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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, USAs--page 4 of 14

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Fluvio-estuarine Facies Assemblage (continued)

Facies C: Very Coarse Grained to Fine-grained Sandstones with Clay Drapes
Description. This facies consists of light-greenish- to yellowish-gray, very coarse grained to fine-grained, calcite- and dolomite-cemented, glauconitic and quartzose sandstones with low-angle, planar cross-stratification and ripple cross-lamination (fig. 7). Mud drapes, horizontal mud laminae, flattened mud lenses, and contorted mud laminae are present (fig. 8). Wavy bedding is dominant, but flaser bedding (single and flaser-wavy) is relatively common. Reactivation surfaces are locally present. Stylolites and variably oriented fractures are common. Facies C includes single intervals towards the bases of the Gaskill (6,065.4-6,051.11 ft; 1,848.7-1,844.4 m) and Kendrick cores (5,431.8-5,444.5 ft; 1,655.6-1,659.5 m). At Gaskill, a scour filled with pebble conglomerates occurs at the base of the unit, coincident with the Mississippian-Pennsylvanian (fig. 9). The whole unit displays a fining-upward trend and consists of stacked, erosionally bounded, fining-upward intervals. A package at the top of the unit in the Gaskill core (6,051.11-6,056.2 ft; 1,844.4-1,845.9 m) is a massive to normally graded, medium-grained sandstone with few mud drapes (fig. 10). In the Kendrick core, facies C overlies fine-grained sandstones of facies A. Cross-lamination dipping in opposite directions has been detected at this locality. Pyrite replacements are also very common in the Kendrick core. Bioclasts are very scarce to absent. A low-diversity assemblage of conodonts is present throughout the whole unit, except in the basal interval, which is devoid of microfossils. The conodont assemblage includes Neognathodus, Idiognathoides, and Adetognathus.

Figure 7.   Figure 8.   Figure 9.   Figure 10.

Ichnology. Facies C is characterized by its extremely low degree of bioturbation (typically 0-1 to very rarely 2 at the top of the package in Kendrick) and its low diversity of trace fossils. Palaeophycus isp. (fig. 11) is the dominant form, but occurs in low abundance. Skolithos isp. (fig. 12) and robust specimens of Arenicolites isp. (fig. 13) are rarely present. Monocraterion isp. (fig. 14) occurs towards the top of the unit at Kendrick. The assemblage includes very simple dwelling structures produced by opportunistic suspension-feeders. Ichnodiversity and degree of bioturbation increase slightly towards the top of the package, particularly at Kendrick. This ichnofauna is suggestive of the Skolithos ichnofacies.

Figure 11.   Figure 12.   Figure 13.   Figure 14.

Interpretation. Facies C is interpreted as having been deposited in upper estuary channels within a bay-head delta. Presence of low-angle cross-stratification indicates migration of subaqueous dunes. A channel-fill interpretation is supported by the presence of erosional basal contacts, fining-upward trends, and upward decrease in bedform size. Dominance of vertical dwelling structures of suspension-feeders also indicates strong currents that keep organic particles in suspension. Presence of mud drapes indicates sediment fallout during slack-water intervals and suggests tidal influence. Tidal action is also indicated by the sporadic presence of cross-lamination dipping in opposite directions and reactivation surfaces. The low diversity of trace fossils and the presence of simple forms produced by opportunistic animals suggest a brackish-water setting (Pemberton and Wightman, 1992). Similar assemblages dominated by dwelling structures of suspension-feeders have been commonly recorded from high-energy zones of estuarine systems, typically estuarine channels (e.g., Bjerstedt, 1987; Benyon and Pemberton, 1992; Pemberton, Reinson et al., 1992). Ichnocoenoses from facies C are clearly different from ichnofaunas recorded from tidally influenced, but freshwater settings (Buatois et al., 1997). A restricted environment is also supported by the conodont fauna (specifically by its low diversity) and by the presence of the genus Adetognathus, dominant form of the Adetognathus biofacies, which indicates shallow marginal-marine environments (Merrill, 1973; Heckel and Baesemann, 1975; Boardman et al., 1995; Merrill and von Bitter, 1976). The whole interval probably records brackish-water conditions. The increase in degree of bioturbation and trace-fossil diversity is consistent with a transgressive infill of the estuarine channels. The structureless to normally graded sandstone package at the top of this unit at Gaskill is similar to the structureless sandstone facies described by Walker (1995) from estuarine deposits of the Cardium Formation. Walker interpreted that facies as having been formed by underflow currents. Sedimentologic characteristics and stratigraphic relationships suggest a similar origin for the upper interval of facies C. Overall, facies C resembles facies 6 (crossbedded sandstone with shale drapes) described primarily from the upper Morrow by Wheeler et al. (1990).

