U.S.
Geological Survey
Open-File Report 01-402
Sedimentology of the Pennsylvanian and Permian Strathearn Formation, Northern Carlin Trend, Nevada
By
Vladimir I. Berger, Donald A. Singer, and Ted G. Theodore
With a
section on Microfossil controls on age of the Strathearn Formation
By Anita G. Harris and Calvin H. Stevens
ABSTRACT Two framework-supported,
poorly bedded conglomerate units of the middle Upper Pennsylvanian and
middle Lower Permian Strathearn Formation belonging to the overlap assemblage
of the Antler orogen are prominent in the northern Carlin trend. These
horizons stratigraphically and temporally bracket thrust emplacement of
a major allochthonous thrust plate of mainly quartzarenite of the Ordovician
Vinini Formation. Lithologic and shape-ratio data from approximately 4,200
pebbles and cobbles at 17 sites as well as biostratigraphic data in the
Strathearn, and their geologic implications, are included in this report.
Conodont biofacies throughout the Strathearn Formation are normal marine
and suggest middle shelf or deeper depositional environments. The conglomerate
units roughly are similar in that they contain only chert and quartzarenite
pebbles, but they differ in compositional proportions of the two lithologies.
The relative proportion of quartzarenite pebbles increases sixfold in
the middle Lower Permian upper conglomerate unit versus its content in
the middle Upper Pennsylvanian lower unit, whereas chert pebbles predominate
in both units. Various roundness categories of chert pebbles in both conglomerate
units of the Strathearn show that the equant pebble class (B/A) = 1 clearly
is represented strongly even in the subangular category, the lowest roundness
categories for the pebbles. Thus, development of equant pebbles cannot
be ascribed totally to a rounding process during predeposition transport.
The equant character of many pebbles might, in part, be an original feature
inherited from pre-erosion rock fractures and (or) bedding that control
overall form of the fragments prior to their release to the transport
environment. The allochthon of the Coyote thrust has been thrust above
the lower conglomerate unit of the Strathearn during a regionally extensive
contractional event in the late Paleozoic. The middle Lower Permian upper
conglomerate unit, highest unit recognized in the Strathearn Formation,
as well as similarly-aged dolomitic siltstone, onlap directly onto quartzarenite
that comprises the allochthon of the Coyote thrust. The conglomerate units
thus represent submarine fanglomerates whose quartz grains and quartzarenite
fragments of variable roundness and shape were derived from a sedimentologically
restored largely southeastward advancing late Paleozoic allochthonous
lobe of mostly quartzarenite of the Ordovician Vinini Formation. Chert
fragments in the conglomerates probably were derived mostly from Devonian
Slaven Chert, including a widespread thick mélange unit of the Slaven
in the footwall of the Coyote thrust. Some chert pebbles may have been
derived from the Ordovician Vinini Formation. |
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Maintained by: Michael Diggles
Created: 12-05-01
Last modified: September 23, 2004 (mfd)