Stratigraphic Architecture and Depositional Environments of A Mud-Rich, Fine-Grained Early Foreland Basin Source-to-Sink System: Pennsylvanian Atoka Formation in Arkansas

Abstract

Abstract

The Pennsylvanian Atoka Formation is a major reservoir unit in the Arkoma Basin, one of the largest and most prolific hydrocarbon basins in the US. The Atoka consists of sediments deposited in deep to shallow water settings, and recorded the transition from a passive margin basin to a rapidly subsiding foreland basin. This study focuses on the lithologic facies, stratigraphic architecture, ichnofacies and paleocurrent indicators of this early foreland basin depositional system based on 21 outcrop sections and 3 quarries in western Arkansas.

The Lower Atoka deepwater system is unique for being a mud-rich, fine-grained system in an active margin setting. It is stratigraphically dominated by fine-grained turbidite sandstones and mudstones. Architectural elements are predominantly lobes with some channel and over-bank deposits. Both sand-dominated and mud-dominated mass transport deposits are present. Surface traces are common throughout all localities. Paleocurrent indicators show an overall east to west or northeast to southwest direction of deposition and flow, except the southernmost outcrops at Antoine area which show southeast to northwest flow indicators. The shallow water Middle Atoka (represented at Webco Quarry and Cabot) is characterized by thick successions of finely-laminated, dark mudstones and thin- to medium-bedded, fine- to medium-grained sandstone sheets. Some intervals show steady progradation from southwest to northeast and bidirectionally north to south. Combined wave and current influences are common. Marine bioclasts, plant fragments and trace fossils are abundant. The Upper Atoka deltaic system (represented at Gum Log Quarry and Morrilton) is characterized by thin- to medium-bedded sandstones and finely-laminated mudstones and siltstones. Some intervals show tidal influence. Trace fossils are abundant. Marine bioclasts are highly concentrated in some thin sandstone beds. Paleocurrent indicates north-to-south bidirectional flow.

The changes throughout the Lower, Middle and Upper Atoka indicate evolutionary response to tectonic compression and subsidence, natural maturing of the basin feeder systems and variations in basin geometry and localized accommodation during progression of the Ouachita Trough to Arkoma Basin transition. Such passive to active margin transitions are common throughout basin evolution records worldwide and the Atoka lends an opportunity for improved understanding of source-to-sink system response to such changes.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016