BIOLOGICAL MEDIATION OF STOCHASTIC PERITIDAL CARBONATE ACCUMULATION

Exponential thickness frequencies of peritidal carbonate units in the Lower Ordovician Kindled and West Spring Creek Formations at Ardmore, Oklahoma, are readily interpreted in a context of probabilities of ups ection transition from one lithology to another, These largely reflect Poisson (random) processes of deposition from suspended load, tractio n load, and microbialitic accumulation, Although grainy to muddy parti culate and cyanobacterial elements exhibit nearly equal ranges of unit thickness, carbonate generation and/or entrapment via algally mediate d processes was less likely to lapse and therefore led to lower probab ility of transition to some other sediment type. The mean thickness of microbially bound units is roughly double those from the physical tra nsport and deposition of particulate material, Greater persistence of algal accumulation probably related to intrinsically higher biological ly induced rates of carbonate precipitation and/or binding by cyanobac teria. Stratigraphic intervals between successive occurrences of suspe nded load, traction Bead, and microbial units are also closely approxi mated by exponential frequency distributions for which regression slop es define probabilities of upsection recurrence of a particular sedime nt type. Values for grainy and algal carbonates are similar and are ne arly twice that of muddy suspended-load units, Although biological pro cesses resulted in significantly lower transition probabilities for th rombolitic bioherms and cryptalgal laminites, spatial dominance of car bonate mud across the region led to higher rates of stratigraphic recu rrence and a volumetric dominance of muddy lithologies in the Ardmore sequence. Poissonian distributions of unit stratigraphic duration and recurrence suggest a significant component of haphazard variation in t he type and amount of accumulated carbonate sediment, If deposition wa s influenced by extrabasinal forcing, such control must have been near ly random in both secular and spatial dimensions of water depth change , Stratigraphic durations and recurrences in this sequence more closel y reflect the inherently stochastic nature of carbonate accumulation i n epicratonic platformal settings than any influence of rhythmic eusta tic forcing.