ROBERTS MOUNTAINS FM.
Silurian Roberts Mountains Formation, Challis Idaho stratigraphy and depositional
environments.
Matthew Chapin
Geology 324
University of Idaho
Abstract
Silurian carbonate rocks of the Bradshaw basin in central Idaho have been
measured and analyzed to interpret the past depositional environments of Idaho. During
the experiment, thicknesses of strata were measured using a Jacob staff and Brunton
compass to accurately construct a stratigraphic column. After measurements the facies
were described by allochem content, rock color, and sedimentary structures. Coarsening
up cycles within the stratigraphy indicate progradational shallowing that are linked to a
net drop in sea level. Conclusively, deep shelf environments are associated with the
mudstone strata, and shallower slope environments are associated with the wacke-stone
and pack-stone strata. Throughout the measured section progradational parasequence sets
are seen and repetitive. These parasequence sets also support the net drop in sea level.
ROBERT MOUNTAINS FM.
Introduction
Four hundred and forty million years ago Idaho was in a completely different
location with different depositional environments than today. The Silurian stratigraphy of
central Idaho is composed of marine carbonate dolostones. These dolostones are found
five thousand plus feet above sea level, and hundreds of miles from the ocean. This report
investigates the depositional environments of central Idaho during the Silurian time
period. Sedimentary structures, overall stratigraphic changes, and paleoenvironmental
information in sections of measured stratigraphy are recorded to interpret the ancient
depositional environments of Idaho. This study was conducted by Dillon Bailey, Ian
Tester, and Matthew Chapin in the Roberts Mountains Formation south of Challis Idaho.
Methods
To collect data, first the orientation of strata was measured by taking the strike
and dip with a Brunton compass. The Brunton compass was then mounted on a Jacob
staff to allow accurate measurements of thicknesses equal to 1.5m. The Jacob staff is
marked at the 10cm and 1m intervals to measure strata thickness less than 1.5m. After
each measurement of unit thickness, data was plotted on a stratigraphic column, and the
strata were described. Data such as sedimentary structures, thickness, and allochems were
all recorded and plotted on the stratigraphic column. Descriptions of the rocks included
color, grain size, sedimentary structures, and fossil content.
Results
Marine carbonate dolostones found in the Bradshaw basin of central Idaho, south
of Challis, belong to the Silurian Roberts Mountains Formation. The dolostone strata
repeat a pattern of mudstone, wacke-stone, and pack-stone up slope. The facies measured
are oriented 355/9 E, and are nearly horizontal. Between beds both sharp and gradual
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contacts are present. Generally, gradual contacts are present between wacke-stone and
pack-stone strata. Fossils are plentiful throughout the facies. Brachiopods, Rugose coral,
and crinoids are abundant in wacke-stone, and pack-stone strata along with fossil hash.
Whole fossils become restricted up section, and fossil hash becomes more prevalent.
Mudstone layers are thinly laminated, and lack fossil presence. At large, fresh surfaces of
the strata are grey in color, and weathered surfaces of the strata are red-brown in color;
however, color varies.
Discussion
The dolostone strata measured for this study were dominantly mudstone, wackestone, and pack-stone. The mudstone strata are characterized by fine grained, thinly
laminated (millimeter scale) beds with less than ten percent grains, and were most
common. Ideally, marine carbonate sediments are deposited primarily on shallow shelf
platforms. The mudstone strata are results of oxygen poor deep shelf settings due to the
lack of fossils, and bioturbation. Wacke-stone strata are characterized by having more
than ten percent grains and more than ten percent allochems; however, the strata are
matrix supported. The pack-stone strata were the least common and are characterized by
having more than ninety percent grains as well as abundance of fossils. The fossils found
in both the wacke-stone and pack-stone strata are equally consistent; including crinoids,
brachiopods, and rugose coral. Both the wacke-stone and pack-stone strata are results of
deposition from sedimentary gravity flows, at large turbidity currents and debris flows.
Also, both quiet and turbid water conditions are recognized by the condition of the
fossils.
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Distinctive shallowing upward cycles within the stratigraphy indicate
progradational parasequences. The stratigraphy includes fossiliferous, laminated and
bioturbated facies. These characteristics are interpreted as subtidal and open marine
settings. During progradational sequences the sediment supply exceeds the rate of
addomodation and the sediment builds into the basin. Progradation is common during sea
level fall and is often associated with glaciations cycles; however, there was no evidence
found in this study to support glaciation as a mechanism for net drop in sea level.
The sediments described in this study were deposited in a passive margin between
oceanic and continental crust. The lithologies consist of carbonate sediments that were
deposited in shallow dipping slope and shelf environments. Because there is lack of
evidence from underwater canyon and channels in the measured sections, turbidites are
best explained as a product of a carbonate slope apron. The coarsening up cycles found in
the stratigraphy also reflect a carbonate slope apron. Furthermore, the coarsening up
trends reflect progradational shallowing caused by a net drop in sea level. The
stratigraphy exhibits evidence of progradational shallowing in these coarsening up cycles,
presumably a deep shelf environment with the influence of anoxic bottom waters, up
slope into a shallow slope environment with oxygenated waters.
Conclusion
Stratigraphy of the Bradshaw basin in central Idaho has been analyzed to interpret
the depositional environments of Idaho throughout the Silurian. During the experiment,
thicknesses of strata were measured and data was plotted on a stratigraphic column.
Following measurements, the strata were named by rock characteristics using the
Dunham classification system. Later, lithologic analysis indicated that these carbonate
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sediments were deposited in slope and shelf environments that were largely influenced by
sedimentary gravity flows. Finally, coarsening up cycles in the stratigraphy indicated
progradational shallowing linked to a net drop in sea level.
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