Table 1. Facies scheme of the lower Morrow Sandstone in the Arroyo and Gentzler fields.
|Facies||Depositional Process||Sedimentary 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|
|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.
|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).
Kansas Geological Survey
Web version November 9,1999