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Lithostratigraphic and biostratigraphic framework for the Mancos Shale (Late Cenomanian to Middle Turonian) at Black Mesa, northeastern Arizona

By
James Ian Kirkland
James Ian Kirkland
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Published:
January 01, 1991

Detailed analysis of three sections of the Late Cretaceous Mancos Shale at Black Mesa in northeastern Arizona demonstrates that sufficient data are preserved within the thick, seemingly monotonous, fine-grained clastic sequences of the western foreland basin to develop a conceptual framework comparable to that developed for the carbonate-dominated central Western Interior Seaway. The Mancos Shale ranges from 146 m in southwestern Black Mesa to form 200 to 210 m in northern and eastern Black Mesa and is herein divided into four members in ascending order: lower shale member, middle shale member, Hopi Sandy Member, and upper shale member, of which only the Hopi Sandy Member is formally described. In addition, a prominent sandstone tongue of the overlying Toreva Formation at Blue Point in southwestern Black Mesa is herein designated the Blue Point Tongue of the Toreva Formation. Based on recovered fauna, a total of 13 biostratigraphic subdivisions are recognized, in ascending order: the upper Cenomanian Metoicoceras mosbyense Zone, Sciponoceras gracile Zone divided into the Vascoceras diartianum and Euomphaloceras septemseriatum subzones, and Neocardioceras juddii Zone divided into the Euomphaloceras irregulare, Neocardioceras juddii, and Nigericeras scotti subzones; the early Turonian Watinoceras coloradoense Zone divided into the Pseudaspidoceras flexuosum and Vascoceras birchbyi subzones and the Mammites nodosoides Zone; the Middle Turonian Collignoniceras woollgari Zone divided into the Collignoniceras woollgari woollgari/Mytiloides hercynicus, Collignoniceras woollgari woollgari, and Collignoniceras woollgari regulare subzones and the Prionocyclus hyatti Zone. Within this lithostratigraphic and biostratigraphic framework, 77 chronostratigraphic marker beds (BMI, BM2, . . . BM77) have been recognized, consisting mainly of bentonites along with laterally continuous concretion horizons, bioturbated marlstones, and sediment bypass intervals (calcisilts).

The Mancos Shale disconformably overlies the Dakota Formation, recording the partial to complete destruction of a series of northwest- to southeast-trending barrier coastlines. The development and preservation of these barrier systems were tectonically controlled by subtle movements of northwest- to southeast-trending folds developed over deep-seated basement structures. The seaway transgressed across Black Mesa from the northeast to the southwest during the Metoicoceras mosbyense Zone through the Sciponoceras gracile Zone. Water depth increased rapidly through the early Turonian, with the lower shale member recording the greatest influence of pelagic sedimentation at Black Mesa. The lower shale member ranges from 54 to 67 m thick, being thickest in the northwest toward the area of maximum development of the western foreland basin proximal to the Sevier Orogenic Belt. The middle shale member thins from 55 m in the southwest to 42.5 m in the northeast and records the first demonstrable effects of regression at Black Mesa, marked by a decrease in carbonate content and an increase in silt and fine sand content representing distal storm deposits. Significant quantities of sand were first transported across the shelf during deposition of the Hopi Sandy Member (uppermost Collignoniceras woollgari woollgari subzone) by a combination of delta progradation from the southwest and storm-induced reworking. The Hopi Sandy Member ranges from 12 to 22 m in thickness, being thickest to the north and northeast. Sandstone abundance and thickness increase in the southwestern shoreward direction, indicating winnowing and offshore reworking of much of the clay. Renewed subsidence is indicated by a rapid decrease in silt and sand content in the lower part of the upper shale member. The upper shale member measures 34 m in the southwest (including the interval of the Blue Point Tongue) and thickens rapidly to the northeast, where it is 85 m thick. Regression of a wave-dominated shoreline complex across the area of southwestern Black Mesa during the Collignoniceras woollgari regulare subzone led to the development of the shallow coastal shoals of the Blue Point Tongue and beach sequences of the lower sandstone member of the Toreva Formation. Continued fine clastic sedimentation offshore reflects dominantly longshore sediment transport. Regression of the sea continued with the final progradation of wave-dominated delta systems across northern and eastern Black Mesa during the Prionocyclus hyatti Zone.

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Contents

GSA Special Papers

Stratigraphy, depositional environments; and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway

J. Dale Nations
J. Dale Nations
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Jeffrey G. Eaton
Jeffrey G. Eaton
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Geological Society of America
Volume
260
ISBN print:
9780813722603
Publication date:
January 01, 1991

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