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Tidal Inlets, Deltas and Flats
Tidal and Seasonal Cycles in a Lower Cambrian Shallow Marine Sandstone (Hardeberga Fm.), Scania, Southern Sweden
The Lower Cambrian Hardeberga Formation in Scania (southern Sweden) is 120 m thick and rests directly on the weathered and peneplained crystalline Proterozoic basement. The formation comprises a basal, 15 to 25 m thick, arkosic sandstone unit, overlain by three, 30 to 50 m thick, vertically stacked marine sequences. The basal unit consists of an immature, conglomeratic sandstone grading into crossbedded, arkosic sandstone. Paleocurrents were consistently toward the south. The basal unit consists of a fluvio-deltaic deposit, truncated by a marine erosion surface (a ravinement).
The three marine sequences are separated by ravinements and were deposited as a result of a northerly directed regional transgression over the southern part of the Baltic Shield. Each sequence contains three major facies associations that document the progradation of a mixed tide-and wave-influenced barrier island shoreline. Each ravinement surface is mantled by a 10 m thick unit, initiated by muddy heterolithic strata overlain by a hummocky cross-stratified and coarse grained wave-rippled quartzose sandstone. This succession represents the transition from an inner shelf to shoreface environment. The paleoshoreline was oriented approximately east-west, parallel to the orientation of the coarse grained wave ripples. The inner shelf-to-shoreface association is truncated by a 10 m thick tabular sandstone body, which shows giant-scale epsilon crossbedding, occasionally interrupted by wave ripples. This deposit is interpreted to have been formed by lateral migration of a tidal inlet delta-barrier complex. The tidal inlet sandstone is erosionally overlain by spit shoreface bar-trough crossbedded deposits or a 1.5 to 2 m thick wedge of epsilon crossbedded sandstone that represents an on-barrier creek deposit. The inlet, spit shoreface and back-barrier creek deposits constitute the barrier island facies association. This association is erosionally overlain by a backbarrier tidal channel facies association consisting of stacked vertical sequences of A and B units. The A units are 2 to 7 m thick, crossbedded sandstone beds displaying regular mud drapes, acceleration-deceleration structures, and a bipolar, east-west paleocurrent pattern, parallel to the paleocoastline. The A units reflect deposition in active tidal channels. The B units typically comprise 2 to 10, stacked 0.2 to 0.8 m thick microsequences of crossbedded sandstone passing upward into bioturbated or tidal-bedded and burrowed sandstone. This cyclic alternation between bedform migration and dense bioturbation is interpreted as indicating deposition during winter storms alternating with longer periods of low-energy deposition in abandoned tidal channels, during the calm summer season.
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