KGS Home Current Research Home Article Start

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 9 of 14

Prev Page--p. 8 || Next Page--p. 10


Open-marine Facies Assemblage (continued)

Facies L: Thoroughly Burrowed, Fine-grained to Very Fine Grained Silty Sandstones with Starved Ripples
Description. This facies consists of dark-greenish-gray, fine-grained to very fine grained, dolomite- and quartz-cemented, glauconitic and quartzose silty sandstones. Phosphatic particles are relatively common. Primary structures are obscured for the most part by bioturbation (fig. 50), but starved ripples are locally detected (fig. 51). Ripples are symmetrical to quasisymmetrical, with irregular and undulating lower boundaries. Cross-laminae are strongly tangential towards the lower boundary, commonly sweeping up and ascending on to the adjacent bedform forming offshoots. Mud drapes are abundant and relatively thick. At Nell, this facies occurs in two intervals--from 6,048.3 ft to 6,046.7 ft (1,843.5-1,843.0 m) and 6,057.7 ft to 6,055.6 ft (1,846.4-1,845.7 m) towards the upper part of the succession. The lower interval is present between two thin packages of facies M. The upper interval is overlain by facies K and underlain by facies J, forming the base of coarsening-upward parasequences. At Gaskill, facies L comprises a single interval (6,015.8-6,013.9 ft; 1,833.6-1,833.0 m). This interval is separated from the underlying facies H by a transgressive lag and is overlain by facies M. The conodont fauna at Gaskill is relatively abundant and diverse and includes the genera Neognathodus, Idiognathoides, Adetognathus, and Idiognathodus. Crinoid fragments are common in both cores.

 
Figure 50.   Figure 51.


Ichnology. This facies displays more intense bioturbation than facies K, ranging from 3 to 5. The assemblage includes Planolites montanus, Chondrites isp. (figs. 51-52), Rhizocorallium isp. (fig. 50), Diplocraterion isp. (figs. 50, 52), Teichichnus isp., Zoophycos isp., Cylindrichnus concentricus, Asterosoma isp., Palaeophycus isp., Ophiomorpha irregulaire, Skolithos isp., and Arenicolites isp. (fig. 51). Although some dwelling traces of suspension-feeders are present, the assemblage is dominated by feeding structures of deposit feeders. The association is an example of the Cruziana ichnofacies. The high density of trace fossils obliterates all evidence of primary bedding and physical sedimentary structures in most beds. Individual beds typically host several ichnotaxa. As a result of high bioturbation, individual forms are sometimes difficult to recognize. At Gaskill, a mottled indistinctive Palaeophycus and Planolites background ichnofabric is overprinted by discrete vertical U-shaped, medium-tiered Diplocraterion, which in turn are crosscut by very well defined specimens of deeper-tiered Rhizocorallium. At Nell, the tiering structure seems to be slightly different. Rhizocorallium is not very distinctive and Chondrites commonly occurs forming small clusters of up to 20 individuals in mudstone pockets or reworking other burrows, such as U-shaped Arenicolites, suggesting emplacement in a deep tier.

Figure 52.


Interpretation. Facies L is interpreted as a distal lower-shoreface deposit. The occasional presence of starved wave ripples indicates sporadic distal storm events. However, most evidence of physical sedimentation has been obliterated by bioturbation. Homogenization of event deposits by burrowing activity is a very common phenomenon in shoreface environments (e.g., Dott, 1988; Pemberton, MacEachern et al., 1992; Pemberton and MacEachern, 1997). Nevertheless, the nature of the ripples and the intensity of bioturbation suggest that sand was deposited in relatively thin beds. The conodont fauna is suggestive of nearshore settings (cf. Boardman et al., 1995). Trace-fossil assemblages from this facies resemble other lower-shoreface, fairweather ichnofaunas described from the Cretaceous of Canada (e.g., MacEachern et al., 1992; MacEachern and Pemberton, 1992). This facies is partially equivalent to facies 4 (interlaminated to bioturbated sandstone and shale) of Wheeler et al. (1990).

