Permian foramol carbonates from a variable salinity shelf environment: the Elm Creek limestone (Artinskian) of North-Central Texas

The Elm Creek Limestone, a division of the Wichita-Albany Group, is part of a series of Artinskian, Lower Permian, strata that crop out in North Central Texas. The Elm Creek is one of several regionally extensive platform carbonates deposited on the western limb of the Bend Arch across the Eastern Shelf of the Midland basin. The Elm Creek outcrop is roughly linear, trending north/south along depositional strike with a regional dip of only 1° or less basinward (westward). Along outcrop the formation exhibits a general transition from terrestrial facies north of the Brazos River to more marine fades southward, with the thickest carbonate beds near the Colorado River.

The carbonates of the Elm Creek are atypically thick packstone to wackestone foramol (foraminifera-mollusca) limestones. The two dominant contributors of carbonate sediments were small benthic foraminifers and mollusks(bivalves). Other groups typical of normal marine Permian fauna, such as fusulinids and brachiopods, were either missing or an extremely small component of the Elm Creek community. Normal marine conditions apparently did not exist in this part of the Eastern Shelf during Elm Creek deposition, unlike other carbonates above and below the Elm Creek.

A model is proposed of relatively continuous, slow, shelf subsidence gradually providing accommodation space for slowly accumulating foramol carbonates in shallow waters. Immense shallow areas existed containing low, but large carbonate bars with movement controlled primarily by wind-driven and storm-driven currents. Minor variations in sedimentation patterns shifted across the broad shelf area. Salinity varied from hypersaline to hyposaline conditions in the restricted waters as monsooris periodically inundated the normally evaporative environment. These harsh conditions severely limited faunal diversity. This is superimposed on the broad pattern of eustatic sea-level rise and fall, which both initiated and ended production of Elm Creek carbonates.

The foramol carbonates of the Elm Creek signal the initiation of a general trend toward more restricted marine environments on the Eastern Shelf. Signatures of this trend are evidenced by the increasing evaporite and terrestrial deposits present later in the Permian. This long-term trend is also seen on published coastal-onlap curves.