Facies D: Parallel-Laminated Black Shales with Fading Ripples
Description. Facies D comprises dark-gray to black, monotonous, parallel-laminated, carbonate-cemented shales interlaminated with thin, very fine grained sandstone lenses that form isolated or connected starved ripples (fig. 15). This facies includes two packages at both the Gaskill and Kendrick cores. At Gaskill, the intervals are from 6,035.3 ft to 6,051.11 ft (1,839.6-1,844.4 m) and 6,033.5 ft to 6,034.9 ft (1,839.0-1,839.4 m), and at Kendrick, they are from 5,426.3 ft to 5,430 ft (1,653.9-1,655.1 m) and 5,419.1 ft to 5,420 ft (1,651.7-1,652.0 m) cores. The packages interfinger with the interbedded sandstones and conglomerates of facies H (figs. 16-17). In Gaskill, sandstone lenses are more abundant towards the base of the lower package and at the top of the upper package. Conodonts are abundant but not diverse, including the genera Idiognathoides, Neognathodus, and Adetognathus.

Figure 15.   Figure 16.   Figure 17.

Ichnology. As in the case of facies C, trace-fossil diversity is low. No biogenic structures were detected at Gaskill. The ichnoassemblage at Kendrick includes Planolites montanus (fig. 18), Diplocraterion isp. (fig. 19), Teichichnus rectus (fig. 19), Cruziana problematica, and Palaeophycus isp. The degree of bioturbation is highly variable, ranging from 0 to 3. The ichnofauna comprises a mixture of feeding structures of deposit-feeders and dwelling structures of suspension-feeders. The assemblage is an example of a depauperate Cruziana ichnofacies.

Figure 18.   Figure 19.

Interpretation. This facies is interpreted as having been deposited in the central basin of the estuarine system. Dominance of parallel-laminated mudstones indicates the fallout of fine-grained material in a low-energy environment. The fading ripples may record sand deposition due to tidal currents. Absence of root traces and pedogenic slickensides suggests subaqueous conditions. At Kendrick, the presence of an impoverished trace fossil assemblage indicates harsh conditions, most likely a stressful brackish-water environment. In contrast to the assemblage in facies C, this ichnofauna includes feeding structures of deposit-feeders, suggesting a protected, low-energy environment with abundant organic matter in the sediment. Similar ichnofaunas have been reported from restricted bay settings (e.g., Benyon and Pemberton, 1992; Pemberton, Reinson et al., 1992; Pattison, 1992). A restricted estuarine setting is also supported by the presence of the marginal-marine Adetognathus biofacies (Merrill, 1973; Merrill and von Bitter, 1976; Boardman et al., 1995). Central-bay mudstones typically accumulate close to or at the turbidity maximum and have been widely recognized in estuarine valleys (e.g., Reinson et al., 1988; Pattison, 1992; Lanier, 1993; Wood, 1994). The turbidity maximum zone represents an area of high mud accumulation linked to high flocculation rates and results from the vertical circulation patterns established in the estuary (Kranck, 1981). Muddying- to sandying-upward vertical trends similar to those identified in this study have been recognized in other estuary-funnel sequences by Pattison (1992), who suggested that they probably reflect fluctuations in sea-level changes or changes in sedimentation rate. In the present case, this vertical succession most likely reflects sand from fluvial sources (at the base of the package), followed by suspension fallout in the deepest areas of the bay (middle mud-dominated portion), and sand brought in via washovers from the estuary mouth (upper part of package).

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Kansas Geological Survey
Web version November 9,1999