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, <Dster Voldgade 10, DK-1350 Copenhagen K, Denmark rlbJNKlK. Ul.ohiv f RONALD C. BLAKEY Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 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 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 760 CLEMMENSEN AND OTHERS SEDIMENTARY FACIES General A detailed study of the Wingate-Moenave transition revealed five main sedimentary suc- cessions in the ancient erg-margin area. These rocks are assigned to seven major facies depos- ited during conditions ranging from dry to damp, transitionally dry/wet (ephemerally wet- ted), to wet. Dry to damp facies include the deposits of dunes and sand sheets, and ephemer- ally 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-strati- fied 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 maxi- mum 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 succes- sion, they are intimately associated with ephem- eral 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- 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 JURASSIC ERG-MARGIN DEPOSITS, UTAH 761 1U° 112° 110° Johnson Canyon Buckskin Wash \ * % \ .„?"""• Paria •I •'•••.•.