AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Channelization and Stratigraphic Architecture of a Tide-Dominated, High Tidal Resonance Basin Succession: Upper Eocene Temple Member of the Qasr Al Sagha Formation, Egypt
(1) Australian School of Petroleum, Adelaide, SA, Australia.
The search for tidal analogues has historically been biased towards transgressive estuarine deposits. The main reason for this is that drowned river valleys form funnel-shaped bodies of water that efficiently amplify tides. Less well studied are tidal shorelines associated with high tidal resonance basins, such as some wide shelves, large embayments and continental seaways. In such environments, significant tidal influence can be exerted on a shoreline independent of its shape or whether it is affected by transgression or regression. Even though, globally, a large number of petroleum reservoirs are associated with such basins, suitable outcrop analogues are rare. We present a case study from the well exposed Upper Eocene Temple Member of the Qasr Al Sagha Formation, Egypt, as one such example.
Detailed mapping of stratigraphic packages from an 11km long, strike-oriented outcrop belt resulted in the identification of over twenty transgressive-regressive, vertically-stacked allomembers. Correlation was greatly aided by the presence of excellent marker beds, such as diagnostic fossil beds, phosphate beds, and regional scoured surfaces. The lower and upper portions of the Temple Member are dominated by ichnologically stressed offshore mudstones, unconformably overlain by highly bioturbated, fossil hash-bearing transgressive sandstones. The middle portion is dominated by meter thick, wave and combined-flow rippled, coarsening-upward heterolithic successions which are also overlain by transgressive sandstones. The coarsening-upward successions often appear associated with channels that are tide dominated and highly variable in terms of fill and dimensions. Individual channels can be sandstone-filled, inclined heterolithic stratified (IHS), or mudstone-filled and can show complicated vertical and lateral stacking relationships. The thicknesses of some of the channels can be significant, reaching up to 40m and truncating a number of underlying allomembers. Even though of unusual size, these channels are single storey, have IHS that extend from top to bottom, and are likely attributed to enhanced tidal scour rather than base level fall and valley incision. The existence of such channels has important implications for sequence stratigraphy as these can easily be confused with incised valleys in subsurface correlation. Oversized channels relative to parasequence thickness may be a common occurrence in high tidal resonance basins.