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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 8 of 14

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Open-marine Facies Assemblage (continued)

Facies J: Rarely to Moderately Burrowed, Planar-crossbedded, Medium- to Fine-grained Sandstones
Description. Facies J consists of light-greenish-gray, planar-crossbedded, medium- to fine-grained, quartzose, calcite-cemented sandstones (fig. 40). Ripple cross-lamination occurs locally. Clay lenses and laminae and stylolitized clay drapes are very common (fig. 41). This facies forms several discrete packages at the Nell core, which typically overlie facies K and underlies facies I. Crinoid fragments are locally present.

 
Figure 40.   Figure 41.


Ichnology. This facies displays a higher diversity and degree of bioturbation than facies I. However, physical sedimentary structures clearly dominate over biogenic structures. Facies J typically displays a low to, very rarely, moderate degree of bioturbation (commonly 1 to rarely 2). The assemblage includes Asterosoma isp. (fig. 42), Ophiomorpha irregulaire (fig. 40), Skolithos isp. (figs. 43-44), Cylindrichnus concentricus (fig. 41), Palaeophycus isp. (fig. 43), Teichichnus isp., Rosselia isp. (figs. 41, 45), Conichnus isp., Bergaueria isp. (fig. 46), Diplocraterion isp., thick-walled burrows (fig. 47), and escape structures. Although overall ichnodiversity is moderate, individual beds commonly exhibit only one or two ichnotaxa. Thick-walled burrows tend to occur in loose bundles of four to five individuals. Trace-fossil assemblages in facies J are characterized by the dominance of vertical burrows, most commonly dwelling structures of suspension-feeders (e.g., Skolithos, Ophiomorpha) and carnivores (e.g., Conichnus, Bergaueria). However, some dwelling structures of deposit-feeders, such as Asterosoma, Rosselia, and Cylindrichnus, are very common. Spreite feeding burrows of deposit-feeders are represented only by extremely rare occurrences of Teichichnus. Facies J is dominated by elements of the Skolithos ichnofacies, but members of the Cruziana ichnofacies are also present.

         
Figure 42.   Figure 43.   Figure 44.   Figure 45.   Figure 46.   Figure 47.


Interpretation. Facies J is interpreted as a middle-shoreface deposit. Planar-crossbedding and cross-lamination indicate tractive processes, essentially migration of subaqueous sand dunes and unidirectional ripples. Dominance of physical structures over biogenic ones gives evidence of rapid sedimentation in a high-energy setting. Presence of abundant vertical burrows of suspension-feeders indicates the high energy of waves and currents that kept organic particles in suspension in a mobile, sandy substrate. Rapid sedimentation is also indicated by the escape structures. The presence of dwelling traces of deposit-feeders, however, suggests local accumulation of organic detritus in the sediment, which has been used in the construction of the concentrically laminated burrows (cf. Nara, 1995). Facies J represents the finer-grained components of facies 5 (crossbedded, fossiliferous sandstone) of Wheeler et al. (1990).

Facies K: Moderately to Thoroughly Burrowed, Rippled, Fine-grained Sandstones
Description. Facies K comprises light-greenish-gray, locally ripple-cross-laminated, fine- to (very rarely) medium-grained, quartzose, glauconite-cemented sandstones (figs. 48-49). Glauconite pellets and phosphatic coating of grains are relatively common. Bioturbation obscures most physical structures, but remnants of both symmetric and asymmetric ripples occur rarely. Clay lenses, lamina, and drapes are abundant, commonly giving the sandstone a wispy appearance. This facies is poorly developed and occurs as several discrete intervals only at the Nell core. Facies K is invariably overlain by facies J. Towards the middle part of the Nell core, it overlies facies I, representing the lower building block of the base-missing, coarsening-upward parasequences. At the top of the succession, facies K tends to overlie finer-grained deposits of facies L. Crinoid and inarticulate brachiopod fragments are scarce.

 
Figure 48.   Figure 49.


Ichnology. While ichnodiversity in this facies is similar to that in facies J, the degree of bioturbation is remarkably higher, typically between 2 and 5. The ichnofauna includes Ophiomorpha irregulaire, Arenicolites isp., Palaeophycus isp., Rhizocorallium isp., Planolites montanus, Rosselia isp., thick-walled burrows, and nonspecific burrow mottling. In contrast to facies J, feeding structures of deposit-feeders are more important components of the ichnocoenoses, and beds tend to host multispecific assemblages. The trace-fossil assemblage combines elements of both the Skolithos and Cruziana ichnofacies. Overall, the high degree of bioturbation obliterates the original primary fabric and physical sedimentary structures (figs. 48-49). The wispy appearance of this facies is probably due to biogenic activity. In some cases, the wispy streaks can be identified as wall linings of Ophiomorpha (see also Raychaudhuri and Pemberton, 1992).

Interpretation. Facies K is interpreted as a proximal lower-shoreface deposit. Rare preservation of remnants of current and wave ripples suggests the activity of oscillatory and translatory currents over the substrate. However, the intense bioturbation, particularly due to the activity of deposit-feeders, which obliterates the primary fabric gives evidence of a low-energy, nutrient-rich environment. Similar facies and ichnofaunas have been described from Cretaceous lower- to middle-shoreface cores by Raychaudhuri and Pemberton (1992). This facies is a partial equivalent of facies 4 (interlaminated to bioturbated sandstone and shale) of Wheeler et al. (1990).


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Kansas Geological Survey
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