Flood surfaces and deflation surfaces within the Cutler Formation and Cedar Mesa Sandstone (Permian), southeastern Utah RICHARD LANGFORD 1 „ „ . tr, , . „ , . „ . , rr . r , , , . 0,,,, MARJORIE A CHAN J Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 ABSTRACT of dune masses and intervening interdune deposits results in the formation of first-order bounding surfaces (Mckee and Moiola,1975; Brookfield, Areally extensive erosion surfaces in eolian deposits have been 1977; Rubin and Hunter, 1982). First-order surfaces are limited in areal interpreted as long hiatuses in eolian deposition. Such erosion surfaces extent (several square kilometers) and form during relatively continuous form during deflationary episodes or during periods of erg stabiliza- eolian deposition. tion or contraction). They are called "super surfaces" and can be useful for stratigraphic correlation because of their regional extent. TOie Permian Cedar Mesa Sandstone and Cutler Formation con- tain a type of super surface (herein termed "flood surfaces"), -10-400+ tan2. Flood surfaces form as a result of fluvial floods into active dune seas. The surfaces may expand through nonclimbing mi- gration of eolian dunes but do not imply long hiatuses in eolian deposi- tion as do other types of super surfaces. Cutler and Cedar Mesa flood surfaces are overlain by shales and sandstones which thicken laterally and merge with fluvial channel-fill deposits. Flooding of active dune fields in the Cedar Mesa Sandstone is suggested by intertonguing of eolian dune and fluvial deposits. Flood surfaces can easily be mistaken for deflationary super surfaces but are distinguished by evidence of dune migration coincident with flood events and by an increase in the number of surfaces adjacent to associated aqueous deposits. INTRODUCTION Purpose How are eolian strata deposited and preserved? Does the deposition of an eolian sandstone proceed relatively continuously, or is it episodic with long hiatuses? Areally extensive erosion surfaces which truncate dune deposits are a distinctive and common feature of eolian sandstones and are the keys to answering these questions. This paper documents a type of areally extensive erosion surface, herein termed "flood surfaces" (hundreds of square kilometers), in the Cutler Formation and Cedar Mesa Sandstone that formed during relatively short breaks in eolian deposition. Previously these same surfaces were interpreted as deflation surfaces (super surfaces), formed during extended hiatuses (Loope, 1985). The criteria that distin- guish flood surfaces are identified. Background Theories for the formation of extensive erosion "bounding" surfaces Figure 1. Outcrop map of Cutler Formation and Cedar Mesa in eolian sandstone have been derived from two schools of thought on the Sandstone with Paleozoic Uncompahgre uplift. Box depicts study nature of eolian deposition. One school proposed that the subcritical climb area. Geological Society of America Bulletin, v. 100, p. 1541-1549, 13 figs., 1 table, October 1988. 1541 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/10/1541/3379329/i0016-7606-100-10-1541.pdf by guest on 27 September 2021 1542 LANGFORD AND CHAN surface which formed as dunes in the Sahel deflated to a degraded, rolling, vegetated topography following a change to a wetter climate. Blakey and Middleton (1983), Kocurek (1984, 1988), Talbot (1985), and Loope (1985) argued that extensive erosion surfaces can represent significant hiatuses in eolian deposition over significant portions of ergs (sand seas). Kocurek (1988) discussed the origin of regionally extensive erosion surfaces and referred to them as "super surfaces." He discussed three categories of super surfaces: (1) super surfaces formed through the termina- tion of ergs through a change in climate, (2) super surfaces formed through erg contraction due to eustatic or tectonic events, or (3) super surfaces formed through erg migration. All of these mechanisms invoke long hia- tuses in eolian deposition. Rubin.and Hunter (1984) proposed an alternative process for creat- ing areally extensive erosion surfaces. Migrating dunes cease to climb, and moving parallel to the depositional surface, they cut an extensive erosion surface. The Cutler Formation and Cedar Mesa Sandstone contain exten- sive erosion surfaces which appear to have formed in this manner. The surfaces are defined initially through fluvial floods into eolian strata and expand through nonclimbing migration of the dunes. Flood surfaces are a fourth category of super surface. In contrast to the other three categories described by Kocurek (1988), they do not mark significant hiatuses in eolian deposition. Depositional Setting Permian sedimentation patterns in the