[PDF] Depositional Factors Of The Upper Cretaceous Eagle Formation South Central Montana eBook

Author : Neal Auchter
Publisher :
Page : 129 pages
File Size : 37,47 MB
Release : 2012
Category : Electronic dissertations
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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.

Author : Eleanor Spangler
Publisher :
Page : 125 pages
File Size : 31,46 MB
Release : 2012
Category : Electronic dissertations
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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.

Author : Edward Allen Merewether
Publisher :
Page : 104 pages
File Size : 50,11 MB
Release : 1996
Category : Geology
ISBN :

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A multidisciplinary approach to research studies fo sedimentary rocks and their constituents and the evolution of sedimentary basins--both ancient and modern.