Do All Carbonate Parasequences Shallow up? If Not, Why Not?
Small scale parasquences characterize carbonate platforms from the Late Archean to Holocene. These cycles are bound by marine flooding surfaces. On shallow platforms shallowing upward regressive, peritidal parasequences are common. Thin transgressive lags form during transgressions when deepening water (sedimentation less than accommodation) may be tied to submarine erosion, bypass and lag time/or depth, followed by accumulation of regressive subtidal to peritidal sediments under stable or falling relative sea level, and possible subaerial erosion updip. Cycles are capped by barren muds, tidal flat facies and subaerial exposure surfaces, and evaporites in arid settings.
Rapid deepening to deeper subtidal depths, followed by upward shallowing to storm or fair weather wave base as sea level falls on outboard of low and mid-latitude platforms in deeper water, commonly causes regressive coarsening up parasequences of carbonate muds, interbedded muds and sands, grainy tops, capped by marine erosion surfaces.
Transgressive and transgressive-regressive parasequences are common on some platform interiors (e.g. Triassic). Here sea level rise just outpaces sedimentation rate, and low energy and insufficient bioturbation prevent removal of the deepening upward section so a marked transgressive record is preserved, often capped by a paleosol if sea level fall is rapid enough or the peritidal cap is stripped by erosion. On low-latitude greenhouse platforms this transgressive part of a cycle may also contain beaches of ooids, sublittoral sand sheets, and even hypersaline ponds of the platform interior.
On deeper parts of low-latitude platforms, deepening and fining upwards parasequences may form on the deep ramp or within intrashelf basins, near storm wave-base. Cycles typically start with a sharp basal contact, overlain by high energy grainstone grading up through packstone, wackestone to mudstone, capped by the next truncation surface. These are deeper ramp, symmetrical transgressive-regressive cycles whose regressive cap has been stripped by marine erosion when sea level is lowered.
Deepening upwards, but upwards coarsening parasequences occur in mid-latitude epeiric ramps within low energy embayments and estuaries bordering swell wave dominated shelves. The cycles have low energy muddy lower parts and deepen up into high energy open marine skeletal grainstones deposited near the wave abrasion depth.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009