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Fawton, T.F and Buck, B.J.. 2006, Geology Implications of diapir-derived detritus and gypsic paleosols in Lower Triassic strata near the Castle Valley salt wall, Paradox Basin, Utah Timothy F. Lawton* Institute of Tectonic Studies, New Mexico State University, Las Cruces, New Mexico 88003, USA Brenda J. Buck Department of Geoscience, University of Nevada–Las Vegas, Las Vegas, Nevada 89154, USA ABSTRACT conformities on the flanks of diapiric anti- Gypsum-bearing growth strata and sedimentary facies of the Moenkopi Formation on clines (Shoemaker et al., 1958; Elston et al., the crest and NE flank of the Castle Valley salt wall in the Paradox Basin record salt rise, 1962; Cater and Elston, 1963; Trudgill et al., evaporite exposure, and salt-withdrawal subsidence during the Early Triassic. Detrital 2004; Matthews et al., 2004). Elston et al. gypsum and dolomite clasts derived from the middle Pennsylvanian Paradox Formation (1962) described carbonate-clast conglomerate were deposited in strata within a few kilometers of the salt wall and indicate that salt rise of possible Paradox Formation fragments in rates roughly balanced sediment accumulation, resulting in long-term exposure of mobile Permian strata near the crest of the Salt Valley evaporite. Deposition took place primarily in flood-basin or inland sabkha settings that anticline, and Shoemaker et al. (1958) de- alternated between shallow subaqueous and subaerial conditions in a hyperarid climate. scribed gypsum detritus adjacent to the Sinbad Matrix-supported and clast-supported conglomerates with gypsum fragments represent Valley anticline in the Lower Triassic Moen- debris-flow deposits and reworked debris-flow deposits, respectively, interbedded with kopi Formation. These observations indicate flood-basin sandstone and siltstone during development of diapiric topography. Mudstone- diapir exposure, but their significance to the rich flood-basin deposits with numerous stage I to III gypsic paleosols capped by eolian mechanics of salt rise was not recognized. We gypsum sand sheets accumulated during waning salt-withdrawal subsidence. Association present observations here demonstrating that of detrital gypsum, eolian gypsum, and gypsic paleosols suggests that the salt wall provided the evaporite of the Castle Valley diapir was a common source for gypsum in the surrounding strata. This study documents a previ- subaerially exposed throughout the Early Tri- ously unrecognized salt weld with associated growth strata containing diapir-derived de- assic, confirming that passive diapirism was tritus and gypsic palesols that can be used to interpret halokinesis. the primary mechanism of salt rise during that time. We describe the oldest gypsic paleosols Keywords: diapirism, Moenkopi Formation, Paradox Basin, gypsum, salt structures, paleosols. known in the rock record and ascribe them to diapiric exposure. These paleosols indicate ex- INTRODUCTION treme aridity during the Early Triassic and Passive diapirism, or downbuilding, where- represent a previously unrecognized type of in the rise of salt diapirs roughly keeps pace diapir-sourced material; thus, they are an im- with contemporaneous sediment accumula- portant indicator of proximity to the salt walls. tion, has long been inferred from geometric relations of flanking strata in outcrop and seis- GEOLOGIC SETTING mic reflection profiles (Barton, 1933; Rowan, The asymmetric Paradox Basin developed 1995) and experimental models (Vendeville in middle Pennsylvanian time adjacent to the and Jackson, 1991). Nevertheless, confirma- northwest-trending basement-cored Uncom- tion of the process requires evidence for pahgre uplift (Fig. 1; Baker et al., 1933; Wen- diapir-generated topography and possible gerd and Strickland, 1954; Elston et al., 1962). evaporite exposure, whether subaerial or sub- The proximal basin fill constitutes the undif- aqueous, in the form of detritus eroded di- ferentiated Cutler Group, a Pennsylvanian– rectly from the diapir and/or its thin overbur- Permian arkosic alluvial wedge ϳ5 km thick den and deposited in flanking strata (e.g., near the uplift; it grades southwestward Giles and Lawton, 1999; Rowan et al., 2003). through a thick section of cyclic middle Penn- In this paper we describe diapir-derived de- sylvanian evaporite, shale, and carbonate of tritus and the oldest recorded gypsic paleosols the Paradox Formation into middle and Late in growth strata of the Moenkopi Formation Pennsylvanian carbonates of the Paradox and at the northwest end of the Castle Valley salt Honaker Trail Formations on the southwest wall, one of several elongate salt diapirs in the Figure 1. Location map of Paradox Basin flank of the basin (Barbeau, 2003). The Paradox Basin of eastern Utah and south- and