of an evolv- laeogeography, Palaeoclimatology, Palaeoecology, v. 49, p. 237-263. ing ancient river system: Sedimentology, v. 30, p. 599-623. 1. The Lower Jurassic Wingate Sandstone Stear, W. M., 1985, Comparison of the bedform distribution and dynamics of Clemmensen, L. B., and Abrahamsen, K., 1983, Aeolian stratification and modern and ancient sandy ephemeral flood deposits in the southwestern facies association in desert sediments, Arran basin (Permian), Scotland: and the overlying Dinosaur Canyon Member of Karoo region, South Africa: Sedimentary Geology, v. 45, p. 209- 230. Sedimentology, v. 30, p. 311-339. Steel, R. J., and Aasheim, S. M., 1978, Alluvial sand deposition in a rapidly the Moenave Formation in south-central Utah Clemmensen, L. B., and Blakey, R. C., 1989, Erg deposits in the Lower Jurassic subsiding basin (Devonian, Norway), in Miall, A. D., ed., Fluvial sedi- Wingate Sandstone, northeastern Arizona: Oblique dune sedimentation: are composed of erg-margin deposits which mentology: Canadian Society of Petroleum Geologists Memoir 5, Sedimentology, (in press). p. 385-413. Collinson, J. D., 1978, Vertical sequence and sand body shape in alluvial formed at the southwestern edge (that is, lateral Steele, R. P., 1983, Longitudinal draa in the Permian Yellow Sands of north- sequences, in Miall, A. D., ed„ Fluvial sedimentology: Canadian Society east England, in Brookfield, M. E., and Ahlbrandt, T. S., eds.. Eolian side) of the Wingate erg. of Petroleum Geologists Memoir 5, p. 577-586, sediments and processes: Developments in Sedimentology No. 38. Demicco, R. V., and Kordesch, E. G., 1986, Facies sequences of a semi-arid p. 543-550. 2. Erg-margin facies include dune, eolian closed basin: The Lower Jurassic East Berlin Formation of the Hartford Thompson, D. B., 1969, Dome-shaped dunes in the Frodsham Member of the Basin, New England, USA: Sedimentology, v. 33, p. 107-118. sand-sheet, mud-flanked eolian, sabkha, so-called "Keuper" Sandstone Formation (Scythian-?Anisian:Triassic) Due, P. H., 1983, Flood basin/shallow lacustrine sediments in Middle Devo- at Frodsham, Cheshire (England): Sedimentary Geology, v. 3, nian deposits, Hornelen Basin, western Norway: Abstract. 4th IAS RE- ephemeral-stream, and lake deposits. p. 263-289. GIONAL MEETING, Split, Yugoslavia, p. 54-56. Tunbridge, I. P., 1981, Sandy high-energy flood sedimentation—Some criteria 3. Erg-margin deposits are characterized by a Edwards, D. P., 1985, Depositional controls on the Lower Jurassic Dinosaur for recognition, with an example from the Devonian of S.W. England: Canyon Member of Moenave Formation, southern Utah and northern number of 2- to 30-m-thick drying-upward or Sedimentary Geology, v. 28, p. 79-95. Arizona [M.S. thesis]: Flagstaff, Arizona, Northern Arizona University, 1984, Facies mode! for a sandy ephemeral stream and clay playa com- 243 p. drying-wetting-upward local sequences. plex, the Middle Devonian Trentishoe Formation of North Devon. Elliott, T., 1974, Interdistributary bay sequences and their genesis: Sedimentol- U.K.: Sedimentology, v. 31, p. 697-715. 4. Individual sequences can be traced from ogy, v. 21, p. 611-622. Walker, T. R., and Harms, J. C., 1972, Eolian origin of Flagstone Beds, Lyons Friend, P. F., 1983, Towards the field classification of alluvial architecture or the edge of the central erg more than 100 km in Sandstone (Permian), type area, Boulder County, Colorado: The Moun- sequence, in Collinson, J. D., and Lewin, J., eds., Modern and ancient tain Geologist, v. 9, p. 279-288. a slightly upwind direction. The sedimentary fa- fluvial systems: International Association of Sedimentologists Special Publication No. 6, p. 345-354. cies of each sequence (unit) change laterally, re- Fryberger, S. G., 1979, Eolian-fluviatile (continental) origin of ancient strati- flecting changes in the paleogeography away graphic trap for petroleum in Weber Sandstone, Rangely oil field, Colo- rado: The Mountain Geologist, v. 16, p. 1-36. from the erg edge. These paleogeographical Fryberger, S. G., Ahlbrandt, T. S., and Andrews, S., 1979, Origin, sedimentary MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 12,1988 features, and significance of low-angle eolian "sand sheet" deposits. REVISED MANUSCRIPT RECEIVED NOVEMBER 17,1988 changes were most pronounced during arid in- Great Sand Dunes National Monument and vicinity, Colorado: Journal MANUSCRIPT ACCEPTED NOVEMBER 23,1988

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