Nature and Recognition of Limestone Turbidites, Marathon Region, Texas: ABSTRACT

The Dimple Limestone (Pennsylvanian) 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 from north to south of laterally adjacent shelf, slope, and basin facies. The shelf facies is characterized by cross-bedded oolith carbonate grainstone. The slope and basin facies consist of sequences of distinctive limestone turbidite. Paleocurrent analysis indicates the existence of a uniform paleoslope dipping southward, with no apparent slope break. Slope-facies turbidites (proximal turbidites) display a wide variety of internal characteristics. They may be graded, reverse graded, or non-graded in their lower parts, but grade abruptly in their upper parts. Basal parts commonly are conglomeratic, and carbonate-mudstone upper parts commonly are absent. Floating pebbles are common, and 3-foot-thick cross-bedded units have been observed. Small-scale cross-bedding is rare, and large-scale convolutions are abundant. Associated rocks are subaqueous slump conglomerate, spicular carbonate mudstone, and black spicular marl. This facies is 5 miles wide. Basin-facies turbidites (distal turbidites) are nearly always graded. Pebbles are scarce, and the coarsest sizes are sand or silt. Carbonate 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 marl. Small-scale cross-bedding is abundant and convolutions are common. Thick beds are commonly massive, becoming laminated and cross-bedded on a small scale upward. Thin beds are commonly laminated. Associated rocks are black radiolarian-bearing marl and spicular chert. Information gained from the Dimple Limestone was applied to lower Paleozoic rocks in the region, and revealed slope-and-basin-facies limestone turbidites in the Ft. Pean Formation and Maravillas Chert (Ordovician). It appears that some geosynclinal limestone turbidite is the product of major tectonism, and is introduced from the cratonic side. End_of_Article - Last_Page 483------------