PSDepositional Environment and Sequence Stratigraphy of the Dakota Sandstone
(Lower Cretaceous) in the Ridgway Area, SW Colorado*
Hayet Serradji1 and Diane Kamola1
Search and Discovery Article #50055 (2007)
Posted October 1, 2007
*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Geology Department, University of Kansas, Lawrence, Kansas ( [email protected] )
Facies and sequence stratigraphic analysis of the Lower Cretaceous Dakota Sandstone was completed in SW Colorado. Delta-front sandstones, channel-fill sandstone, and deltaplain and/or floodplain siltstone are the dominant facies. Delta-front sandstones are upward-coarsening intervals, with abundant planar-to-current-rippled beds indicating river dominance. Individual deltas can be traced for up to 10 km and show facies changes from distributary channel to proximal delta to distal delta-front settings. The facies belts are usually over 3 km width. Arenicolites, found throughout the deltaic intervals, is interpreted to indicate stressed, possibly brackish-water conditions.
Eight parasequences are present. Parasequence boundaries correspond to a sudden increase of accommodation seen by the vertical stacking of the various depositional facies. In places, a thin (cm-thick) coal horizon defines the Parasequences boundaries, which can be traced up to 5km. Parasequences at the base of the studied interval usually contain floodplain/deltaplain or fluvial deposits. Paresequence toward the top of the studied interval contain deltaic deposits and record the sudden input of coarser material to the study area. Progressively greater wave influence is observed in the deltas that are stratigraphically higher in the section, seen through the presence of thick HCS and (10 to 15 cm) wave-ripple beds towards the top of the formation. This vertical stacking of parasequences (greater marine influence up section) is interpreted to reflect a gradual landward movement of the shoreline across the study area throughout while the Dakota accumulated. Based on this, a retrogradational parasequence stacking pattern is inferred.
The Dakota Sandstone records the transition from continental environments represented by the Jurassic Morrison and Burro Canyon formations to a fully marine environment represented by the Cretaceous Mancos Shale. It was deposited during the transgression of the Western Interior Seaway across the area, and is interpreted to represent a transgressive system in western Colorado. The Dakota Sandstone is described as Early Cretaceous (Aptian and Albian) in the eastern part of Colorado, but as Early to Late Cretaceous (Aptian, Albian and Cenomanian) in the western part of Colorado (Young, 1960). It records the western shoreline of the Cretaceous Western Interior Seaway during this time interval. A facies and sequence stratigraphic study provides a better understanding of its depositional environments.
Paleogeography of Southwestern Colorado during the Early Cretaceous
During the late Mesozoic, convergence between the North American and Farallon plates created a Cordilleran orogenic belt. This orogenic belt extended north-south along the western North American craton (Dickinson and Snyder, 1978). The continuous growth
of the Cordilleran orogenic belt induced the formation of the Western Interior Foreland Basin (Jordan, 1981). This foreland basin occupied the western limit of the Cretaceous Western Interior (KWI) Seaway. The flooding of the K W I Seaway across North America occurred from the Arctic and Tethys simultaneously. During the mid-Cretaceous (~100 Ma), the two seas joined in SE Colorado and progressively spred west from there (Raynolds, 2002). The study area is located near the western margin of the Western Interior Foreland Basin (Ulicny, 1999).
Facies and sequence stratigraphic analysis of the Lower Cretaceous Dakota Sandstone is completed in southwestern Colorado, along Highway 550, between the towns of Montrose and Ridgway.
The Dakota Sandstone was first named by Meek and Hayden (1861) for the basal coarse grained Cretaceous strata near the village of Dakota City, northeastern Nebraska. The unit now known as the Dakota Sandstone has been identified by various other names: Dakota Formation, Dakota Group, and Naturita Formation. In the Colorado Plateau, Young (1960) used the name of Naturita Formation to describe this unit and the name of Cedar Mountain Formation to describe the underlying Burro Canyon Formation. He grouped the two formations into the Dakota Group. In the Gunnison area (South Central Colorado), Bartleson (1989), used the terms of Dakota Sandstone and Burro Canyon to describe these two separate units. Here, the terms of Dakota Sandstone and Burro Canyon Formation will be used.
Although the Dakota Sandstone is well exposed in the study area, its depositional environment is not well documented. This study is based on outcrop investigation. The Dakota Sandstone is studied via eleven measured sections between the towns of Montrose and Ridgway. These sections are oriented N-S and cover a distance of approximately 15 Km. Each section ranges from 30 to 50 m and is measured using a Jacob’s staff. The sedimentological observations include description of lithologies, and physical and biogenic sedimentary structures, as well as bedding attributes such as bed thickness, bedding contacts, etc. Attention is paid to the recognition of major bounding surfaces formed in response to base level changes (i.e., parasequence and sequence boundaries). Photo mosaics were taken to help determine the architecture of beds and bed sets. Correlation of measured sections using a sequence stratigraphic approach document lateral facies changes and extent of deltaic subenvironments across the study area, as well as the connectivity (or lack of ) of the seemingly continuous deltaic sandstones within this formation.
This study provides a detailed depositional setting of the deltaic Dakota Sandstone in southwestern Colorado. The use of a sequence stratigraphic approach allows a better understanding of lateral facies changes associated with these river-dominated deltas.
1- River-dominated delta: delta plain, distributary channel and delta front
2 - Fluvial and associated floodplain
1- Distributary channel:
2- Delta front:
Wave influence at the top of the Dakota Sandstone
Wave influence near the top of the Dakota sandstone is shown by the presence of wave ripple cross laminae, wave-modified current ripple cross laminae and hummocky cross stratification (HCS).
Bartleson, B., 1989, Dakota Sandstone and associated rocks adjacent to San Juan Sag near Gunnison, Colorado (abs.): AAPG Bulletin, v. 73, p. 1147.
Hasiotis, S.T., 2002, Continental trace fossil atlas: SEPM, Short Course Notes Number 51, 32 p.
Meek, F.B., and F.V. Hayden, 1861, Descriptions of new Lower Silurian (Primordial), Jurassic, Cretaceous, and Tertiary fossils, collected in Nebraska Territory, with some remarks on the rocks from which they were obtained: Proceedings Academy Natural Philadelphia, p. 415-447.
Young, R.G. 1960. Dakota Group of Colorado Plateau: AAPG Bulletin, v. 44, p. 156-194.