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Abstract

The Dimple limestone (Atokan) of the Marathon region represents a period of carbonate deposition which interrupted the deposition of a thick terrigenous flysch section in the Ouachita geosyncline. It consists of laterally adjacent “shelf,” slope, and basin facies from north to south.

The “shelf” facies is characterized by cross-bedded fossil and oolith lime grainstones. The slope and basin facies consist of sequences of distinctive limestone turbidites. Paleocurrent analysis indicates a uniform paleoslope dipping southward, with no apparent slope break. A southern source with less contribution to the basin is also indicated based on paleocurrent evidence.

Slope facies limestones (proximal turbidites) may be graded, partly graded, or nongraded. Many beds are nongraded in their lowest parts but grade rapidly in their upper parts. Basal portions are often conglomeratic, and lime mudstone upper portions are often absent. “Floating” pebbles are common, as are large-scale convolutions. Small-scale cross-bedding is rare, and occasional medium-scale (up to 3 feet thick) cross-bedding has been observed, which is interpreted as “dune” bed forms. Associated rocks are subaqueous slump conglomerates and spicular lime mudstones. The slope facies is 5 miles wide.

Basin facies limestones (distal turbidites) are nearly always graded. Pebbles are rare; the coarsest size being generally sand or silt. Lime mudstone beds are well-developed, and were deposited from turbidity currents because (1) thick mudstone beds overlie thick graded beds, and (2) the normal pelagic sediment is radiolarian-bearing mudstone (marl) with a large terrigenous mud component. Convolutions are common, and small-scale cross-bedding is abundant. Associated rocks are black radiolarian-bearing shales and spicular cherts.

The portion of the Ouachita geosyncline revealed in the Marathon region is believed, in Dimple time, to have been a basin about 40 miles wide and perhaps a few hundred feet deep. Turbidity currents moving down a gently-sloping bottom from the north deposited limestones on the slope as well as in the basin. These proximal and distal turbidites display distinctive characteristics, and an awareness of them may assist in reconstructing facies patterns elsewhere.

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