SEDIMENTARY RECORD OF MODERN AND ANCIENT TIDAL INLETS

The sedimentary characteristics of tidal inlets were defined in detail by process –geomorphologic and subsequent stratigraphic investigations primarily along the barrier island shorelines of North and South Carolina during the 1970’s and 80’s. The results of these studies were the basis for tidal inlet facies models providing a spectrum of facies associations and generalized recognition criteria for “inlet –fill” deposits in meso- and microtidal settings.

From a sedimentary facies perspective, inlet-fills are but an end-member of a myriad of tidal channel deposits. Their uniqueness, and hence recognition in ancient successions lies in the fact that tidal inlet deposits erosionally overlie, and are laterally adjacent to, either: a) shoreface facies; or b) lagoonal facies in a prograding or retrograding barrier shoreline setting. Thus, the application of Walther’s Law is imperative in accurately identifying a tidal channel facies association as an “inlet-fill”.

By nature of their depositional origin, tidal inlets are deeply incised and rework previously deposited sediments. Due to their rapid sedimentation and deposition below the depth of wave reworking during shoreface ravinement, tidal inlet-fills have the highest preservation potential of all barrier shoreline facies. This assumption is verified by drilling and subsurface investigations which have documented that roughly 40% of Holocene barrier island shorelines on average are underlain by tidal inlet-fill deposits.

Modern inlet sediments are commonly bioclastic accumulations in a coarser grained sand matrix winnowed free of detrital fines. As a result, they are prone to a high degree of permeability and primary and/or secondary (moldic) porosity. This enhanced reservoir quality, coupled with their preferential preservability, make inlet deposits prime hydrocarbon reservoir targets in the ancient sedimentary record. Numerous examples of tidal inlet oil and gas reservoirs occur, including the Eocene Sparta Fm. in the Gulf of Mexico, and Triassic Halfway Fm. in the Western Canada Sedimentary Basin. The recognition, architecture, predictability and successful drilling of these hydrocarbon pools has been achieved through insight and comparison to modern day analogues from the barrier shorelines of the eastern United States.