AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Borneo – Where Models Go to Die
(1) Petroleum Geoscience Program, Chulalongkorn University, Bangkok, Thailand.
The explicit and implicit assumptions imbedded in nearly all facies, ichnofacies and sequence stratigraphic models cause important aspects of those models to break down when hydrodynamic and climatic conditions in the depositional setting lie considerably outside the “normal” range. The low tidal ranges and extremely low wave heights that occur on much of the Borneo coastline combine with exceptionally high sediment supply rates, and an equatorial climate that minimizes storm activity, to generate major departures from standard models.
Tide-dominant retrogradational successions at the parasequence and parasequence set scale are as common as, and in places more abundant than, progradational units in outcrop and in the subsurface. The high sedimentation rates force deposition of thick (km scale) successions during relative sea level rise that commonly include coarsening-upward, tide-dominated deltaic units and/or fining-upward, back-filled distributaries. The latter closely resemble progradational successions and develop because the low-energy receiving basins with relatively rare storms routinely allow preservation of relic, progradational deltaic sand body and distributary morphology during transgression.
Sedimentation is confined to water depths of ≤10 m along wave-dominated shorelines and there is a strong shoreward ichnofacies shift that results in no clear separation of the Skolithos and Cruziana ichnofacies. Occasionally, a Cruziana assemblage in wave-rippled and/or hummocky cross-stratified storm sands will include a subordinate number (5 - 10%) of open shelf (e.g. Palaeophycus) and deep water traces (e.g. Nereites, Paleodictyon). Presumably, this is because the shelf outboard of the 10 m bathymetric contour is a very low energy environment with an extremely low sedimentation rate and a muddy substrate that mimics the deep water environments in which these traces normally occur. Benthic foraminifera exhibit a similar trend; species that are most commonly occur in outer shelf and slope sediments comprise a subordinate, but significant, percentage in wave-dominated shallow marine sands and living, “mid-shelf” assemblages have been recovered from a water depth of only 5 m.
Stratigraphic successions deposited in extreme hydrodynamic environments can be interpreted successfully with a process-based approach that integrates local conditions. Conversely, model-driven interpretations are prone to failure due to their built-in assumptions.