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Stratigraphy and Depositional Environments of the Mancos Shale (Cretaceous), Black Mesa, Arizona
Five outcrop sections of the Upper Cretaceous Mancos Shale of Black Mesa, Arizona, have been measured and sampled at close intervals, and the contained microfossils used to subdivide the unit into assemblage zones.
On the basis of its contained microfossils, three vertical biostratigraphic units, or assemblage zones, can be recognized, each comprising about one-third of the section. The lower zone, called the Hedbergella delrioensis zone, contains a planktonic and benthonic foraminiferal fauna. The middle unit, called the megaspore zone, is nearly barren except for plant megaspores. The upper zone, called the Gaudryina bentonensis zone, contains an exclusively arenaceous foraminiferal fauna. The zonal names used are all informal.
The Hedbergella delrioensis zone is named for this planktonic foraminifer, which occurs in large numbers throughout the lower unit. This zone contains abundant foraminifera, both planktonic species and calcareous and arenaceous benthonics, and may have been deposited in an estuary or embayment which was at times partially cut off from the open seaway to the east. The lower part of the Hedbergella delrioensis zone also contains the boundary of the Rotalipora and Praeglobotruncana Concurrent-Range Zones, which define the Cenomanian-Turonian boundary as presently recognized in the western interior, Gulf Coast, and California areas of the United States.
The megaspore zone contains two types of megaspores based on form, and appears to have been deposited in a marginal marine mud flat under near subaerial conditions like those in modern Dystichlis-Salicornia mud flats.
The Gaudryina bentonensis zone is named for this distinctive, predominantly Carlile (middle Turonian) species which occurs abundantly in the upper assemblage zone, but has not been found in either of the two lower zones. Numbers of individuals and species of arenaceous foraminifera show an increase in this zone, and megaspores disappear. More flooding during deposition is thus indicated for the G. bentonensis zone than for the megaspore zone, either as a result of a slight decrease in sediment supply or a slight increase in the rate of basin subsidence.
Faunal evidence indicates that the entire Mancos Shale of Black Mesa was deposited in a partially restricted embayment on the western shore of the western interior seaway, beginning in late Cenomanian time. Marine conditions persisted into the early Turonian. The embayment filled with mud, and became a nearly subaerial mud flat and later a partially flooded mud flat by mid Carlile (middle Turonian) time. Maximum water depth during deposition of the Hedbergella delrioensis zone may have been less than 10 fathoms, and probably did not exceed 20 fathoms.
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