Year of Award
Master of Science (MS)
Department or School/College
Department of Geosciences
James R. Staub
Holly A. Thompson, Marc S. Hendrix
Billings, Combined Flow, Cretaceous, Eagle Formation, Facies Architecture, Ichnofauna, Landward Migrating, Montana, Overwash, Regressive to Transgressive, Sandy Shoal, Storm Deposits, Wave-Dominated Delta
University of Montana
The Santonian-Campanian Eagle Formation in south-central Montana was deposited along the western margin of the Cretaceous Western Interior Seaway and is composed of a series of well preserved regressive and transgressive deposits. This study documents the depositional evolution of the informal lower member of the Eagle Formation exposed near Billings, MT. Detailed mapping and characterization of the internal stratigraphy, facies architecture, and bounding surfaces of the lower Eagle provide the basis for reconstruction of a paleodelta system that serves as a valuable reservoir analog within the Cretaceous Seaway and elsewhere. This study demonstrates that the informal lower member of the Eagle Formation in south-central Montana was deposited in a regressive to transgressive wave-dominated deltaic system. Within this deltaic succession the identification of five facies and four bounding surface types are used to establish a two-phase depositional history: 1) a regressive phase characterized by prograding pro-delta to distal delta front deposits emplaced below fair weather wave base but above storm wave base; and 2) a transgressive phase characterized by landward dipping proximal to distal shoal overwash fan deposits derived from combined-flow storm-generated turbidity currents. A maximum regressive surface (MRS) separates underlying prograding deltaic deposits from downlapping shoal overwash fan deposits above. Shoal overwash fan packages dip and stack en-echelon towards the paleoshoreline indicating landward migration of the shoal during delta transgression. A time-transgressive ravinement surface (TRS) developed on the basinward side of the transgressive shoal and migrated landward with the shoreline, truncating of shoal overwash fan deposits.
Spangler, Eleanor, "INTERNAL FACIES ARCHITECTURE OF A REGRESSIVE TO TRANSGRESSIVE WAVE-DOMINATED DELTA IN THE UPPER CRETACEOUS EAGLE FORMATION, SOUTH-CENTRAL MONTANA" (2012). Graduate Student Theses, Dissertations, & Professional Papers. 1373.
© Copyright 2012 Eleanor Spangler