Sedimentation, Stratigraphy, and Paleoenvironments
of the Lower Members of the Carmel Formation
(Middle Jurassic, southwestern Utah)
Kirsten Bannister
The Middle Jurassic Carmel Formation in southwestern Utah is composed
of shallow-marine to peritidal sedimentary rocks, including complexly
interbedded limestones, siltstones, sandstones, and evaporites that
accumulated during two major transgressive cycles. The Carmel is composed
of seven, second-order shallowing-upward cycles and is believed to
have been deposited in off shore and shoreline environments near the
southern margin of the Middle Jurassic Western Interior seaway (Carmel
seaway) that extended into southern Utah from Canada.
Previous interpretations separated the Carmel into six members (A-F),
each of which represents a portion of the shallow marine peritidal
system. Members A through C were studied for this report. Member A,
an oolitic shoal assemblage, represents the highest energy environment
preserved in the Carmel Formation, and contains fossil fragments of
gastropods and bivalves; the fossils of this member are most likely
transported and reworked remains derived from elsewhere in the system
by storms.
Members B and C are indicative of a restricted lagoonal environment
grading into an intertidal sequence. Member B represents a hyper saline
lagoon and represents an arid laggon with little or no water circulation
occurred. It lacks fossils, although a variety of sedimentary structures
are present, including small scale ripple marks, laminated siltstone
and mudstone coated with gypsum hopper crystals. Member C includes
sedimentary structures and fossils representing an intertidal onshore
zone. Structures include crossbedding on small and large scale, ripple
marks, desiccation cracks. An array of fossils including bryozoans,
crinoids, and bivalves are found within a variety of lithologies.
Overall, the Carmel Formation represents the margins of a much larger,
intra-cratonic marine basin and records periods of high to low energy
conditions and normal to hypersaline marine to peritidal environments.
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