Abstract

The process regime of low-gradient coastal plains, delta plains, and shorelines can change during transgression. In ancient successions, accurate assessment of the nature of marine influence is needed to produce detailed paleogeographic reconstructions, and to better predict lithological heterogeneity in hydrocarbon reservoirs. The Campanian lower Neslen Formation represents a river-dominated and tide- and wave-influenced coastal-plain and delta-plain succession that accumulated along the margins of the Western Interior Seaway, USA. The succession records the interactions of multiple coeval sedimentary environments that accumulated during a period of relative sea-level rise.

A high-resolution data set based on closely spaced study sites employs vertical sedimentary graphical logs and stratigraphic panels for the recognition and correlation of a series of stratal packages. Each package represents the deposits of different paleoenvironments and process regimes in the context of an established regional sequence stratigraphic framework. Down-dip variations in the occurrence of architectural elements in each package demonstrate increasing marine influence as part of the fluvial-to-marine transition zone.

Three marine-influenced packages are recognized. These exhibit evidence for an increase in the intensity of marine processes upwards as part of an overall transgression through the lower Neslen Formation. These marine-influenced packages likely correlate down-dip to flooding surfaces in the time-equivalent Îles Formation. The stratigraphic arrangement of these packages is attributed to minor rises in sea level, the effects of which were initially buffered by the presence of raised peat mires. Postdepositional autocompaction of these mires resulted in marine incursion over broad areas of the coastal plain. Results demonstrate that autogenic processes modified the process response to overall rise in relative sea level through time. Understanding the complicated interplay of processes in low-gradient, coal-bearing, paralic settings requires analysis of high-resolution stratigraphic data to discern the relative role of autogenic and allogenic controls.