Current Research--Buatois et al.--page 6 of 14

Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 243, part 1
Sedimentology and Ichnology of Paleozoic Estuarine and Shoreface Reservoirs, Morrow Sandstone, Lower Pennsylvanian of Southwest Kansas, USA--page 6 of 14

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Fluvio-estuarine Facies Assemblage (continued)

Facies G: Laminated Calcareous Mudstones

Description. This facies includes light-gray, parallel-laminated, calcareous mudstones (fig. 29). Facies G occurs only at Fretz, forming a single package at the middle interval of the core (5,425-5,429 ft; 1,645.6-1,654.8 m). A 4-cm (1.6-in)-thick lag of small pebbles, granules, and fragmented shells separates this facies from the underlying paleosol siltstones of facies B (fig. 30).

Figure 29. Figure 30.

Ichnology. No trace fossils have been recorded.

Interpretation. Facies G is interpreted as having been deposited in a restricted, low-energy, subtidal middle- to lower-estuarine setting. The basal, coarse-grained deposit represents a transgressive lag that indicates inundation of the paleosols developed at the interfluves.

Facies H: Poorly to Moderately Fossiliferous, Planar-crossbedded Sandstones and Pebble Conglomerates

Description. Facies H comprises light-gray, massive, normally graded, parallel-laminated, or low-angle planar-crossbedded, coarse- to medium-grained, calcite-cemented, glauconitic and quartzose sandstones and pebble conglomerates with interbedded fine-grained and very fine grained sandstones and carbonaceous mudstone partings. Ripple cross-lamination (fig. 31), stylolites, mud drapes, mud lenses, and flaser bedding (wavy type) are abundant in the finer-grained interbeds. Mudstone intraclasts are also common. This facies forms three packages at the middle part of the Gaskill core--from 6,035.3 ft to 6,034.9 ft (1,839.6-1,839.4 m), 6,033.5 ft to 6,031.6 ft (1,839.0-1,838.4 m), and 6,014.8 ft to 6,025.7 ft (1,833.3-1,836.6 m)--and two packages at Kendrick--from 5,420 ft to 5,426.3 ft (1,652.0-1,653.9 m) and 5,419.1 ft to 5,415 ft (1,651.7-1,650.5 m). It typically interfingers with the shale deposits of facies D (figs. 16-17) and, in the Gaskill core, with the heterolithic deposits of facies E. It is overlain by bioturbated sandstones of facies L at Gaskill and by fossiliferous, crossbedded sandstones of facies I at Kendrick. At Gaskill, this facies is composed of sandstones, whereas conglomerates are dominant at Kendrick. Pebbles are subangular to subrounded and may be up to 2 cm (0.7 in) but are commonly 0.3-0.8 cm (0.1-0.3 in). The matrix consists of fine- to coarse-grained quartzose sand. Bioclast content in this facies is highly variable, commonly ranging from absent to moderate. Crinoid fragments are locally common, and articulate brachiopod shells, rugose corals, and fenestrate bryozoans are also present. Overall, the fossil content increases upwards, with bioclasts becoming more common in the upper packages (fig. 32). A moderately diverse conodont fauna--including the genera Adetognathus, Cavusgnathus, Neognathodus, and Idiognathoides--is present at Gaskill.

Figure 31. Figure 32.

Ichnology. Trace fossils are extremely rare at Gaskill. Discrete, very simple feeding traces of Planolites isp. occur in carbonaceous mudstone partings at the top of the interbedded fine-grained and very fine grained sandstones. Degree of bioturbation ranges from 0 to very rarely 1. At Kendrick, trace fossils are mainly restricted to the uppermost part of the lower package, where the silty matrix of the conglomerate is intensely bioturbated (up to degree 4) (fig. 33). The association includes Palaeophycus isp., Asterosoma isp., Diplocraterion isp., and Skolithos isp. (fig. 34), as well as undistinguishable mottled textures. A large, lined burrow probably produced by crustaceans occurs towards the base of this package. Small Planolites montanus occur at mudstone partings. The assemblage present in facies H probably represents a mixed, depauperate Cruziana-Skolithos ichnofacies.

Figure 33. Figure 34.

Interpretation. Facies H is interpreted as having been deposited in the estuary mouth, comprising the sand plug. Such sand plugs are typical elements of wave-dominated estuaries (e.g., Dalrymple et al., 1992; Pattison, 1992; Zaitlin et al., 1994). The stratigraphic position of this facies, interfingering at the base with the central-bay mudstones (facies D) and lying immediately below open-marine, lower-shoreface sandstones (facies L) or upper-shoreface sandstones (facies I), supports deposition at the seaward end of the estuary. A brackish environment is also suggested by the low-diversity conodont fauna and by the occurrence of the genus Adetognathus, dominant form of the marginal-marine Adetognathus biofacies (Merrill, 1973; Merrill and von Bitter, 1976; Boardman et al., 1995). Facies H includes different subenvironments within the estuary mouth: the coarser-grained facies probably records washovers and flood tidal deltas, and the finer-grained facies represents deposition in more quiet, protected settings. Packages commonly display evidence of increasing marine influence upwards (e.g., more bioclasts) and most likely record retrogradation of the barrier complex that separated the estuary from the open-marine environment. The presence of crinoidal, brachiopod, coral, and bryozoan skeletal debris in the sandstones indicates a seaward source of detritus. The absence of bioturbation in the coarser-grained barrier facies at Gaskill probably is due to high energy and sedimentation rate, rather than brackish-water conditions. Monospecific suites of poorly specialized traces of opportunistic animals in the finer-grained deposits at Gaskill are common features of estuarine systems (e.g., Wightman et al., 1987; Keith et al., 1988; Ranger and Pemberton, 1988, 1992; Pattison, 1992; Pemberton and Wightman, 1992; Benyon and Pemberton, 1992; Pemberton, Reinson et al., 1992; MacEachern and Pemberton, 1994). Planolites seems to reflect opportunistic colonization after rapid deposition of washover sands derived from the barrier. Brackish conditions also probably precluded the establishment of a more diverse ichnofauna. Washover-fan and flood tidal-delta deposits typically display very few traces (e.g., Pemberton, Van Wagoner et al., 1992). The presence of intensely bioturbated sediments at the top of the lower package in Kendrick probably also records colonization by the infauna after deposition of coarse-grained detritus derived from the barrier. Although Pemberton, Reinson et al. (1992) proposed that the presence of Asterosoma is suggestive of periods of normal to quasi-normal salinity conditions, other authors (e.g., MacEachern and Pemberton, 1994; Howell et al., 1996) recorded Asterosoma in restricted, brackish-water settings. This facies is probably partially equivalent to facies 5 (crossbedded fossiliferous sandstone) and 6 (crossbedded sandstone with shale drapes) of Wheeler et al. (1990).

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