Sequence Stratigraphy and Marine to Nonmarine Facies Architecture of Foreland Basin Strata, Book Cliffs, Utah, U.S.A.: Discussion1 Shuji Yoshida2, Andrew D. Miall3, and Andrew Willis4 ©Copyright 1998. The American Association of Petroleum Geologists. All Rights Reserved1Manuscript received July 17, 1996; revised manuscript received March 20, 1997; final acceptance March 26, 1998. 2Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada. Current address: T.H. Huxley School, Imperial College, London SW7 2BP, United Kingdom. 3Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada. 4 Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada. Current address: Rigel Oil & Gas Ltd., 1900 Bow Valley Square 3, 255-5th Avenue, SW, Calgary, Alberta T2P 3G6, Canada. INTRODUCTION Van Wagoner (1995) published a lengthy facies description and sequence interpretation of the Castlegate Sandstone and the underlying Desert Member of the Blackhawk Formation of the Book Cliffs of Utah (Figure 1). We take issue with Van Wagoner in a number of areas, including the following: (1) facies interpretation and regional sequence stratigraphic framework of the Blackhawk-Castlegate succession, (2) the sequence generating mechanism, (3) paleogeographic reconstruction of the lower Castlegate Sandstone, and (4) many of the details of the general nonmarine sequence model that Van Wagoner proposed. Van Wagoners field study was limited to the area east of Green River. In this area the Castlegate Sandstone corresponds to the cliff-forming sandstone unit termed the lower Castlegate Sandstone by Fouch et al. (1983), based on their studies of the type section at Price Canyon, north of Price, Utah. The upper Castlegate Sandstone of Fouch et al. (1983) passes into the Sego and Neslen formations east of Green River (Figures 2, 3). Our detailed mapping of the area west of Green River (Yoshida et al., 1996) clarified the facies and sequence relationships of these units and provides the basis for the comments contained in this discussion. REGIONAL SEQUENCE STRATIGRAPHIC FRAMEWORK Van Wagoner (1995) assumed, without carrying out field mapping west of the Green River, that both the Castlegate and the Desert sequence boundaries are regionally traceable for a great distance, and he extrapolated his observations of the Desert sequence architecture east of Green River westward into updip localities. He stated (1995, p. 146) that lowstand sandstones in both the Desert Member and the lower Castlegate Sandstone sequences are fluvial sheet sandstones contained within incised valleys at least 50 mi (80 km) wide; however, Yoshida et al. (1996) showed that the erosional sequence boundaries of the Desert Member and the lower Castlegate Sandstone sequences differ in lateral extent both in updip and downdip directions. The Castlegate sequence boundary is traceable along the entire Book Cliffs outcrop belt (Figure 2). By contrast, the high-frequency sequences of the upper Blackhawk Formation (Sunnyside, Grassy, and Desert members) have type 1 erosional sequence boundaries with limited lateral extent. These sequences are characterized by well-developed dendritic incised-valley systems and well-preserved interfluve areas (e.g., OByrne and Flint, 1995) within the area overlying the Paleozoic Paradox basin (Figure 1), but in updip and downdip directions these sequence boundaries become less erosional and finally become conformities. No incised-valley systems extend beyond this area (OByrne and Flint, 1995; Yoshida et al., 1996) (Figures 2, 3). The Desert sequence boundary is conformable northwest of Trail Canyon (Yoshida et al., 1996) (Figures 2, 3) and downdip near Sagers Canyon End page 1596---------------- (Van Wagoner, 1995, p. 155). East of Sagers Canyon there is no evidence of subaerial exposure, such as rootlets, paleosols, or widespread erosional surfaces (Van Wagoner, 1995, p. 155). Castlegate sequence boundary 1 (Van Wagoner, 1995, his figure 5) (cf. Figure 2) has pronounced erosional relief between Thompson Canyon and the Paradox basin margin to the east (Figure 1; Van Wagoner, 1995, his figure 5, between the Thompson Canyon and Bull Canyon sections). Downdip, beyond the Paradox basin margin, the Castlegate sequence boundary becomes a flat and conformable surface that is unmappable east of the Horse Pastures area (Van Wagoner, 1995, p. 179). Van Wagoners (1995) Castlegate sequence boundary 2, corresponding to the regionally extensive Castlegate sequence boundary (Van Wagoner et al., 1990, 1991; Yoshida et al., 1996) (Figure 2) also has less erosional relief in the downdip direction. Between Tusher Canyon and the Paradox basin margin, near Sagers Canyon, incision is broad and deep with few to no interfluves (Van Wagoner, 1995, p. 179). Van Wagoners (1995) figures (west of the Bull Canyon section of his figures 5 and 46) show no interfluves except a few intravalley highs. Downdip from the Paradox basin margin, near Sagers Canyon, incision becomes progressively narrower and shallower, with an accompanying increase in number and areal extent of interfluves (Van Wagoner, 1995, p. 179). East of the Horse Pastures area, incision is absent and the sequence boundary is represented by a flat surface. FACIES ARCHITECTURE OF NONMARINE DEPOSITS Van Wagoner (1995) interpreted the lower Castlegate Sandstone west of Sagers Canyon as being made up solely of braided-fluvial deposits with minor brackish-water incursions near the top. We found, however, that this unit includes a significant thickness of tidally influenced fluvial deposits and estuarine facies in its upper part (Figures 3, 4). East of Green River, thin, transgressive shoreface sandstones with abundant Ophiomorpha and tidal-inlet channel deposits rest erosionally