2014 Rocky Mountain Section AAPG Annual Meeting

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Regional sequence stratigraphic analysis of the Upper Cretaceous Telegraph Creek and Eagle formations in south-central Montana


The Santonian to Campanian Telegraph Creek and Eagle formations in south-central Montana contain a series of regressive to transgressive cycles deposited on the western margin of the Cretaceous Western Interior Seaway. This study focuses on depositional environments and stratal stacking patterns. Four cycles are identified and mapped regionally over an area of 6,000 km2 using outcrop and subsurface data to correlate 100 km up paleoslope from their maximum seaward extent. Each cycle of progradation and subsequent transgression left behind an upward-coarsening shoreface and/or deltaic sandstone body, capped by either a transgressive sand, an erosional top or maximum flooding surface. The stratigraphically lowest cycle studied, in the upper Telegraph Creek Formation, contains a single regressive sandstone topped by a transgressive ravinement chert pebble lag. The overlying Eagle Formation is composed of three genetically related regressive to transgressive cycles: 1) a basal cycle consisting of a detached low-stand delta capped by a ravinement surface which is overlain by transgressive glauconite-rich “green” marine sands and a maximum flooding surface; 2) a middle cycle composed of a sharp-based forced regressive shoreface and deltaic deposits that are erosionally incised with the incision surface indicative of base-level fall and representing a sequence boundary, which is overlain by a subsequent transgressive tidally-influenced valley-fill; and 3) a top cycle consisting of a normal regressive shoreface deposit capped by a transgressive ravinement surface containing black-chert pebbles, representing the Claggett transgression. Each successive regressive to transgressive cycle steps consecutively basinward creating a larger progradational wedge.