AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Forced-Regressive Tidal Flats: Response to Falling Sea Level in Tide-Dominated Settings
(1) Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada.
During forced regression the coastline advances irrespective of sediment supply and relocates to a topographically lower position. Forced-regressive deposits are recognized by a basal unconformity, and are capped by another unconformity due to subaerial exposure or wave-ravinement. In wave-dominated settings, the basal ‘regressive surface of marine erosion’ is a scour surface cut by wave action. The response in tide-dominated settings has been hitherto unknown. The stratal architecture of the Gog Group of the southern Canadian Rocky Mountains reveals a new mechanism for the formation of this surface landward of the lever point of balance between sedimentation and erosion in the subtidal environment.
The contact between the Lake O’Hara and Lake Oesa members of the St. Piran Formation represents a regional unconformity. The Lake O’Hara Member records the growth and migration of a subtidal sand-sheet complex during a normal regression. Parasequences are characterized by the transition from intercalated thin-bedded, sandstone and mudstone, hummocky cross-stratified, and densely bioturbated Skolithos pipe rock, overlain by medium- to thick-bedded, cross-stratified sandstone. However, at the top of this member, an erosion surface truncates a pipe-rock interval, and the expected capping cross-stratified sandstones are absent. The Lake Oesa Member is composed of fining-upward parasequences characterized by thin- to medium-bedded cross-stratified sandstone. These are gradationally followed by intercalated mudstone and wavy- and lenticular-bedded sandstone containing interference ripples and desiccation cracks. These facies are typical of tidal flats.
The erosive contact at the base of the tidal-flat package indicates an abrupt seaward facies shift. This is because sand-flat deposits erosively overlie subtidal Skolithos pipe rock. Absent are intervals of non-bioturbated cross-stratified sandstone that normally would have been deposited in the shallower parts adjacent to the tidal flats. This jump in facies indicates a forced regression, and the erosion surface must have formed in response to sea-level fall. The laterally continuous tidal flats advance and the preexisting shallow-subtidal compound dunes are scoured by strong tidal currents that carve gradually a new equilibrium profile. We argue that the accretion of intertidal flats on top of subtidal sands is an overlooked yet predictable component of falling-stage systems tracts in tide-dominated settings.