Late Mississippian (Chesterian)-Early Pennsylvanian (Morrowan) Conodont Biostratigraphy of East-Central Idaho and Southwest Montana*
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Search and Discovery Article #50022 (2005)
Posted November 10, 2005
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1University of Idaho, University of Idaho, Department of Geological Sciences, Moscow, ID 83843, phone: (814) 574 4121 ([email protected])
2Department of Geological Sciences, University of Idaho, Moscow, ID 83844-3022
Abstract
Biostratigraphic
investigation and biofacies analysis of Late Mississippian (Chesterian) to Early
Pennsylvanian (Morrowan) conodonts in east-central Idaho and adjacent Montana
were required to delineate sequences on a platform to foreland basin transect.
Three conodont biozones were encountered and time lines through thin and thick
successions are evident. Formations containing these zones are interpreted to
represent westward thickening shelf margin, cratonic trough or embayment,
shallow shelf, and near-shore deposition on the basis of associated conodont
biofacies, faunal correlatives, and lithology. Conodont faunas present within
the boundary interval are dominated by species of Cavusgnathus,
Adetognathus, Gnathodus, and Rhachistognathus, which compose
nearly 95% of conodonts recovered. Late Mississippian (Chesterian) conodonts
recovered include Cavusgnathus naviculus, C. altus, C.
regularis, C. windsorensis, C. convexus, Adetognathus
unicornis, Gnathodus bilineatus, and G. commutatus commutatus.
Several broadly occurring and correlatable
biostratigraphic
zones are delineated
for the Chesterian and include: 1) the mid-Chesterian Cavusgnathus sp.
zone; 2) the upper Chesterian Adetognathus unicornis/Cavusgnathus
naviculus zone; 3) the uppermost Chesterian Rhachistognathus muricatus/Gnathodus
bilineatus-commutatus commutatus zone. Early Pennsylvanian (Morrowan)
conodonts recovered include Adetognathus lautus, A. spathus, A.
gigantus, Rachistognathus muricatus, and R. primus. The
Mississippian-Pennsylvanian boundary is marked by the overlapping occurrence of
Rachistognathus primus, Adetognathus lautus, and A. spathus
with the concurrent decimation of Cavusgnathus sp. and Gnathodus
bilineatus–commutatus commutatus. These defined
biostratigraphic
zones allow
for the accurate
correlation
of shallow marine strata across the study area.
Location of field area (east-central Idaho and southwest Montana).
Late Chesterian depositional setting (after Rose, 1976)
Late Mississippian facies and CAI (color alteration index [conodont]).
Reference