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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, USA--page 5 of 14

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Fluvio-estuarine Facies Assemblage (continued)

Facies E: Flaser- and Wavy-bedded Sandstones and Siltstones
Description. Facies E consists of light-yellowish-gray, very fine grained, calcite- and dolomite-cemented, glauconitic and quartzose sandstones and siltstones. Heterolithic bedding is dominant, consisting of flaser bedding (typically bifurcated-wavy or, more rarely, wavy), and wavy bedding. Physical sedimentary structures include ripple cross-lamination, mud drapes, low- and high-angle, planar cross-lamination, and reactivation surfaces (figs. 20-21). Cross-lamination dipping in opposite directions is present (fig. 22). Locally interbedded, fine-grained sandstones display planar cross-lamination, with coarser grains tending to concentrate along foresets. Stylolites filled with clays, organic matter, and mica are common. Mudstone intraclasts are rare. Facies E consists of a single package at the Gaskill core (6,031.6-6,025.7 ft; 1,838.4-1,836.6 m), where it is associated with sandstones of facies H. Bioclasts are very scarce to absent. Large carbonized wood logs are abundant.

   
Figure 20.   Figure 21.   Figure 22.


Ichnology. No trace fossils have been recorded.

Interpretation. This facies is interpreted as having been deposited in a restricted tidal-flat environment, most likely representing the lower-intertidal sand flat developed in a lower- to middle-estuarine setting. Presence of reactivation surfaces and cross-lamination dipping in opposite directions suggests tidal influence and flow reversals. Bedload transport during tidal flow and suspension settlement during slack-water periods are indicated by alternating flaser and wavy bedding (Reineck and Wunderlich, 1968; Klein, 1971). Local presence of planar cross-lamination is suggestive of sandwave or dune migration during periods of high-energy currents (Dalrymple, 1992). Open-marine, tidal-flat deposits commonly exhibit abundant and diverse biogenic structures (e.g., Mángano et al., 1996a,b). The paucity of body and trace fossils in facies E supports the interpretation of a tidal flat developed in a restricted embayment, rather than in a coastline directly connected with the open sea. The vertical association with facies H also suggests deposition in a transitional middle- to outer-estuarine environment (see discussion of facies H below). Facies E is similar to facies 4 (interlaminated to bioturbated sandstone and shale) described by Wheeler et al. (1990) from the upper Morrow Sandstone.

Facies F: Inclined, Deformed Sandstones and Siltstones
Description. This facies consists of light-gray, very fine grained to fine-grained sandstones interbedded with dark-gray to black siltstones. Wavy bedding, flaser bedding (single, bifurcated, and flaser-wavy types) are dominant, and lenticular bedding (with connected or single thick lenses) is very common. The diagnostic features of this facies are diverse soft-sediment deformation structures (slumps, ball-and-pillow, pseudonodules, fluid-escape structures, convolute lamination, load casts, and syndepositional fractures) (figs. 23-24). Facies F includes a package at the base of Nell core (6,100-6104.2 ft; 1,859.3-1,860.6 m) and at the top of Fretz core (5,399-5,425 ft; 1,645.6-1,653.5 m). At Nell, inclined-heterolithic stratification is a distinctive structure, with abundant, climbing-ripple cross-lamination, planar cross-stratification, and mud drapes. The whole package at Fretz comprises a rotational slide. The basal part of the unit (5,425-5,420 ft; 1,653.5-1,652.0 m) is highly deformed, slumped and convoluted, with no stratification preserved. The upper part (5,420-5,399 ft; 1,652.0-1,645.6 m) is characterized by inclined strata displaying recurrent variations in lamina thickness and forming symmetric cycles (figs. 25, 26, 27, 28).

         
Figure 23.   Figure 24.   Figure 25.   Figure 26.   Figure 27.   Figure 28.


Ichnology. No discrete trace fossils have been recorded. At Fretz, some indistinct mottled textures (possibly Planolites or Palaeophycus) are locally present.

Interpretation. Facies F is interpreted as having been deposited in tidal channels. The inclined-heterolithic stratification present at Nell is interpreted as having been produced by point-bar accretion, a dominant structure in upper-intertidal channels (e.g., Reineck, 1958; Bridges and Leeder, 1976; de Mowbray, 1983; Thomas et al., 1987). The abundance of soft-sediment deformation structures at this core suggests downslope movement of sediment across bar slopes. The presence of flaser and wavy bedding records alternation of bedload transport during tidal flow and suspension settlement during slack-water periods (Reineck and Wunderlich, 1968; Klein, 1971). Planar cross-stratification probably indicates sandwave or dune migration across the channel floor. The package at the Fretz core is thought to record cut-bank rotational slides in an intertidal channel (cf. Bridges and Leeder, 1976). The presence of symmetric cycles is indicative of tidal action and these deposits may be considered as tidal rhythmites, with thicker sets of strata representing deposition during spring and thinner sets during neap tides (e.g., Kvale and Archer, 1989; Archer, 1991; Archer et al., 1991; Lanier et al., 1993; Kvale and Barnhill, 1994). The scarcity of trace fossils is problematic because intense to moderate bioturbation has been recorded in modern (Bridges and Leeder, 1976) and ancient tidal point bars (Ranger and Pemberton, 1992). This scarcity probably is due to intense syndepositional deformation. Intertidal channels recorded by facies F were probably formed in a middle-estuarine setting. This facies corresponds to facies 9 (ripple-laminated sandstone) of Wheeler et al. (1990).


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