on the estuarine deposits at the top of the lower Castlegate Sandstone (Figure 4). Van Wagoner (1995) suggested that the Desert lowstand shoreline retreated far to the east, beyond Sagers Canyon; however, to date, no detached lowstand deposits of the Desert sequence have been found in this area, and there is no evidence of subaerial exposure, such as rootlets, paleosols, or widespread erosional surfaces east of Sagers Canyon (Van Wagoner, 1995, p. 155). Beyond this point eastward, the Desert sequence boundary was interpreted to coincide with the flat flooding surface that marks the lithostratigraphic top of the marine Desert Member (Van Wagoner, 1995, p. 155). His interpretation of the Desert lowstand fluvial system on the emergent shelf is questionable because perennial river systems in a humid environment (the environment we suggest, as discussed in following paragraphs) do not show a downdip decrease in channel-base erosional relief. Downdip-decreasing Figure 1-Outcrop belt of the Castlegate Sandstone and the location of the Paradox basin. JV = Joes Valley reservoir, PC = Price Canyon, SS = Sunnyside, HC = Horse Canyon, TR = Trail Canyon, NT = Nefertiti, TU = Tusher Canyon, CC = Coal Canyon, CR = Crescent Canyon TH = Thompson Canyon, SC = Sagers Canyon, HP = Horse Pastures area, PR = Prairie Canyon. Paleocurrent data are taken from Van de Graaf (1972), and the edge of the late Paleozoic Paradox basin with the listric basement fault (Uncompahgre fault) on its northern margin is taken from Friedman et al. (1994). End page 1597---------------- Figure 2-Schematic sequence architecture of the Castlegate Sandstone and equivalent units, Book Cliffs, Utah (modified from Yoshida et al., 1996). Systems tract designation indicated at Horse Canyon refers to the low-frequency sequence of Olsen et al. (1995). Sequence numbering and systems tract designation at right is that of Van Wagoner et al. (1990), and refers to the high-frequency sequences present east of Tusher Canyon. DSB = Desert sequence boundary, CSB1 and CSB2 = Castlegate sequence boundaries 1 and 2, respectively, of Van Wagoner (1995). CSB1 is truncated by CSB2 west of Crescent Canyon, whereas CSB2 is traceable throughout the study area. For this reason, CSB2 is called the Castlegate sequence boundary in this paper, following the usage of Olsen et al. (1995) and Yoshida et al. (1996). End page 1598---------------- channel-base erosional relief is observable, however, in some coastal environments, such as distributary channel systems of ebb-dominated deltas at the mouths of estuarine valleys (e.g., Yang and Nio, 1989); therefore, we interpret the final Desert lowstand shoreline did not retreat east beyond Sagers Canyon. Van Wagoners (1995) Castlegate sequence boundary 2 also has less erosional relief in the downdip direction. Downdip from the Paradox basin margin near Sagers Canyon, incision becomes progressively narrower and shallower, with an accompanying increase in the number and areal extent of interfluves (Van Wagoner, 1995, p. 179). East of the Horse Pastures area, incision is absent and the sequence boundary is represented by a flat surface that Van Wagoner (1995, p. 180, his figure 5) interpreted as an interfluve overlain by freshwater deposits that formed in ephemeral lakes and swamps; however, he failed to show clear indicators of an emergent shelf, such as rootlets, desiccation cracks, calcretes, or soil horizons. We (Yoshida et al., 1996) interpreted the channel systems with downdip-decreasing erosional relief as a distributary channel system in deltaic or fluvial-dominated estuarine deposits in the lower part of an incised valley mouth. The downdip decrease in the erosional relief of the Castlegate lowstand channel system near Sagers Canyon also could be explained by the deviation of the present outcrop belt from the major distributary pathway of the Castlegate river system. We interpreted the overall increase in tidal indicators in the upper part of the lower Castlegate Sandstone (Figures 3, 4) as recording the drowning of the incised valleys prior to the major Buck tongue transgression (Yoshida et al., 1996). For this reason we assume that during the formation of Castlegate sequence boundaries 1 and 2 of Van Wagoner (1995), the shoreline did not retreat far downdip from the Horse Pastures area. SEQUENCE GENERATING MECHANISM Tectonism Van Wagoner (1995) suggested climatically induced eustatic sea level changes as the driving mechanism that generated the sequence architecture observed in the Desert Member and the lower Castlegate Sandstone; however, as we detail in the following paragraphs, we found no evidence for climate change during deposition of these units, and eustasy alone cannot explain the following observations: (1) Localization of high-frequency

Shuji Yoshida, Andrew D. Miall,
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AAPG Bulletin, V. 82 (1998), No. 8 (August 1998), P. 1596-1606.

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Sequence Stratigraphy and Marine to Nonmarine Facies Architecture of Foreland Basin Strata, Book Cliffs, Utah, U.S.A.: Discussion¹

Shuji Yoshida², Andrew D. Miall³, and Andrew Willis⁴

¹Manuscript received July 17, 1996; revised manuscript received March 20, 1997; final acceptance March 26, 1998.
²Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada. Current address: T.H. Huxley School, Imperial College, London SW7 2BP, United Kingdom.
³Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada.
⁴ Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada. Current address: Rigel Oil & Gas Ltd., 1900 Bow Valley Square 3, 255-5th Avenue, SW, Calgary, Alberta T2P 3G6, Canada.

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