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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 3 of 14

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Facies Descriptions and Interpretations

Fifteen facies, grouped in two facies-assemblages (fluvio-estuarine and open marine), were identified (table 1). The sedimentology and ichnologic content of each of these facies is discussed below, along with its implications in terms of depositional conditions and sedimentary environments. Facies descriptions include thin-section information. Samples from Gaskill were processed for conodonts and a list of taxa was available for this study (H. R. Lane, unpublished report, 1983). Information on the degree of bioturbation is based on the scheme by Taylor and Goldring (1993). Terminology for skeletal accumulations is based on Kidwell et al. (1986).


Table 1. Facies scheme of the lower Morrow Sandstone in the Arroyo and Gentzler fields.

FaciesDepositional ProcessSedimentary Environment
A: Fine-grained sandstones Tractive currents Fluvial channels
B: Rooted siltstones Pedogenic processes Paleosol
C: Very coarse grained to fine-grained sandstones with clay drapes Migration of subaqueous, unidirectional flow dunes and slack-water sediment fallout Upper-estuarine, bay-head delta channels
D: Parallel-laminated black shales with fading ripples Sediment fallout and low-energy tidal currents Central-estuarine bay
E: Flaser- and wavy-bedded sandstones and siltstones Tidal currents Restricted tidal flat
F: Inclined, deformed sandstones and siltstones Tidal currents, sediment downslope movement and lateral accretion Tidal channel
G: Laminated calcareous mudstones Sediment fallout Lower estuary
H: Poorly to moderately fossiliferous, planar-crossbedded sandstones Migration of subaqueous, unidirectional flow dunes Estuarine mouth
I: Highly fossiliferous, planar-crossbedded, very coarse grained to medium-grained sandstones and pebble conglomerates Migration of subaqueous, unidirectional flow dunes Upper shoreface
J: Rarely to moderately burrowed, planar-crossbedded, medium- to fine-grained sandstones Migration of subaqueous, unidirectional flow dunes Proximal middle shoreface
K: Moderately to thoroughly burrowed, rippled, fine-grained sandstones Migration of subaqueous, unidirectional flow ripples and sediment fallout Distal middle shoreface
L: Thoroughly burrowed, fine-grained to very fine grained silty sandstones with starved ripples Migration of subaqueous, unidirectional flow ripples and sediment fallout Lower shoreface
M: Thoroughly burrowed, very fine grained silty sandstones and siltstones with interbedded, normally graded sandstones Storm action and sediment fallout Offshore transition
N: Thoroughly burrowed siltstones Sediment fallout Offshore
Parallel laminated black shales Sediment fallout Shelf


Fluvio-estuarine Facies Assemblage

Facies A: Fine-grained Sandstones
Description. This facies consists of light-gray to yellowish gray, fine-grained, calcite-cemented, glauconitic and quartzose sandstones (fig. 4). Sandstones are typically massive, with poorly defined, high-angle, planar crossbedding only preserved locally. Some foresets are delineated by small, flattened coal intraclasts. Stylolites and rare, very thin siltstone drapes occur towards the top of the interval. Pyrite replacements are very common (fig. 5). Facies A is present only at Kendrick, forming a single interval at the base of the core (5,459.7-5,444.5 ft; 1,664.1-1,659.5 m).

 
Figure 4.   Figure 5.


Ichnology. Scarce and very small (2-3 mm; 0.08-0.12 in) Palaeophycus isp. were recorded.

Interpretation. Facies A is interpreted as having been deposited in fluvial channels. Local presence of planar crossbedding indicates migration of unidirectional, subaqueous dunes. A freshwater setting is supported by the paucity of bioturbation. Palaeophycus is a facies-crossing form, and no definite marine indicators are present in this interval. Fluvial facies recorded in the upper Morrow by Wheeler et al. (1990) are typically coarser grained.

Facies B: Rooted Siltstones
Description. Facies B consists of dark-gray siltstones with textures and structures indicative of pedogenic processes, such as prismatic peds and very fine grained cutans with striated and smeared surfaces (fig. 6). Root marks and plant debris are present. Scarce fragments of abraded, marine bivalve shells were also detected. Facies B is only present at Fretz, forming a single interval at the base of the core (5,429-5,449 ft; 1,654.8-1,660.9 m). The upper part of the unit (5,431-5,429 ft; 1,665.4-1,654.8 m) consists of laminated shales.

 
Figure 6.  


Ichnology. No animal traces were detected.

Interpretation. This facies is interpreted as a paleosol, which most likely developed in interfluve areas. The local presence of bivalve shells indicates pedogenic modification of marine sediments. The striated and smeared surfaces of the cutans are regarded as slickensides, and the cutans, therefore, are considered to be stress cutans. Although stress cutans may also form simply by the crushing of peds against one another during compaction (Retallack, 1990), the swelling and shrinking of clays during repeated wetting and drying episodes is consistent with the envisaged depositional environment of the associated facies. This facies is comparable to upper Morrow facies 10 of Wheeler et al. (1990).


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