northeastern Colorado Plateau were influenced by the Uncompahgre Uplift (Fig. 1). A thick sequence of fluvial arkosic sandstones and conglomerates were shed from the south- western flank of the uplift to form the Cutler Formation (Fig. 1). Paleocurrent indicators suggest west and northwest dispersal of Cutler fluvial arkoses, oblique to the Uncompahgre range front (Baars, 1970). Over most of the exposure of the Cutler Formation in eastern Utah, fluvial arkosic sandstones and shales are interbedded with eolian sand- stones. Fluvial strata predominate near the Uncompahgre Uplift, northeast of Castle Valley (Fig. 1). Here eolian strata compose only 3% of the strata in 2 measured sections. Eolian strata, however, make up 51% to 78% of 12 measured sections southwest of Moab, farther from the Uncompahgre Uplift (Fig. 1). The eolian deposits occur as discontinuous tabular bodies, surrounded by fluvial sandstones and shales. The Cutler Formation intertongues with the Cedar Mesa Sandstone to the southwest (Figs. 1, 2, 3), a cliff-forming sandstone which has been Figure 2. Geologie map of study area showing locations referred interpreted both as marine and eolian in origin (Baars, 1962; Loope, to in text. Line A-A' is location of Figure 3. B-B' is location of Figure 1984a). Strata within the Cedar Mesa Sandstone intertongue with ma- 5, and C-C' is location of Figure 8. rine limestones on the west, and fluvial and eolian strata of the Cutler Formation on the east (Fig. 1) (Baars, 1962). Loope (1981, 1984a) has argued convincingly for an eolian setting. In 21 measured sections, the Cedar Mesa Sandstone consists of 90% to 97% cross-stratified sandstone in The alternative model is based on episodic eolian deposition. Stokes thick tabular beds (Fig. 4). Loope (1981) described erosion surfaces and (1962) suggested that extensive parallel erosion surfaces formed through thin lenticular shales and limestones between the tabular bodies. He also deflation of dunes to the water table. Loope (1981, 1984b, 1985) sug- observed terrestrial plant and vertebrate remains and sedimentary struc- gested that regionally extensive erosion surfaces within the Cedar Mesa tures formed through desiccation and eolian deposition (Loope, 1981, Sandstone formed as water-table-controlled deflation surfaces. Kocurek 1984a). Eolian deposition in the Cedar Mesa Sandstone was contempo- (1984) reported a similar deflation surface from the Jurassic Entrada raneous with the fluvial and eolian deposition in the Cutler Formation. Sandstone. In discussion papers, Kocurek (1984), Loope (1984), and Rubin and Stratigraphic Relations Hunter (1984) agreed that both theories were correct and that deflation surfaces might form during interruptions in the climbing migration of Eolian and fluvial strata within the Cutler Formation and Cedar eolian dunes. Thus, deflation surfaces should truncate first-order surfaces. Mesa Sandstone were examined in an area extending from Moab to the Talbot (1985) described a second type of regional erosional bounding Needles District in Canyonlands National Park (Fig. 2). Thirty-five strati- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/10/1541/3379329/i0016-7606-100-10-1541.pdf by guest on 27 September 2021 FLOOD AND DEFLATION SURFACES, SOUTHEAST UTAH Moab Fault 10 Km Indian Creek Needles Moenkopl Formation .Organ Rock Formation/ '/y^Fluvlai and eolian Figure 3. North-to-south cross section showing Permian stratigraphie relations along a folton and fluvial north-south lime (A-A\ see Fig. 2) across the study area. Eolien W8& lower Cutler beds (Elephant Canyon Fm WÊÊmEollan, Fluvial, and Marine tmm-. v.,- J. graphic sections were measured through the Cedar Mesa and Cutler For- Cedar Mesa is 15-18 m higher stratigraphically in the south (Figs. 2, 3) mations in this area. Near Indian Creek, in the southern part of the study (McKnight, 1940). area, cross-stratified eolian sandstones of the Cedar Mesa Sandstone inter- The limestones mark the top of a section of intertonguing fluvial, tongue with fluvial deposits of the Cutler Formation (Figs. 1, 2, 3). eolian, and marine strata. These beds have been the subject of some debate The Cedar Mesa Sandstone and stratigraphically equivalent Cutler and have been referred to as the "Elephant Canyon Formation" (Baars, Formation are -250 m thick throughout the study area. Both formations 1962) and as the "Rico Formation" (Loope, 1981, 1985). Due to the overlie and intertongue with thin fossiliferous limestones (McKnight, nature of this debate, we informally refer to this section as the "lower 1940). In the study area, the base of the Cedar Mesa Sandstone rests on Cutler beds" (suggested by Loope, 1987, personal commun.). From the and intertongues with two limestones.