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Sedimentary Processes and Depositional Dynamics of the Atokan (Lower Pennsylvanian) 13 Fingers Formation in Eastern Texas Panhandle, Anadarko Basin, Texas, USA


The organic-rich 13 Fingers Formation in the Anadarko Basin (Texas & Oklahoma, Lower Pennsylvanian) owes its name to a typical Resistivity wireline log signature showing over a dozen very high peaks (the “fingers”) alternating with troughs of very low values. Other log suites such as gamma-ray and neutron density also show this pattern. Though the number of “fingers” varies, the lithologic horizon is present across nearly the entire Anadarko basin. The basal 13 Fingers Formation contains two sandstone beds overlain by two sapropelic layers alternating with organic-rich argillaceous mudrocks. In the upper part, organic-rich argillaceous mudrocks intercalate with organic-poor limestone beds. The sandstone beds represent delta front deposits, linked to sea-level lowstand deposition. This is the only facies showing evidence of a relative oxygenation of the water column. The two sapropelic layers, which reach up to 64% TOC (mixed type II and III organic matter), were deposited during times of maximum flooding, with limited clastic input. The organic-rich argillaceous mudrocks reach up to 15% TOC (mostly Type III OM) and show evidence of deposition below storm-wave base under the influence of bottom currents. The limestone beds contain two sedimentary facies of two different sedimentary origins. The first consists of shallow water or slope carbonate material deposited downdip by mass wasting processes ranging from debris-flows to transitional concentrated density flows. The second represents material from mass wasting events reworked and sorted laterally by bottom currents. All facies except sandstones were deposited under dysoxic to anoxic settings evidenced by the absence of bioturbation and sustained benthic colonies. Isopach mapping of the area shows an asymmetrical, fan-like structure developing from the northwest towards the southeast, with a larger eastern lobe. This morphology is coherent with the reworking of mass wasting events by bottom currents forming “modified fan-drifts”, elongated along the main current direction. Core description reveals that the apparent cyclic stacking of clay rich and clay poor lithologies is responsible for the oscillations of the wireline log suites “fingers” of the 13 Fingers Formation. The deposition of this formation occurred on the slope as a time-transgressive facies mixing bottom-current influenced accumulation and episodic mass wasting events.