CARBONATE TIDAL FLATS - A DIFFERENT PERSPECTIVE

Carbonate tidal flat models have been defined over the last 40 years and applied to a number of ancient stratigraphic systems, especially where large volumes of hydrocarbons are trapped. Here, focus is to re-assess the sequential order of facies and the applicability of modern systems to ancient examples.

Stratigraphic sequences of Permian age in the Permian Basin of west Texas and New Mexico trap enormous volumes of hydrocarbons and have been a frequent subject of studies relating tidal flat models to trapping mechanisms. Trapping occurs in one or more low order sequences in which the reservoir is in subtidal or channel deposits and seal is in evaporite-rich, frequently laminated interior intertidal sediments. Between the evaporitic facies of the interior and the subtidal reservoir is a well-laminated, often mud-cracked supratidal facies. Vertical sequence and lateral progradational facies mimic those of modern tidal flats on Andros and North Caicos Islands. The supratidal zone is the key element in separating reservoir and seal; it forms a low beach ridge in modern tidal flats in interior bank positions. Beach sediment is composed mainly of mud, often in the form of winnowed pellets. The beach ridge might be termed the bank margin levee where storm generated layers of pelleted mud are stranded adjacent to their subtidal source. The layers are typically mud cracked and can support only sparse colonies of bacterial mat. Between the beach ridge and the ultimate landward limit is a ponded area, somewhat starved of transported sediment. This is the intertidal, rich in cyanobacterial mats and in arid settings, the locus of evaporite deposition.

The beach ridge concept may be applied in reservoir-evaporite seal modeling of tidal flats in the Upper Mississippian of the Williston Basin of North Dakota and Saskatchewan. In a similar pattern with higher energy beaches, the supratidal may be aeolian dunes. Intertidal ponds would still exist between beach ridge and dry land and are the site of evaporite deposition, constituting a lateral seal. Modern and ancient examples of shoreface oolitic systems illustrate a higher energy case.