About This Item
- Full TextFull Text(subscription required)
- Pay-Per-View PurchasePay-Per-View
Purchase Options Explain
Share This Item
The AAPG/Datapages Combined Publications Database
Rocky Mountain Section (SEPM)
Abstract
Stratigraphic Evolution of the Lower Cretaceous Dakota Group, Western Interior, USA
Abstract
The Lower Cretaceous Dakota Group of the Western Interior, USA, consists of a complex stacking of four major sequences that reflect regional base level fluctuations created by tectonic events and eustatic changes. Over 2 BBOE of Dakota Group hydrocarbons have been produced to date, primarily from paleotopographically controlled stratigraphic traps. Multiple unconformities and smaller scale sequences occur within each major stratigraphic interval. This paper provides a general overview of the four major sequences and provides evidence for multiple sequences within the basal Lakota Formation.
The lowermost megasequence (1-Lakota) consists primarily of oxidized red and green sandstones and shales with ubiquitous chert pebble conglomerates. The Lakota is everywhere underlain by a regional unconformity which bevels through Morrison to Mississippian strata. Fluvial and lacustrine strata (dominantly playa lakes) and associated paleosols dominate this sequence. Multiple vertically stacked gravel layers separated by fine-grained shales and limestones indicate numerous periods of incision and deposition. Alluvial fan facies occur to the west in the foredeep to the Overthrust Belt. In northwestern Montana, the basal pre-Cretaceous unconformity has been folded into the footwall of the Overthrust Belt prior to deposition of overlying alluvial fan and plain facies. In Central Wyoming, gravely sheet sandstones generally less than 40 ft (12 m) thick are interpreted as relict pediment gravels. These pediments appear to be incised laterally by younger valley networks which contain thick shale intervals in their upper intervals, suggesting at least two regional unconformities exist within the Lakota interval.
Megasequence 2 (Fall River Sandstone) consists of on lapping transgressive packages of carbonaceous sandstone and shale which formed as coastal deposits accompanying early Skull Creek transgression. At least one sea level lowstand during this overall transgression created the incised valley fill deposits of the Fall River and equivalent sandstones. Maximum sea level highstand following the Fall River lowstands culminated in deposition of the widespread black shales and basal marine sandstones of the Skull Creek and Plain view Formations (megasequence 3). At least one relative lowstand occurred after deposition of the Skull Creek Shale, creating a regional lowstand surface of erosion which was onlapped and infilled during the ensuing Mowry transgression (megasequence 4, Muddy Formation). Traditional lithostratigraphic correlation of the Muddy Formation includes deposits formed both before and after formation of this surface.
Local variation in cyclicity related to sediment supply, climatic variations and local structural movements create numerous smaller scale sequences which we do not elaborate in this paper. A regional drainage divide developed in the vicinity of the Transcontinental Arch in Colorado during each lowstand. Oil and gas production occurs form a variety of reservoirs and trapping geometries. Valley fill traps are the most common, but generally have small field sizes. Progradational shoreline sandstones deposited during overall transgression and rare buried hill traps provide the best reserves per field. With the exception of the giant Cutbank Field (Lakota) in Montana, the bulk of the oil and gas is produced from oils generated in the black shales of the Skull Creek and Mowry sequences.
Pay-Per-View Purchase Options
The article is available through a document delivery service. Explain these Purchase Options.
Watermarked PDF Document: $14 | |
Open PDF Document: $24 |