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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 69 (1985)

Issue: 2. (February)

First Page: 313

Last Page: 313

Title: Reexamination of Bengal Fan Model for Turbidites of Frontal Ouachitas: ABSTRACT

Author(s): Michael B. Underwood

Article Type: Meeting abstract


The lower member of the Pennsylvanian Atoka Formation outcrops within the frontal Ouachitas of Arkansas. Strata are comprised of turbidites and related deep-marine deposits, which locally exceed 5.5 km (18,000 ft) in structural thickness. Several previous workers have drawn analogies between Ouachita turbidites (or flysch) and the present-day Bengal Fan, located in the eastern Indian Ocean. As a first-order approximation, this model is basically correct, especially in terms of overall tectonic setting. Yet, when examined in detail, there are striking dissimilarities between the frontal Ouachitas and the Bengal Fan.

The dimensions of the Bengal Fan are staggering, it measures roughly 1,000 km (620 mi) by 3,000 km (1,860 mi). The main feeder channel is 13 km (8 mi) across and 850 m (2,790 ft) deep. Channels within the mid-fan region are up to 2-3 km (1.2-1.9 mi) in width and 100 m (330 ft) in depth, and some channels maintain continuity for well over 2,000 km (1,240 mi). Rates of vertical sediment accumulation are no more than 75 m/m.y., with isopach data showing only 3.5 km (11,500 ft) of post-Eocene accumulation. Limited sampling also shows rather low sand-mud ratios over much of the fan.

Lithofacies data and depositional cycles within the lower Atoka Formation are suggestive of middle-fan, outer-fan, and basin-plain environments. If Atoka channels were as large as those of the Bengal Fan, they certainly remain unrecognized in the rock record. Instead, the entire length of the outcrop belt in Arkansas is less than 250 km (155 mi), and facies changes define a clear east-to-west transition from middle fan to basin plain. Significantly, Atoka sedimentation rates were approximately 10 times higher than Bengal rates. It is evident that the Atoka fan system was much more confined than the Bengal Fan; it probably formed within a narrow, rapidly filling, remnant ocean basin rather than on an unrestricted abyssal floor.

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