Conodont fauna of the Dimple Limestone (late Morrowan-early Atokan, early Pennsylvanian) in the Marathon Basin, Texas



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Texas Tech University


The Dimple Limestone is a structurally deformed Lower Pennsylvanian carbonate unit in the Ouachita orogenic belt that crops out in the Marathon Basin of southwestern Texas. It is bounded above and below by terrigenous clastic flysch sediments derived from a southeastern source area, and it represents a period of tectonic quiescence during which progradation of the southern margin of the North American craton led to deposition of carbonate turbidites in the basin.

Conodont distribution within the Dimple turbidites is not uniform. While silicification obscures some of the pattern, detailed sampling of a number of turbiditic sequences demonstrates that conodonts are common only in the lower part of the basal oobiolithoclastic packstones. The overlying finer-grained spiculitic packstone, overlying spiculitic shales, and intervening hemipelagic shales do not process well, and no conodonts were found. Despite their occurrence in a turbiditic sequence, the regular order of first occurrence of species in each section indicates that superpositional order has been preserved.

Conodont faunas of the Dimple Limestone are dominated by Idiognathoides (55%), Idiognathodus (14%), Declinognathodus (7%), and Neognathodus (2%). Ramiform elements, predominately the genus Idioprioniodus, make up about 12 percent of the fauna, and older reworked conodonts comprise about 7 percent of the faunas. Several discrete intervals of reworking of older conodonts are recorded. Frasnian- Fammenian and Kinderhookian forms common in the Woodford shale, and Chesterian and lower Monro wan forms, including Declinognathodus noduliferns, common in the Bamett Shale occur. During submarine canyon downcutting, shelf allochems of late Morrowan-early Atokan age were mixed with the debris from the older cratonic shales and transported down submarine channels into the basin for incorporation into the Dimple turbidites.

Evolution within the genus Idiognathodus provides a biostratigraphic basis for subdivision of the Dimple into five faunal intervals. Intervals A and B are recognized on the occurrence of/. sinuosusMI and the appearance of I. sinuosusMl, respectively. Interval C is recognized by the appearance of I.klappeii, Interval D by the first occurrence of I. incurvus Ml, and Interval E by the first occurrence of I. incurvusMl. The appearance of Idiognathodus incurvusMl occurs at or near the base of the Atokan, and in the absence of Zone 21 foraminifers, may be a reliable indicator of this boundary.

Apparently, a continuous sequence across the Morrowan-Atokan boundary is preserved in the Dimple. The Dimple Limestone is correlative with part of the Wapanucka Formation and the lower Atoka Formation of southeastern Oklahoma and part of the Marble Falls Formation of the Llano region of Texas. However, in both areas regional unconformities occur at the critical Morrowan-Atokan boundary.