Regional Context 

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.

 

Figure1-1. Stratigraphy of the area (Billy Creek, Ridgway)

  

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).

 

	Figure 1-2. Paleogeography of North America during Early Cretaceous (115Ma) (Blakey, 2006.http://jan.ucc.nau.edu/~rcb7/)
	Figure 1-3. Paleogeography of North America during Late Cretaceous (90Ma) (Blakey, 2006.http://jan.ucc.nau.edu/~rcb7/)

 

Study Area 

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.

 

	Figure 2-1. Map of southwestern Colorado showing the study area.
	Figure2-2.  Map of the Ridgway area showing the eleven sections.

 

Previous Work 

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.

 

Methods 

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.

 

Scientific Importance 

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.

 

Depositional Facies 

1- River-dominated delta: delta plain, distributary channel and delta front

 

Figure 2-3. Distributary channel and delta front (proximal and distal).

 

2 - Fluvial and associated floodplain

• This poorly exposed facies occurs at the base of the Dakota Sandstone and contains channel-fill sandstone, floodplain silty shale and thin beds of coal.

• Channel-fill sandstone average 0.5 m in thickness, and consists of upward-fining sandstones. Individual channel-fill sandstones are traced for 3 km laterally.

• The floodplain is represented by gray-colored silty shale. Thin beds of coal occur sporadically.

 

Figure 2-4. Dakota Sandstone, above Burrow Canyon Formation, consisting of channel-fill sandstone, floodplain shale and thin beds of coal.

 

Deltaic Subenvironments 

1- Distributary channel:

• Erosionaly truncates delta front sandstone.

• It is characterized by a basal scour.

• It is comprised of medium to coarse grained sandstone.

• Lag deposit of mud-clasts or pebbles commonly seen.

• It is dominated by wedge-shape and trough cross bedded sandstone.

• Up to 2 m thick.

 

	Figure 2-5. Distributary channel showing trough cross bedded sandstone.
	Figure 2-6. Rip up clasts at the base of the distributary channel.

 

2- Delta front:

• Upward-coarsening and thickening succession of sandstone beds.

• Silty-shale at the base (distal delta front) grading upward into upper-medium to lower-coarse sandstone at the top (proximal delta front).

• It is the thickest and the most extensive facies in the Dakota Sandstone.

• It exhibits lateral continuity between the different measured sections.

• Delta front facies are approximately 1.4 m to 6 m in thickness, with an average of about 4 m thick.

• Sandstone beds range from 1 cm at the base of this succession to 10 or 20 cm at the top.

• The sandstone beds are characterized by well-defined planar bedding and current ripple laminations that occur as a couplet. Some current ripples are wave-modified.

• Arenicolites are abundant locally; Diplocraterion occurs randomly.

 

	Figure 2-7. Proximal delta front overlying distal front.
	Figure 2-8. Proximal delta front with upward thickening beds of sandstone.
	Figure 2-9. Planar bedding in proximal delta front.
	Figure 2-10. Distal delta front showing coarsening-upward beds of sandstone
	Figure 2-11. Wave ripple laminae.
	Figure 2-12. Arenicolites are vertical U-shaped dwelling burrows. We seldom find the complete burrow. It usually appears as a single, J-shape burrow. This vertical burrow varies from 5 cm to 10 cm long. Arenicolites usually indicate a brackish environment. They are indicators of stressed environments.
	Figure 2-13. Diplocraterion is described in only one section. “It is a U-shaped burrow oriented perpendicular to the bedding surface with spreite apparent between the limbs of the U. Its length reaches 60 cm. This burrow is developed primarily in sandy sediment where relatively high levels of wave or current energy are typical” (Hasiotis, 2002 )

 

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).

 

The HCS:

• Have been traced for up to 12 km.

• They are either present as individual beds 20 cm thick or amalgamated beds 2 m thick.

• HCS beds consist of fine grain sandstone.

• Burrowing and escape structures can be present.

 

	Figure 2-14. Almalgamated HCS showing swales and hummocks individual beds of HCS.
	Figure 2-15. Individual beds of HCS.
	Figure 2-16. Wave ripples on bedding plane.
	Figure 2-17. Wave modified current ripple laminae.

 

Facies Distribution 

Figure 2-18. Fluvial channel and floodplain deposit are present at the base of the formation. Deltaic deposits dominate the upper half of the formation. Wave influence is seen in the upper 7m of the formation. Wave influence is interpreted from the presence of HCS, wave ripples, and wave-modified current ripples.

 

Sequence Stratigraphy 

• Nine parasequences and two sequence boundaries have been identified.

• Parasequences are separated by parasequence boundaries. Parasequence boundaries represent a relative rise in sea level.

• Parasequences are characterized by lateral facies changes (from distributary channel to proximal delta front to distal delta front).

• Parasequences average 4 m in thickness and have been correlated across the 15 km of outcrop exposure.

• When coal or paleosol surface is present, the parasequence boundary corresponds to the top of either one of these surfaces.

• Two sequence boundaries are identified by relative falls in sea level. Two incised valley-fills are described.

 

	Figure 3-1. Vertical stacking pattern of parasequences.
	Figure 3-2. Interpreted section in Chaffee Creek. - White lines are formation contacts. - Blue lines are parasequence boundaries. - Red lines are sequence boundaries - PS 1, 2 and 3 cannot be distinguished.
	Figure 3-3. Interpreted section in Dallas Creek showing two incised valley-fills. - White lines are formation contacts. - Blue lines are parasequence boundaries. - Red lines are sequence boundaries - PS 1, 2 and 3 cannot be distinguished. - PS 6 (part of it) and PS7 are cut by incised valley-fills.

 

Conclusions 

• The Dakota Sandstone in southwest Colorado is dominated by river dominated deltas.

• It is subdivided into nine parasequences (PS). The lower PS contains fluvial and floodplain deposits. The upper eight parasequences are deltaic, with wave influence at the top of the succession.

• The vertical trend identified from the parasequence stacking pattern is interpreted to reflect an overall gradual landward movement of the shoreline across the study area.

• The parasequences are characterized by complex lateral facies changes.

• Parasequence boundaries are correlated between measured sections. When coal is present, the parasequence boundary is placed above the coal surface.

• The parasequence stacking pattern is complex and contains progradation, aggradation and retrogradation of facies.

• Two sequence boundaries have been identified. The sequence boundaries represent a fall in sea level and formation of incised valley-fill cutting into the underlying parasequences.

• Facies analysis and sequence stratigraphic approach allows for a better understanding of the complexity related to the deltaic Dakota Sandstone. Using the same approach, reservoir predictability in other deltaic environments will be easier to study.

 

Sequence Stratigraphic Interpretation 

	Figure 4-1. Correlation-stratigraphic-sedimentologic section.
	Figure 4-2. Channel-fill sandstone (incised valley-fill).

 

References 

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.