AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Tidal Bars: An Overview
(1) Geol. Sci. & Geol. Eng., Queens University, Kingston, ON, Canada.
Tidal bars are one of the most important morphological elements in tidal environments, and their deposits should form a significant component of most tidal successions. However, confusion exists in some circles about the nature of tidal-bar deposits. At least two common misconceptions exist: tidal bars occur only as elongate, current-parallel features; and tidal-bar deposits consist predominantly of sand. Recent work shows that there is a morphological continuum between the free-standing elongate tidal bars that occur on continental shelves, elongate tidal bars that are attached to the bank at one end, and tidal point bars. Moreover, seismic data from many places show that all of these bar types migrate laterally, not in a forward direction as shown by the Mutti tidal-bar model. As result, all of these bar types create broadly similar deposits: they begin with an erosion surface formed by migration of the adjacent channel thalweg or inter-bar trough, and generally fine upward because of decreasing flow velocities toward the bar crest. In shelf settings, wave action on the bar crest may reverse the upward-fining trend. As a result of these similarities, tidal point bars and elongate tidal bars may be difficult to distinguish. Only the details of the facies, as controlled by the systematic variation in depositional conditions through the fluvial-marine transition will allow the location of origin to be determined. In many locations, elongate tidal bars consist of muddy heterolithic stratification, either because the environment had a high suspended-sediment concentration, or because the bar form is due to erosion of pre-existing deposits. Therefore, the indiscriminant use of the term “tidal sand bar” is inappropriate.