Utah

LOWER CRETACEOUS CEDAR MOUNTAIN AND DAKOTA FORMATIONS IN AND AROUND THE UTAH PORTION OF DINOSAUR NATIONAL MONUMENT AND EVIDENCE OF A LATE ALBIAN MARINE INCURSION OF THE CRETACEOUS WESTERN INTERIOR SEAWAY INTO UTAH
by Douglas A. Sprinkel, Scott K. Madsen, James I. Kirkland, Gerald L. Waanders, and Gary J. Hunt -1
Utah Geological Survey Consulting Palynologis

DISCUSSION

The Cedar Mountain Formation in northeastern Utah consists of the basal Buckhorn Conglomerate Member or a basal interval of mottled, yellow-orange mudstone with scattered chert pebbles, overlain by a carbonate-rich mudstone that grades upward into a capping noncalcareous mudstone. The basal mottled chert-pebble mudstone beds may represent soil development on floodplain deposits and likely grade laterally to the Buckhorn Conglomerate Member, as has been demonstrated with the Yellow Cat-Buckhorn Members in central Utah (Greenhalgh and Britt, 2007). The bulk of strata exhibited in our sections of the Cedar Mountain Formation best fit the Ruby Ranch Member. The Ruby Ranch Member in its type area (between Crescent Junction and Green River, Utah) is typically drab green and mauve variegated slope forming mudstone whose most conspicuous features are numerous carbonate nodules that cover the surface and ribbon sandstone lenses (Kirkland and others, 1997; Ludvigson and others, 2010a).

The uppermost part of the Cedar Mountain in northeastern Utah is reminiscent of the Mussentuchit Member of the Cedar Mountain Formation of the western San Rafael Swell, but radiometric data, dinosaur biostratigraphy, and palynomorph assemblages from sections in

Current: June 6, 2012 Sprinkel, Madsen, Kirkland, Waanders, and Hunt
Original: June 1, 2005 Sprinkel-etal_CedarMtn-Dakota_06062012.docx
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central Utah indicate these beds are not time equivalent (Tschudy and others, 1984; Cifelli and others, 1997; Kirkland and others, 1997; Kirkland and others, 1999; Sprinkel and others, 1999; Garrison and others, 2007; Kirkland and Madsen, 2007). Radiometric (40Ar/39Ar sanidine) ages obtained from the lower part of the Mussentuchit Member of the Cedar Mountain Formation on the San Rafael Swell are 98.37 ±0.07 Ma (Cifelli and others, 1997; 1999) and 98.5 ±0.06 Ma (Garrison and others, 2007). These ages are similar to the radiometric age of the Arrow Creek Bentonite (Obradovich, 1993). A radiometric (40Ar/39Ar sanidine) age of 97.2 ±0.06 Ma was obtained from near the top of the Mussentuchit Member (Garrison and others, 2007) and is similar to the age of the Clay Spur Bentonite (Obradovich, 1993). Thus, these dates indicate that the Mussentuchit Member in its type area on the southwest side of the San Rafael Swell is equivalent to the Mowry Shale in Wyoming. In addition, the discontinuous conglomerate that separates the Mussentuchit from the underlying Ruby Ranch member along the west side of the San Rafael Swell (Kirkland and Madsen, 2007; Lawton and others, 2010; Paul Kuehne, Utah Geological Survey, 2010 written communication) may correlate with the Dakota Formation in northeastern Utah, based on late Albian radiometric age and palynomorph assemblage reported here.

Currently, we believe it is unwarranted to use the member names of the Cedar Mountain Formation defined by Kirkland and others (1997) and Kirkland and others (1999) in central Utah for the Cedar Mountain Formation in northeastern Utah because not all members may be represented. However, if the relationship between the Buckhorn Conglomerate Member and the basal mottled chert-pebble mudstone unit can be demonstrated in northeastern Utah, as Greenlaugh and Britt (2007) have shown for the Buckhorn Conglomerate and Yellow Cat Members in central Utah, perhaps the terms Yellow Cat and Ruby Ranch Members can both be

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Original: June 1, 2005 Sprinkel-etal_CedarMtn-Dakota_06062012.docx

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applied to the Cedar Mountain Formation in northeastern Utah. If those terms are used, we believe the Mussentuchit Member should not be applied to the uppermost part of the Cedar Mountain Formation in northeastern Utah because the Mussentuchit in central Utah is time equivalent to the Mowry Shale in northeast Utah.

The initial transgression of the Cretaceous Western Interior Seaway in the Rocky Mountain region is recorded in Albian-age marine strata, and the transgressive shoreline approached and possibly encroached into the northeastern corner of Utah by late Albian (Weimer, 1984; Ryer, 1993; Cobban and others, 1994; Dickinson, 2006). Our palynomorph assemblage from the Dakota Formation (table 1) is similar to the assemblage from the Thermopolis Formation of central and southern Wyoming, and the Dakota Formation of Kansas, Nebraska, and Iowa (Oboh-Ikuenobe and others, 2007; Scott, 2007; Scott and others, 2009; Ludvigson and others, 2010b). The middle to late Albian microplankton recovered from the dark-gray, organic-rich shale in the basal Dakota Formation and the irregular thickness of this interval indicate the initial marine incursion into northeastern Utah was likely restricted to incised valleys and other low-lying areas, similar to the Dakota Formation found elsewhere along the Western Interior Seaway (Ludvigson and others, 2010b).

