LOWER-MIDDLE MISSISSIPPIAN SHALLOW-WATER, MID-LATITUDE HETEROZOAN CARBONATE AND SILICEOUS SPONGE-SPICULE FACIES IN A MIDCONTINENT SETTING, USA (Invited Presentation)

Mixtures of heterozoan-dominated carbonate and biosiliceous deposits that formed in middle-to low-latitude neritic settings are increasingly being recognized. This study documents Lower-Middle Mississippian (Osagean) biosiliceous and heterozoan carbonate facies that were deposited in a mid-latitude (~ 20° south) shallow water, inner ramp setting in Kansas. Primary facies in two sequences (DS1 and DS2) are: 1) Mudstone-Wackestone (MW); 2) Sponge Spicule-Rich Wackestone-Packstone (SWP); 3) Echinoderm-Rich Wackestone-Packstone-Grainstone (EWPG); and 4) Dolomitic Siltstones and Shale facies.

The abundance of EWPG with diverse fauna, evidence of extensive reworking by burrowing organisms, and only rare occurrence of evaporites suggest subtidal deposition in a normal to slightly restricted marine inner-ramp setting for DS1. An increase of SWP with less diverse fauna, abundance of early evaporites (replaced by silica), and subaerial exposure horizons throughout DS2 suggest deposition in restricted inner ramp, lagoon, and sabkha environments. Middle ramp to ramp-margin areas are characterized by heterozoan facies, and an increase in silica and biosiliceous accumulations, including sponge buildups on the ramp margin. A regional control is suggested by other Osagean–Meramecian shallow-water mid-latitude locations in North America that show similar facies associations, and a general lack of photozoan association facies. Distributions of heterozoan and biosiliceous facies, including the abundance of biosiliceous facies at margin locations, suggest upwelling of basinal waters rich in nutrients and dissolved silica as a primary control. Terrestrial sources may have contributed in innermost shelf and ramp locations.

Results from this study have implications from a petroleum reservoir standpoint. The sponge spicule, heterozoan carbonate, and silica-replaced evaporite facies in the study location and surrounding areas form reservoirs. The interpreted regional control and occurrence of similar facies elsewhere in North America could have more widespread reservoir implications. Similar middle-to low-latitude neritic heterozoan and biosiliceous deposits are documented in other Paleozoic systems and continuing study can contribute to advances in development of predictive models.