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Recent work on the Dimple Limestone (Atokan) of the Marathon tectonic belt of West Texas indicates that it is predominantly a turbidite sequence deposited in relatively deep water below normal wave base. A narrow upperslope facies of nongraded, festoon cross-stratified grainstone, packstone, and chert conglomerate beds deposited in relatively shallow water occurs in the northwest part of the area. Wherever exposed, the formation is part of an overthrust complex which moved northwestward a distance of several miles or more.
The basin facies to the southeast consists of interbedded pelagic and turbidite beds. The pelagic beds are dark gray to black terrigenous shale, calcilutite, and chert containing a sparse fauna of radiolaria and sponge spicules. The turbidite beds are graded lithoclastic fossiliferous limestones ranging in thickness from less than an inch to more than 9 feet. Lithoclasts are mostly angular fragments of Devonian and Ordovician cherts. The fauna is characteristic of shallow shelf and slightly deeper slope environments, and indicates redeposition by turbidity currents. Small quantities of ooliths, pellets, glauconite, and quartz sand grains occur commonly in the turbidite beds.
The turbidite beds contain current direction features including aligned fossil fragments (particularly sponge spicules and brachiopod spines), cross laminae, ripple marks, and flame structures; and slope indicators such as convolutions and medium-scale slump structures. Sole markings are not evident. Delicate laminae in upper parts of graded beds commonly consist of siliceous sponge spicules aligned parallel with the current direction.
Numerous measured sections of the basin facies indicate a record of continuous deposition from the flysch-like Tesnus Formation, through the Dimple Limestone, and into the overlying flysch-like Haymond Formation. Submarine slides containing large blocks of white Devonian chert locally produced erosional surfaces. A comparison of the Tesnus Formation, Dimple Limestone, and Haymond Formation with the Scaglia-Brecciola-Macigno sequence of the northern Apennines shows striking similarities.
Current direction indicators, bed thickness, grain size, facies, and over-all formation thickness indicate that the Dimple Limestone was transported down a slope from the northwest. During early Atokan time, a carbonate shelf in the vicinity of the Glass Mountains provided shallow-water fossils and other carbonate debris for later redeposition in a relatively deep basin on the southeast.
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