High-resolution estuarine sea level cycles from the Late Cretaceous: Amplitude constraints using agglutinated foraminifera

摘要

Agglutinated foraminifera provide high-resolution proxies for relative sea level change in Late Cretaceous coal-bearing strata. Three foraminiferal assemblages are recognized where Trochammina (trochospiral) occurs in abundance with either one of the following: 1) Miliammina (quinqueloculine) associated with carbonaceous shale, interpreted as the marsh; 2) Ammobaculites (uncoiled) and estuarine ostracodes associated with shelly mudstones, interpreted as the central, muddy estuary; and 3) Verneulinoides and Textularia (serial) associated with gray mudstones, interpreted as distal estuary (open bay). The marsh represents 0-1 m water depth and this approximates absolute mean sea level, the central estuary represents 5-8 m water depth, and the distal estuary (open bay) represents water depths of 10 m or greater. Alternations between foraminiferal associations in a 25-m section of the upper middle Turonian Smoky Hollow Member, Straight Cliffs Formation, indicate sea level amplitude changes that ranged from 1-10 meters.