AUTOGENIC DYNAMICS IN CARBONATE SEDIMENTATION - METER-SCALE, SHALLOWING-UPWARD CYCLES, UPPER CAMBRIAN WESTERN NEWFOUNDLAND, CANADA

Meter-scale, shallowing-upward cycles in Upper Cambrian carbonate stra ta of western Newfoundland, Canada, are muddy at the base, clean upwar d to oolite, and are capped by hardgrounds and small, microbial bioher ms. These cycles are interpreted to be fossil shoals. Their origin as shoals intrinsically explains (1) why cycles show a predictable, shall owing-upward lithofacies organization; (2) why this organization can c hange parallel and perpendicular to depositional strike (resulting in many apparently 'incomplete' cycles, but retention of some predictabil ity); and (3) why cycles can either persist laterally for many kilomet ers or terminate abruptly. These latter two points are a function of t he relationships between the wedge-shapes df the fossil shoals, contin uity of lithofacies, the orientation/sinuosity of the fossil shoal bod ies, and modern outcrop geometry. A cycle is produced from In-place oo id and calcisilt creation (termed 'shoaling-mode'). Shoals grow, migra te and accrete and may reach equilibrium by feedback with the local fl ow regime. Shoaling increases the potential for early lithification of the sea floor, locally stabilizing the shoal surface. Cessation of sh oaling can be autogenic, for example, as a result of the shoal migrati ng out of the principal sediment transport path or through submarine l ithification of the shoal surface. Alternatively, shoaling can be arre sted through allogenic effects, such as changes in atmospheric/oceanog raphic dynamics that alter platform circulation patterns. Rock cycles suggest that upon termination of shoaling, sedimentation switched loca lly to 'microbialite mode'. Microbes Invaded the shoal field and infes ted the lithified sea floor (capping the cycle as small stromatolite/t hrombolite bioherms). Cycle tops are distinct principally because they were cemented early. Using modern platforms as a rough guide, it is r easonable to assume that microblalite-mode at one locality was synchro nous with active shoaling elsewhere on the platform (or with any other sediment transport system on the platform, such as tidal flat accreti on). Given long-term subsidence, variable lateral timing and positioni ng of shoaling (and shoal abandonment) would result in creation and de struction of accommodation space, at the same scale as the shoal bodie s, and a rock record of spatially staggered lenticular bodies. These m eter-scale carbonate cycles, therefore, are not a metronome of high-fr equency eustatic changes. In this interpretation, stratal cyclicity re flects autogenic, self-limiting, organization in the depositional dyna mics; an organization that is inherent to the creation and dispersion of carbonate sediments. Cyclicity is not an allogenically-imposed, pan -platform, temporal rhythm. Cycles mean only that the sediment was mov ed by water and lithified early. High-amplitude, Pleistocene-like fluc tuations in sealevel may actually interfere with this organization and blur the autocyclic signal in the rock record, suggesting that regula r, meter-scale patterns in platform carbonates may be more common in G reenhouse rather than Icehouse intervals.