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Nearshore and Shelf Systems
Depositional Facies and Controls on Parasequence Development in Siliciclastic Tidal Deposits from the Lower Proterozoic, Upper Mount Guide Quartzite, Mount Isa Inlier, Australia
The upper Mount Guide Quartzite is a quartz arenite unit (2 km thick) of predominantly tidal origin. Sedimentation took place in a thermally subsiding intracratonic basin over a 20 to 25 m.y. time span between 1720 and 1800 Ma. Facies in the upper Mount Guide Quartzite consist of crossbedded arenite deposits capped by thin bedded arenites, and contain stacked, tabular parasequences ranging from 0.5 to 10.2 m thick.
Three types of crossbedded arenite subfacies are recognized: 1) tabular crossbedded sets and cosets (0.2-5.5 m thick) with common sigmoidal-shaped pause planes; 2) compound crossbed cosets (up to 9.9 m thick) characterized by three hierarchical orders of bounding surfaces; and 3) large-scale trough crossbed cosets (3.5-5.1 m thick). Internal structures within crossbed sets consist of ripple stratification and grainflows arranged in accelerating-decelerating flow cycles typical of modern and ancient tidal subaqueous dune deposits. Paleocurrent azimuths are dominantly toward the west, indicating strong asymmetrical tidal flow. The thin-bedded arenite caps contain a variety of structures: 1) current, combined-flow and wave ripples; 2) modified ripples including ladderback, round-crested, flat-topped and washed-out forms; and 3) emergence structures including aeolian ripple stratification, adhesion ripples and warts, and desiccation cracks. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal and, rarely, supratidal conditions.
Fischer plots of cumulative parasequence thickness (corrected for linear thermal subsidence) versus time show deviations in thickness from that expected for pure linear subsidence. The deviations can be attributed to changes in relative sea level rather than “jerky” subsidence. The Fischer plot permits recognition of systems tracts that are associated with type-2 sequences.
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