MICROFOSSILS AS PROXIES FOR MARINE FLOODING EVENTS ALONG THE WESTERNMOST MARGIN OF THE WESTERN INTERIOR SEAWAY AT THE TIME OF THE CENOMANIAN-TURONIAN BOUNDARY INTERVAL (LATE CRETACEOUS)

Microfossils from Cretaceous coal-bearing strata can be used to establish key stratigraphic surfaces that mark sea level rises and fall where marine flooding events maintain intermediate (10⁵-10⁶) and high frequency (10³-10⁴) periodicities. We document several examples of this cyclicity from the transgressive and regressive facies at the land-sea transition of the Greenhorn Marine Cycle. 

Estuarine strata from the upper Cenomanian Dakota and Turonian Straight Cliffs Formations yield 4 primary fossil assemblages: 1) proximal estuary comprising the brackish ostracode Fossocytheridea, charophytes, and smooth admetopsid gastropods within coal zones; 2) central estuary comprising a rich agglutinated foraminiferal population of Trochammina and Verneuilinoides and brackish ostracodes and mollusks in a skeletal shell accumulation; 3) distal estuary/bay comprising the ostracodes F. posterovata, Cytheromorpha, Looneyella, and Cytheropteron, the foraminifera Trochammina and Ammobaculites, and ornate brackish mollusks in calcareous shelly mudstones; and 4) estuarine marsh comprising an exclusive population of the foraminifera Trochammina, Miliammina, and Ammobaculites in rooted lignites.  

Intermediate flooding surfaces are marked by normal marine taxa that superimpose the background of a primary marginal marine assemblage. In general, major flooding events approximate lithologic and biostratigraphic boundaries and record basin-wide paleoenvironmental changes with the advancing Greenhorn Sea. We correlate coal zones from the coast to calcium carbonate and planktic foraminiferal maxima in the offshore. The intermediate cycles approximate ammonite biostratigraphic zones and therefore maintain periodicities within the 10⁵ yr bandwidth and suggest a regional and perhaps global sea level mechanism controlled their stratigraphic position. Superimposed on the intermediate cycles are higher frequency cycles that contain short-lived flooding events that often overlie coal seams. As many as 6 high frequency cycles may overly an intermediate cycle and therefore periodicities fall with the 10⁴-10⁵ yr range. The general asymmetry of the packages suggests a combination of oceanographic, climatic, and autogenic processes exerted primary control on stratal architecture.