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Recognition Criteria and Facies Models
Tidal Estuary and Marine Facies of the Glauconitic Member, Drayton Valley, Central Alberta
The Lower Cretaceous Glauconitic Member near Drayton Valley, Alberta, consists of regionally extensive marine strata incised by laterally confined, estuarine channel fills. Marine facies consist of burrowed and laminated mudstone, muddy sandstone, sandy mudstone, and clean sandstone. These facies comprise three, widespread, coarsening-upward lithosomes, which represent three transgressive-regressive shelf-to-shoreface depositional Cycles (Cycles 1, 2 and 3). Estuarine channel-fill sequences are fining-upward successions of crossbedded sandstone, interbedded sandstone and mudstone, and massive mudstone deposited in tidally-influenced point-bar, channel margin, and abandoned channel environments. Evidence of tidal estuarine processes in the channel deposits includes: 1) rhythmically-bedded inclined heterolithic stratification (IHS), and wavy, lenticular, and flaser bedding, which infer a prominence of tidal currents; and 2) restricted suites of ichnofossils and microfossils, which indicates brackish water conditions.
Petrographic and sedimentological evidence suggest contemporaneous deposition of the tidal estuarine channel fills and marine facies within the two youngest depositional Cycles (Cycles 1 and 2). Evidence for this genetic relationship between tidal estuarine and marine facies includes: 1) the oldest channel fill and the middle marine succession (Cycle 2) are quartzose-lithic sandstones, whereas the youngest channel fill and the uppermost marine succession (Cycle 3) are both feldspathic-lithic sandstones; and 2) cored sequences document the presence of an outer estuary facies that is transitional with middle and upper shoreface facies of depositional Cycle 2. Outer estuary facies comprise burrowed, muddy sandstones that occur in channel-fill sequences and crossbedded shoreface sandstones that contain wood and shale intraclasts.
Cycle 3 represents the last stage of tidal estuarine deposition in the Glauconitic Member of the study area, and is characterized by feldspathic-lithic tidal point-bar deposits. Significant reworking of the sedimentary record by channel scour and migration occurred at the close of Cycle 3 deposition. It is likely that a number of tide-influenced rivers representing the mid to upper portions of an estuary extended across the entire study area.
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