Facies M: Thoroughly Burrowed, Very Fine Grained Silty Sandstones and Siltstones with Interbedded, Normally Graded Sandstones
Description. Facies M comprises dark-greenish-gray, very fine grained, quartzose and glauconitic, dolomite-cemented silty sandstones and siltstones (fig. 53). Matrix consists of silt, organic matter, and clay. Interbedded mudstone laminae are rich in organic matter and silt grains and are commonly stylolitized. Pyrite is locally present. Facies M includes interbedded, erosive-based, normally graded, coarse- to medium-grained sandstones that are capped by very fine grained silty sandstones with symmetric to quasisymmetric ripples. Lower boundaries of ripples are irregular and undulating. Internal structure consists of strongly tangential cross-laminae that may form offshoots, and swollen lenslike sets. Although dispersed throughout the normally graded division, bioclasts are more abundant towards the base, forming a lag of crinoid and gastropod fragments. Basal surfaces may display relatively deep and steep scours. Fading symmetric to quasisymmetric ripples also occur isolated within fine-grained packages. This facies is not very well represented in the studied cores. It forms two thin intervals at Nell--from 6,057.7 ft to 6,058.2 ft (1,846.4-1,846.5 m) and 6,054.1 ft to 6,055.6 ft (1,845.3-1,845.7 m)--and another one at Gaskill (6,013.6-6,013.9 ft; 1,832.9-1,833.0 m). Facies M is typically associated with facies L and O, being part of both coarsening- and fining-upward successions. At Gaskill, the top of the facies M interval is sharply attenuated by an erosive surface overlain by facies I.

Figure 53.


Ichnology. The degree of bioturbation in facies M is highly variable. Interbedded coarse-grained beds are essentially unbioturbated, but finer-grained tops may exhibit low to moderate bioturbation (2 to 3). Conversely, the degree of bioturbation in very fine grained silty sandstones and siltstones is high (5). Trace-fossil assemblages include Planolites montanus (fig. 53), Chondrites isp. (fig. 53), Teichichnus isp., and Palaeophycus isp. Palaeophycus tends to occur in the finer-grained sandy tops, while the other forms are more abundant in the siltstone and silty sandstone intervals. The association is dominated by feeding traces of deposit-feeders and represents an example of the Cruziana ichnofacies.

Interpretation. This facies is interpreted as having been deposited in the transition zone between the offshore and the shoreface. Sedimentologic and ichnologic analyses reveal the interplay of storm-event deposition and background fairweather sedimentation. The erosional and depositional history of this facies can be summarized in four stages: (1) the initial erosive phase of the storm event recorded by the basal scoured surface, (2) the phase of main storm deposition represented by the coarse-grained, normally graded division, (3) the phase of waning storm deposition recorded by the wave rippled, finer-grained sandstone formed under lower-energy oscillatory conditions, and (4) the post-storm, fair-weather deposition phase characterized by the presence of interbedded, very fine grained silty sandstones and siltstones, which may record the latest stage of sediment fallout after the storm or background sedimentation (see also Pemberton, Van Wagoner et al., 1992; Pemberton and MacEachern, 1997). Biogenic activity was restricted to the third and fourth phases. Palaeophycus represents opportunistic colonization immediately after waning storm deposition, while feeding structures of deposit-feeders were emplaced after the storm and record the activity of the resident fairweather assemblage. Bioturbation by this later suite obliterated the upper contact, leading to the formation of diffuse gradational tops. This facies is a partial equivalent of facies 4 (interlaminated to bioturbated sandstone and shale) of Wheeler et al. (1990).

