Boyd, Ron1, Robert Dalrymple2, Brian Zaitlin3
(1) University of Newcastle, Callaghan, Australia
(2) Queen's University, Kingston, ON
(3) Encana Corporation, Calgary, AB
ABSTRACT: Estuaries and Incised Valleys
Estuaries are complex depositional environments that are characterized by the following key features. Firstly, they owe their origin to fluvial processes that form a valley. Secondly, estuaries are the direct result of sea level rise, depositing estuarine sediments in the valley during transgression and subsequent highstand. Thirdly, estuaries receive sediment from both marine and fluvial sources. Fourth, as defined geologically, estuaries contain a unique combination of facies controlled by wave, tide and river processes, and can be best classified with a ternary plot of these three factors. Both wave and tide dominated estuaries accumulate sedimentary facies in a tripartite zonation resulting from the down-valley variation in river and marine processes. Wave dominated systems appear to be more common in the ancient record, but tidal features are widespread and an important estuary recognition criterion. Estuaries are particularly important in low accommodation basins where they are preferentially preserved. Initial valley locations are often controlled by underlying structure and paleotopography. Hence the resulting estuary fills may be compound, reoccupying the valley over multiple cycles. Advances in understanding estuaries and incised valleys over the past decade have emphasized the initial development of comprehensive facies models for these settings. Other advances focused on the relationship of estuaries to sequence stratigraphic concepts, the establishment of key recognition criteria such as tidal facies and estuarine ichnology, the detailed and complex 3-D nature of the estuarine valley fill, and the development of mathematical and statistical models that simulate estuarine classification, sedimentation and circulation.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.