salt anticline province. Salt walls: CV, strongly progradational Early Permian part of western Colorado (Fig. 1). The salt walls Castle Valley; FV, Fisher Valley; GV, Gypsum the alluvial wedge grades southwestward into Valley; LVD, Lisbon Valley–Dolores; MSV, marginal marine, eolian, and fluvial siliciclas- formed by migration of middle Pennsylvanian Moab–Spanish Valley; PV, Paradox Valley; evaporite of the Paradox Formation during ST, Salt Valley; SV, Sinbad Valley. Commu- tic and subordinate carbonate strata of the Cut- Late Pennsylvanian, Permian, and Triassic nities: GJ, Grand Junction; GR, Green River; ler Group (Condon, 1997). The Lower Triassic time; minor salt movement continued beyond M, Moab. Edge of salt is from Condon Moenkopi Formation, which unconformably the Triassic. Evidence for syndepositional salt (1997); distribution of salt in salt walls is af- overlies the Cutler Group and was likewise ter Shoemaker et al. (1958). Explanation rise includes thinning of strata and angular un- shows distribution of selected geologic fea- derived from the Uncompahgre uplift, post- tures near end of Pennsylvanian. Inset rect- dated important shortening-related flexural *E-mail: [email protected]. angle indicates area of Figure 2. subsidence (Barbeau, 2003), but records con- ᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; October 2006; v. 34; no. 10; p. 885–888; doi: 10.1130/G22574.1; 4 figures. 885 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/34/10/885/3531439/i0091-7613-34-10-885.pdf by Rice University user on 27 February 2020 Figure 2. Geologic map and cross section of northwest end of Castle Valley anticline. Plunging crest of anticline is indicated by anticlinal symbol. Locations of measured sections of Figure 3: EM, East monocline; PM, Pariott Mesa; WM, West monocline. Map units of Moenkopi Formation are stratigraphic intervals dominated by lithologic associations (see text for details), except for Pariott Member, which is a formal lithostratigraphic unit (Shoemaker and Newman, 1959). Use of informal ‘‘lower Cutler beds’’ as a lithostratigraphic unit follows the recommendation of Condon (1997). tinued diapirism in the salt anticline region in opposite sides of the structure dip steeply and gion represents terrestrial strata deposited in a the northeastern part of the Paradox Basin face outward away from the weld; (2) the hyperarid basin of low regional relief. On the (Shoemaker et al., 1958; Shoemaker and New- structure terminates downdip at exposures of northeast flank of the Castle Valley anticline, man, 1959; Trudgill et al., 2004). Paradox gypsum, shale, and dolostone; (3) the Moenkopi unconformably overlies arkosic The Castle Valley salt wall and associated strata on both sides of the structure thin to- sandstone and conglomerate of the Cutler For- anticline (Fig. 1) trend 310Њ from Oligocene ward it; (4) updip Cutler beds adjacent to the mation with ϳ10Њ of angular discordance. The intrusions on the southeast to a plunge ter- structure contain dolostone clasts derived basal bed consists of structureless fine- to mination at the Colorado River on the north- from the Paradox Formation, although the coarse-grained gypsum-cemented sandstone. west. Gypsum, black shale, and dolostone of Paradox is no longer present there. The struc- This is overlain on both sides of the Castle the Paradox Formation crop out locally on the ture loses stratigraphic displacement upward Valley by a distinctive 0.7–1-m-thick bed of flanks and near the middle of the eroded salt and terminates into the upper hinge of a finely crystalline gypsum (Dane, 1935; Shoe- wall where not covered by a thin veneer of monoclinal panel of growth strata in the maker and Newman, 1959), locally with faint Neogene surficial deposits. Permian and Tri- Moenkopi Formation (Fig. 2). Maximum tabular cross-beds as high as 30 cm. We in- assic strata dip northeast and southwest away structural relief on the monocline decreases terpret this extensive bed of gypsum as an eo- from the diapir; dips are steep to overturned northward from 250 m at its southernmost ex- lian sand sheet. ϳ near the diapir and decrease away from the posure to 20 m near the Colorado River (a Four major lithologic associations are pres- diapir and upward through the section, indi- distance of 1600 m). On the western upthrown ent in the Moenkopi Formation adjacent to the cating decreasing amounts of halokinetic ro- block of the weld, laminated dolomicrite and Castle Valley salt wall (Figs. 2 and 3): (1) het- red sandstone that we interpret as upper tation in progressively younger strata. erolithic thin-bedded siltstone and sandstone; Pennsylvanian–Lower Permian ‘‘lower Cutler Structural and stratigraphic relations at the (2) thick-bedded sandstone; (3)
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