The middle Cretaceous within the Western Interior basin includes the Kiowa-Skull Creek marine depositional cycle (Albian) and Greenhorn marine depositional cycle (Cenomanian) (Brenner and others, 2000). Recently, Ludvigson and others (2010b) proposed a new Muddy-Mowry depositional cycle that separates the Kiowa-Skull Creek and Greenhorn cycles. The Cedar Mountain Formation in northeastern Utah is, in part, the landward time-equivalent of the Kiowa-Skull Creek cycle (figure 13). The basal dinoflagellate-bearing mudstone and shale unit of the Dakota Formation represents peak sea level during the Kiowa-Skull Creek cycle and the Current: June 6, 2012 Sprinkel, Madsen, Kirkland, Waanders, and Hunt Original: June 1, 2005 Sprinkel-etal_CedarMtn-Dakota_06062012.docx

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initial marine incursion into northeastern Utah (figure 13). The overlying non-marine part of the Dakota Formation and marine Mowry Shale represent the newly recognized Muddy-Mowry cycle (figure 13). Marine environments did not transgress into central Utah until the late Cenomanian (Elder and Kirkland, 1993; Cobban and others, 1994; Elder and Kirkland, 1994).

CONCLUSIONS
The Lower Cretaceous section exposed in northeastern Utah includes the Cedar Mountain and Dakota Formations. The Cedar Mountain Formation consists of fluvial-lacustrine and pedogenic carbonate beds and includes important dinosaur sites. The basal mottled chert-pebble mudstone beds may represent soil development on floodplain deposits and likely grade laterally to the Buckhorn Conglomerate Member, as has been demonstrated with the Yellow Cat-Buckhorn Members in central Utah (Greenhalgh and Britt, 2007). The bulk of strata exhibited in our sections of the Cedar Mountain Formation best fit the Ruby Ranch Member. The uppermost part of the Cedar Mountain in northeastern Utah is reminiscent of the Mussentuchit Member of the Cedar Mountain Formation of the western San Rafael Swell, but radiometric data, dinosaur biostratigraphy, and palynomorph assemblages from sections in central Utah indicate these beds are not time equivalent (Tschudy and others, 1984; Cifelli and others, 1997; Kirkland and others, 1997; Kirkland and others, 1999; Sprinkel and others, 1999; Garrison and others, 2007; Kirkland and Madsen, 2007).

The age of the Cedar Mountain Formation in northeastern Utah is early to late Albian from a radiometric age (detrital U-Pb zircons) of 104.46 ±0.95 Ma associated with a well preserved sauropod skull, chemostratigraphic analysis, and palynology in the overlying Dakota Current: June 6, 2012 Sprinkel, Madsen, Kirkland, Waanders, and Hunt Original: June 1, 2005 Sprinkel-etal_CedarMtn-Dakota_06062012.docx

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Formation. Part of the Cedar Mountain Formation was deposited during the Kiowa-Skull Creek depositional cycle.

The Dakota Formation along the flanks of the Uinta Mountains typically consists of lower and upper light-colored and cross-bedded sandstone ledges separated by dark-colored carbonaceous mudstone and shale. The sandstone and mudstone beds are nonmarine; however, locally the basal Dakota Sandstone includes a thin interval of marine mudstone and shale beds.

Our palynomorph assemblage recovered from the basal Dakota Formation is similar to the assemblage from the Thermopolis Formation of central and southern Wyoming, and the Dakota Formation of Kansas, Nebraska, and Iowa (Oboh-Ikuenobe and others, 2007; Scott, 2007; Scott and others, 2009; Ludvigson and others, 2010b) and the irregular thickness of this interval indicate the initial marine incursion into northeastern Utah was likely restricted to incised valleys and other low-lying areas, similar to the Dakota Formation found elsewhere along the Western Interior Seaway (Ludvigson and others, 2010b).

Dinoflagellate cysts recovered from this basal marine interval represent peak sea level during the Kiowa-Skull Creek depositional cycle and the first marine incursion of the Cretaceous Western Interior Seaway into Utah. The age for this event is middle late Albian. An ash bed in the middle Dakota yielded a U-Pb zircon age of 101.4 ±0.4 Ma, which correlates to the newly defined Muddy-Mowry depositional cycle.

The Mowry Shale consists of siliceous marine shale that represents widespread openmarine conditions for the area. The radiometric age of the Mowry is between 98.5 ±0.5 Ma and 97.2 ±0.7 Ma (40Ar/39Ar sanidine) from bentonite beds in Wyoming. However, the biostratigraphic age is controversial because of downward revision to key neogastroplitid Current: June 6, 2012 Sprinkel, Madsen, Kirkland, Waanders, and Hunt Original: June 1, 2005 Sprinkel-etal_CedarMtn-Dakota_06062012.docx

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ammonite zones, revision of the Albian-Cenomanian boundary age to 99.6 Ma, and recently published palynostratigraphic work.