MULTIPLE SHALLOWING CYCLES GENERATED WITHIN OVERALL SEA LEVEL HIGHSTANDS

In ancient sequences, shallowing upwards sequences are commonly thought to represent a distinct high-stand cycle of sedimentation during times of prolonged sea level stability, with the shallowing cycles as an integral part of coastal progradation. Recent research on late Holocene sea level documents small sea level oscillations or steps embedded within the overall sea level trend. A study of sedimentation in response to the latest small rise (23 cm since 1930) affecting the broad, low-gradient, southwest coast of Florida shows that these embedded small rapid sea level rises can trigger rapid sedimentation and generation of shallowing-upwards sequences within the coastal system. Combined with the sea level oscillation at 3,200-2,400 YBP, this single overall Holocene rise of sea level may generate multiple shallowing-upwards sediment cycles. These multiple cycles can either be stacked or offset, complete or partially eroded by the subsequent cycle. Importantly, the nature of each embedded cycle may be quite different, being controlled by the nature and pattern of pre-existing sedimentary environments and the nature and extent of the sea level rise or oscillation. A more careful examination of ancient shallowing-upwards cycles will undoubtedly reveal that a composite set of cycles is a common feature of a sea level high stand.