Facies N: Thoroughly Burrowed Siltstones
Description. Facies N consists of dark-gray, intensely bioturbated siltstones (figs. 54-55). Physical sedimentary structures have not been preserved and the only evidence of the primary fabric is the presence of a relic, irregularly parallel lamination. This facies only occurs at Nell, forming a single interval (6,053-6,054.1 ft; 1,844.9-1,845.3 m), which overlies facies M and is sharply attenuated at the top by an erosional surface that separates facies N from the overlying facies J.

 
Figure 54.   Figure 55.


Ichnology. Degree of bioturbation is extremely high, ranging from 5 to 6. Tiny Phycosiphon incertum and Chondrites isp. represent the background ichnofabric. This assemblage is overprinted by relatively large and slightly compressed Thalassinoides isp. (fig. 54). The assemblage records the activity of a stable deposit-feeder community and may be regarded as an example of a depauperate Cruziana ichnofacies.

Interpretation. This facies is interpreted as representing offshore deposits. Deposition was dominated by sediment fallout of silt particles. Intense bioturbation resulted in complete to nearly complete obliteration of the original fabric. The dominance of feeding structures of deposit-feeders indicates a low-energy, nutrient-rich environment. Martin and Pollard (1996) have documented similar low-diversity Phycosiphon and Chondrites ichnofabrics from offshore deposits of the Fulmar Formation. The low diversity and small size of the burrows are suggestive of stressful environmental conditions, with low-oxygen content being the most likely candidate (cf. Savrda and Bottjer, 1986; Savrda, 1992). The facies-N ichnoassemblage seems to be represented by forms more or less typical of relatively deep tiers. In oxygen-depleted settings, uppermost tiers are replaced by deeper tiers (Bromley and Ekdale, 1984). However, Bromley (1996) urged caution in invoking a low-oxygen content when deeper-tier forms are responsible for total bioturbation. Extensive burrowing activity may have actually lead to the destruction of the shallower traces. This facies is a partial equivalent of facies 4 (interlaminated to bioturbated sandstone and shale) of Wheeler et al. (1990).

Facies O: Parallel-Laminated Black Shales
Description. This facies comprises black, parallel-laminated, commonly calcareous shales. It was observed only in the Gentzler field, where it occurs in both Nell and Gaskill wells. In Gaskill, it forms a very thick package at the top of the succession (6,000.6-5,952 ft; 1,828.9-1,814.2 m), overlying the coarse-grained sandstones of facies I. Towards the base of the interval, normally graded, conglomeratic, very coarse grained to medium-grained, carbonate-cemented sandstones are interbedded with the fine-grained deposits (fig. 56). Bioclast content is very high, consisting of crinoid and coral fragments. In some beds, bioclasts and pebbles tend to concentrate at the base, forming lags. In other cases, they are dispersed throughout the bed. The conodont fauna is relatively diverse and abundant and includes the genera Idiognathodus, Neognathodus, Adetognathus, Rhachistognathus, Declinognathodus, and Idiognathoides. In Nell, facies O forms a single interval (6,058.2-6,063 ft; 1,846.5-1,848.0 m) in the upper half of the succession. This package is underlain by coarse-grained sandstones of facies I and overlain by facies M.

Figure 56.


Ichnology. No trace fossils have been recorded.

Interpretation. This facies is interpreted as having been deposited in a shelf environment. Sediment fallout of fine-grained particles was the dominant depositional process. Interbedded coarse-grained beds may record transgressive ravinement. Preservation of thin laminations, absence of bioturbation, and dark colors are suggestive of anoxic bottom-water conditions. The conodont fauna represents the Idiognathodus-Streptognathodus biofacies, most likely the Idiognathodus subfacies of Boardman et al. (1995). According to these authors, this subfacies characterizes deposition in oxygen-depleted, open-marine areas. Facies O is an equivalent of facies 1 (dark-gray, thinly laminated shale) of Wheeler et al. (1990).


Previous Page--p. 8 || Next Page--p. 10

Kansas Geological Survey
Web version November 9,1999
http://www.kgs.ku.edu/Current/1999/buatois/buatois9.html
email:lbrosius@kgs.ku.edu