Trends of Sedimentary Microfabrics of Ooid Tidal Channels and Deltas
Tidal channels within ooid sand shoals are involved in the longshore transport of ooids, the offbank and onbank transport of ooids and perhaps the generation of ooids. Channels are oriented perpendicular to the axis of highest ooid concentration and are linked to the transport system by their ebb-tidal deltas.
On Joulters Cays, Bahamas, surface samples and cores from four channels provide information on trends of microfabrics that should be useful for interpretation of ancient oolites. The percent ooids is maximum at the ebb-tidal delta and decreases both seaward and bankward where peloids and skeletal grains increase in abundance. Bioerosion of ooids is minimal at the ebb-tidal delta and increases into more stabilized areas both bankward (up channel) and seaward (offshore). Ooids are well sorted, unimodal and have fewer laminae (are smaller) on the ebb-tidal delta; whereas within the channel, ooids are bimodal in size (large ooids have numerous laminae and small ooids have fewer laminae) and therefore less well sorted. Nuclei of ooids are dominated by peloids in all environments. A trend of increasing size of ooids on ebb-tidal deltas and along intervening beaches in the direction of longshore transport has been noted. Size variation of ooids is not dependent on the size of the nucleus. Bimodality of ooids may result from in-place ooid generation and/or transport under variable energy conditions.
Tidal channels can migrate, be blocked for variable time periods, and can rapidly fill with sediment. Lime mud forms or accumulates as layers (several centimeters thick) within high-energy tidal channels. These layers may result from storm transport of lime mud from interior lagoons or from in situ accumulation during a period of blockage.
KeywordsTidal Channel Offshore Area Sedimentary Petrology Sand Shoal North Channel
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