Pisolites & Oncoids ( Pis/9/Jur/Pak )

5x cross nicols
5x wave plate
10x wave plate
Outstanding feature Marine algal oncolites nucleated about pelecypod valves
Folk Classification Poorly washed Fossiliferous Pisosparite
Dunham Classification Fossiliferous Pisolitic Packstone
Common Grains Oncoids, pelecypod valves, grapestones, pellets and spicules
Sorting Bimodal sorted
Rounding rounded grains
Micritization & envelopes Common alteration of margins of bioclasts
Cement & diagenetic fabric Calcite spar & micrite cement. Marine & then burial diagenesis
Depositional Setting Moderate energy
Diagenetic history Micritized, then a rim cement followed by equant spar during burial.
This oncoid rich carbonate accumulated in a moderate energy open marine setting, since there are both bioclasts and algae associated. The bimodality suggests there were several sources for the sediment, which shows some level of winnowing of the fine grained micrites. The setting of these sediments appears to have ranged from higher energy marine, like a channeld to some level of protection on an adjacent shoal. The sediment grains show evidence of micritization in a marine setting followed by the effects of a bladed rim cment preciptated during early burial and a later final void fill equant spar.

Thursday, November 11, 2021
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