Year of Award

2012

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

Christopher P. Palmer, Michael H. Hofmann

Keywords

Composite Surfaces, Cretaceous, Incised Valley

Abstract

The Santonian-Campanian Eagle Formation in south-central Montana is composed of a series of exceptionally well preserved regressive-transgressive cycles deposited on the western margin of the Cretaceous Interior Seaway. This study focuses on the genesis of a regionally correlative basal incision surface and the subsequent fill architecture of the informal middle member of the Eagle Formation. Three stratigraphic surfaces are traced continuously for up to 15 km and include the regressive surface of marine erosion (RSE), sequence boundary (SB), and transgressive surface of tidal-fluvial erosion (TSE). Within this sequence stratigraphic framework, 4 facies association, internal lower-order architectures (i.e. channel forms, lateral accretion sets, crevasse splays) and higher-order regional surfaces (i.e. sequence boundaries) are used to establish a three-phase depositional history of valley evolution: 1)fluvial incision during decreasing accommodation concomitant with fluvial deposition and sediment storage on abandoned terraces within the valley, 2) rapid increase in accommodation significantly outpaced sediment influx concurrent with seasonal fluctuations in fluvial discharge and landward migration of the turbidity maximum, producing a flood-dominated mud-rich central valley deposit, and 3) deceleration in the rate of relative sea level rise marked by a depositional shift to tidally influenced fluvial deposition, extensive valley widening, and coal development. The resulting valley architecture is a highly diachronous composite basal incision surface. The time-transgressive multi-phase nature of valley formation and infilling produced a fill profile with a mud-rich valley center flanked by stacked, sand-rich fluvial and tidal-fluvial deposits toward the valley margins. These findings contribute to the sequence stratigraphic interpretation of incised valleys along the Late Cretaceous Western Interior Seaway, serve as an outcrop analog for potential incised valley reservoirs, and have direct application in developing reservoir models for the Eagle Formation in south-central Montana and elsewhere.

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© Copyright 2012 Neal Auchter