Year of Award

2014

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

James R. Staub

Commitee Members

James W. Sears, Christopher P. Palmer

Keywords

Lake Basin Fault Zone, Sequence Stratigraphy, Eagle Formation, Telegraph Creek Formation, Cretaceous

Abstract

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, stratal stacking patterns, and syndepositional faulting. Four cycles are identified and mapped regionally over an area of 30,000 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, the Telegraph Creek Formation, contains a single regressive sandstone topped by a transgressive ravinement surface with chert and quartz 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 chamosite-rich “ green” marine sands and a maximum flooding surface; 2) a middle cycle composed of a highstand normal regressive delta and a sharp-based forced regressive shoreface that are erosionally incised into 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 normal regressive shoreface and deltaic deposits 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. Sediment dispersal patterns indicate two separate and distinct controls on deposition, eustatic sea level fluctuations and tectonic overprint. Eustatic fluctuations affect the paleodip distribution of sediment (i.e. systems tracts) with a drop in eustatic sea level attributed to the valley incision within the middle member of the Eagle. The tectonic overprint, caused by activation of the Lake Basin Fault Zone, controls the paleostrike variability with an overall creation of accommodation space to the south of the fault zone.

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© Copyright 2014 Alexander Bradford Brekke