Estuarine Tidal-Accretion Bar Subfacies Recognition and Geometries

Roderick W. Tillman

Tidal-accretion bars are a significant part of tidal estuarine sand bodies in Willapa Bay, Washington and the Muddy Cretaceous sandstones of Sun Ranch field and in outcrops near Newcastle, Wyoming. However, subfacies useful in recognizing tidal-accretion bars have been largely ignored. Subfacies of tidal accretion bars are designated herein as (1) active tidal-accretion bars, (2) incipient slumped-sand facies, (3) slumped-sand facies, (4) slurried bank-margin collapse deposits and (5) bottom-channel tidal-fill sand. These subfacies all occur in the examples to be discussed.

Active tidal accretion bars differ from predominantly cross-laminated traction-deposited fluvial point bars in that they appear to have almost no traction component and appear to have been deposited dominantly from suspension as one to five meter thick bars composed entirely of gently tilted parallel sets up to 2^Prime thick. Interlaminated with the sand on a more-or-less regular basis are distinct dark colored silty shale laminae.

During periods of low tides sandy surficial portions of the bars commonly slump downward towards the adjoining channels. Initial slumping is recorded as incipient soft-sediment deformation of the accreted laminae. With increasing downslope movement of the slumps they break up into irregular blocks. With further slumping gravity-falls form a slurry and the sands lose all evidence of bedding and laminations. Clay clasts and plant roots and stems may be scattered through the otherwise massive appearing slurried sand.

The architecture of these features is well illustrated in selected outcrops and cores. Petrophysical characteristics and flow unit geometries of the different subfacies are distinctly different and predictable. Subsurface and outcrop gamma-ray log characteristics vary among the subfacies dependent in part on the amount and distribution of the clay as drapes, clasts and beds.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California