--> ABSTRACT: Conodont Biostratigraphy and Biofacies of the Wahoo Limestone (Carboniferous), Sadlerochit Mountains, Northeast Brooks Range, Alaska, by Andrea P. Krumhardt, Anita G. Harris; #91003 (1990).

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ABSTRACT: Conodont Biostratigraphy and Biofacies of the Wahoo Limestone (Carboniferous), Sadlerochit Mountains, Northeast Brooks Range, Alaska

Andrea P. Krumhardt, Anita G. Harris

The Wahoo Limestone forms the upper part of the Lisburne Group (Carboniferous) in the Sadlerochit Mountains. The Lisburne Group is a thick (> 600 m) sequence of platform carbonate rocks that extends across the Brooks Range of northern Alaska and beneath the North Slope. At Prudhoe Bay, the Lisburne Group forms a major hydrocarbon reservoir.

In the easternmost Sadlerochit Mountains, the Wahoo Limestone is divisible into informal lower (64 m) and upper (192 m) members. The basal 46 m is chiefly bryozoan and pelmatozoan packstone that formed on a relatively shallow platform during the latest Mississippian lower muricatus subzone (as shown by the occurrence of the zonal index with representatives of Cavusgnathus). Cavusgnathus is dominant in this part of the section and occurs with representatives of Kladognathus, Ghathodus, Adetognathus, Hindeodus, and Rhachistognathus (in order of decreasing abundance). Declinognathodus noduliferus, the index for the base of the Pennsylvanian, first occurs at 49 m above the base of the Wahoo and 1 m above a discontinuity surface that marks the Mississippian-Pennsylvanian boundary. The unco formity represents the highest conodont subzone of the Mississippian and probably part of the earliest Pennsylvanian. Previously, the Mississippian-Pennsylvanian boundary was placed at the lower-upper Wahoo contact based on endothyroids; conodont data now indicate that this boundary is 15 m lower. The remaining lower Wahoo is possibly of noduliferus-primus zone age and chiefly yields, in order of decreasing abundance, species of Adetognathus, Declinognathodus, and Rhachistognathus, as well as redeposited Mississippian conodonts. The lower 15 m of the upper member of the Wahoo contains silty (5-40%) carbonate rock types that yield very few conodonts. Conodonts no older than the minutus-sinuatus zone are relatively abundant from 15 to 106 m above the base of the upper Wahoo. These faunas a e dominated by robust representatives of Rachistognathus, particularly R. minutus, and lesser numbers of Adetognathus and Declinognathodus. Most of these collections are from medium- to coarse-grained oolitic to skeletal grainstones. The various carbonate microlithofacies in the succeeding 86 m of the uppermost Wahoo yield mainly species of Adetognathus, Declinognathodus, Idognathodus, and Rhachistognathus in variable relative abundance. These conodonts merely indicate an age no older than the sinuosus zone and no younger than the Atokan. Endothyroids from this section suggest an Atokan age for this interval.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990