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Abstract
Journal of Sedimentary Research, Section
B: Stratigraphy and Global Studies
Vol. 65B (1995)No.
1. (February), Pages 61-79
Cyclostratigraphy of Middle Devonian Carbonates of the Eastern Great Basin
Maya Elrick
ABSTRACT
Middle Devonian carbonates (250-430 m thick) of the eastern Great Basin
were deposited along a low energy, westward-thickening, distally steepened
ramp. Four third-order sequences can be correlated across the ramp-to-basin
transition and are composed of meter-scale, upward-shallowing carbonate
cycles (or parasequences). Peritidal cycles (shallow subtidal facies capped
by tidal-flat laminites) constitute 90% of all measured cycles and are
present across the entire ramp. The peritidal cycles are regressive- and
transgressive-prone (upward-deepening followed by upward-shallowing facies
trends). Approximately 80% of the peritidal cycle caps show evidence of
prolonged subaerial exposure including sediment-filled dissolution cavities,
horizontal to vertical desiccation cracks, rubble and ka st breccias, and
pedogenic alteration; locally these features are present down to 2 m below
the cycle caps. Subtidal cycles (capped by shallow subtidal facies) are
present along the middle-outer ramp and ramp margin and indicate incomplete
shallowing. Submerged subtidal cycles (64% of all subtidal cycles)
are composed of deeper subtidal facies overlain by shallow subtidal facies.
Exposed subtidal cycles are composed of deeper subtidal facies overlain
by shallow subtidal facies that are capped by features indicative of prolonged
subaerial exposure (dissolution cavities and brecciation). Average peritidal
and subtidal cycle durations are between approximately 50 and 130 k.y.
(fourth- to fifth-order).
The combined evidence of abundant exposure-capped peritidal and subtidal
cycles, transgressive-prone cycles, and subtidal cycles correlative with
updip peritidal cycles indicates that the cycles formed in response to
fourth-to fifth-order, glacio-eustatic sea-level oscillations. Sea-level
oscillations of relatively low magnitude (< 10 m) are suggested by the
abundance of peritidal cycles, the lack of widely varying, water-depth-dependent
facies within individual cycles, and the presence of noncyclic stratigraphic
intervals within intrashelf-basin, slope, and basin facies. Noncyclic intervals
represent missed subtidal beats when the seafloor lay too deep to record
the effects of the short-term sea-level oscillations.
Exposure surfaces at the tops of peritidal and subtidal cycles represent
one, or more likely several, missed sea-level oscillations when the platform
lay above fluctuating sea level, but the amplitude of fourth- to fifth-order
sea-level oscillation(s) were not high enough to flood the ramp. The large
number of missed beats (exposure-capped cycles), specifically in Sequences
2 and 4, results in Fischer plots that show poorly developed rising and
falling limbs (subdued wave-like patterns); consequently the Fischer plots
are of limited use as a correlation tool for these particular depositional
sequences. The abundance of missed beats also explains why Milankovitch-type
cycle ratios ( 5:1 or
4:1) are not observed and why such ratios would not be expected along many
peritidal-cycle-dominated carbonate platforms.
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