WHERE PLATFORM LIMESTONE MEETS DELTAIC DEPOSITS: OUTCROP STRATIGRAPHIC ARCHITECTURE OF THE OREAD CYCLOTHEM (UPPER PENNSYLVANIAN), SE KANSAS AND NE OKLAHOMA

The ~{!0~}layer-cake~{!1~} model for late Paleozoic cyclothems cannot fully explain the juxtaposition between the Upper Pennsylvanian ~{!0~}Kansas-type~{!1~} Oread Cyclothem and the deltaic cycles at the southern margin of the Kansas Shelf in SE Kansas and NE Oklahoma. On the shelf, the Oread Cyclothem is composed of mixed marine and nonmarine siliciclastic and carbonate rocks. It juxtaposes with deltaic rocks in the south. We investigated the outcrop facies architecture of juxtaposition to understand the controlling processes. This study focused on the Leavenworth Limestone-Heebner Shale-Plattsmouth Limestone interval in the Oread Cyclothem, and documented facies and thickness changes by correlating 15 measured sections in a 10-km2 area. From north to south in 5 km, the transgressive Leavenworth Limestone is persistent in lithology and thickness; the maximum-transgressive Heebner Shale changes from 2-m anoxic black shale to 30-m deltaic deposits; the regressive Plattsmouth Limestone changes from 8-m phylloidal algal mound-dominated facies to 1-m arenaceous grainstone, and pinches out into deltaic deposits. Syndepositional growth faulting and differential compaction of deltaic sediments increased local topographic relief. In detail, the Plattsmouth Limestone changes from arenaceous grainstone, shale, fossiliferous sandstone, limestone pebble conglomerate, to arenaceous packstone in 500 m from north to south. Specifically, the medium-grained sandstone with large scale tabular cross-beds interfingers with the conglomerate. The conglomerate is 40 cm thick, composed of mixed quartz sand, limestone intraclasts, and skeletal fragments, in a 10-m wide zone. Lithology, sedimentary structure, and bedding geometry suggest facies changes and mixing occurred in a syndepositional low with strong uni-directional current. We interpret that the facies juxtaposition and mixing were caused by hydrographic partitioning by alongshore currents associated with oceanic upwelling, similar to the modern analogs in offshore NW Africa and the Mahakam Delta region in Java Sea. Syndepositional topography and structural deformation also played important roles in determining the facies distribution, thickness, and boundary relationship in the transition zone.