COMBINED FLOW ORIGIN OF EDGEWISE INTRACLAST CONGLOMERATES - SELLICK HILL FORMATION (LOWER CAMBRIAN), SOUTH-AUSTRALIA

Unusually thick, coarse grained edgewise intraclast conglomerates occu r at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the Lower Cambrian Sellick Hill Formatio n, South Australia. The intraclast beds are flat based and laterally d iscontinuous, forming barlike structures that must have exhibited bath ymetric relief of as much as 1 m. The internal fabrics of these beds a re variable. Thinner beds are dominated by flat-lying intraclasts, thi cker beds contain both chaotic, randomly oriented, steeply inclined in traclasts and clusters of fan-shaped, vertically stacked edgewise intr aclasts. The Sellick Hill Formation intraclast conglomerates are infer red to have been formed by intense, storm-generated combined flows on a broad, subtidal carbonate ramp. Superimposition of wave-induced osci llatory motions on geostrophic bottom flows during large storms genera tes short-lived, but exceptionally high instantaneous shear stresses i n the bottom boundary layer. Entrainment of the relatively large intra clasts occurs through sliding, rather than pivoting. Edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclas ts during passage of waves and the chaotic nature of collisions betwee n intraclasts moving within the boundary layer. Collisions between int raclasts impart a rotating moment, causing intraclasts to tip up durin g maximum fluid shear stress. Lodgement or packing of clasts in vertic al or steeply inclined positions occurs within scours, where intraclas ts can wedge between other vertically inclined clasts, or where intrac lasts are pinned in steep orientations by collisions with shallowly in clined intraclasts. Differential erosional resistance of the intraclas t deposits probably led to the development of sharp lateral changes in thickness. The Sellick Hill Formation intraclast conglomerates record erosion and reworking of subtidal, subfairweather wave base environme nts by exceptionally intense and presumably rare storm flows. The intr aclast horizons represent a substantial loss in stratigraphic resoluti on due to widespread erosion of the ramp.