TRANSITION FROM DELTAIC TO CARBONATE PLATFORM DEPOSITION – RAMP CREEK FORMATION (MISSISSIPPIAN) OF CENTRAL INDIANA

Detailed examination of more than 25 cores in Monroe and Lawrence Counties of central Indiana provides new insight into the transition from Borden delta deposition to shallow carbonate platform deposition. This change occurred during deposition of the Ramp Creek Formation (Valmeyeran, Mississippian). Definition and internal stratigraphy of the complex Ramp Creek interval in Indiana was the subject of considerable confusion from the late 1800s until it was finally settled in 1975.

Internally, the Ramp Creek consists of interbedded crinoidal-dominated skeletal grainstones ranging in thickness from a few centimeters to nearly 3 m. Between the grainstones are individual fine-grained units consisting of argillaceous dolostone, argillaceous limestone, siliceous siltstone, or very fine-grained sandstone. The dolostone beds often contain 40-70% ankerite (CaFeMgMnCO₃). All the fine-grained beds also contain silicified fossiliferous layers, nodules, and true geodes, indicating that the diagenetic system was rich in both iron and silica. The fine-grained beds range in thickness from less than 2 cm to nearly 6 m.

There is a high degree of lateral variability within the Ramp Creek and individual beds cannot be correlated, even over short distances; however, the unit can be divided into an upper division dominated by skeletal grainstone beds and a lower division dominated by fine-grained beds. The lower division is highly variable in thickness when compared to the upper division. Overall, the Ramp Creek thickens to the south and southwest from slightly over 3 m to more than 15 m in the two-county area. The fine-grained beds become almost entirely very fine-grained sandstone to the south, possibly related to increased proximity to a source of sediment on the Borden delta.