Abstract

The Dimple Limestone (Early Pennsylvanian) of the Marathon region, Texas, is a sequence of limestone, shale, conglomerate, and chert which can be referred to shelf, slope, and basin facies. The formation ranges in thickness from 200 feet in the shelf facies to 905 feet in the basin facies. A generalized isopach map indicates the axis of the basin had a northeast-southwest trend during Dimple time. Facies belts as defined by rock associations parallel the thickness contours. Both the upper and lower contacts of the formation are gradational with adjacent formations.

The shelf facies occurs as a band 4 miles wide along the north and northwest margins of the basin. Shelf facies limestones are typically nongraded fossil and oolith lime grainstones cemented by sparry calcite. Individual beds are normally a few feet thick and may contain a set of cross-laminae which passes completely through a bed. Lenticular beds of chert-pebble conglomerate locally truncate the limestones. Shales are composed mainly of lime mud.

The slope facies occurs as a narrow band about 4 miles wide seaward (south) of the shelf facies. Slope facies limestones are typically nongraded and cross-laminated in their lower parts, but graded, laminated, and convoluted in their upper parts. They are classified as fossil-lithoclast lime packstones. Conglomerates of the shelf facies are heterogeneous mixtures of material from mud to boulder size. They normally loaded the underlying sediment and often squeezed soft mud as much as 3 or 4 feet up into the conglomerate. Shales are mainly composed of lime mud but may have a terrigenous mud component.

The basin facies is the most widespread and occupies the remainder of the basin. It is transitional into rocks of the slope facies. Limestones of the basin facies are graded bedded almost without exception. The lower portions of beds are fossil-lithoclast lime packstones, and the upper portions are spicular lime mudstones or spiculites. Individual beds possess a sequence of internal sedimentary structures including massive, laminated, cross-laminated and/or convoluted, and laminated layers in ascending order. Terrigenous shales containing a sparse pelagic fauna and spicular cherts are interbedded with the limestones.

Rocks of the shelf facies were deposited in a high-energy shallow marine environment. Rocks of the basin facies represent slowly deposited pelagic muds periodically interrupted by rapidly travelling turbidity currents which deposited limestone turbidites. Rocks of the slope facies have intermediate characteristics.

Directional structures in the limestones indicate that paleo-currents flowed down a gentle slope from the north and northwest and a minor gentle slope from the south. Near the axis of the basin the currents were deflected eastward parallel to the axis of the basin.