KING, PETER R., GREG H. BROWNE, and TIM R. NAISH
P.O. Box 30-368, Lower Hutt, New Zealand

Abstract: Deep Water Sedimentation And Sequence Boundary Development During The Failing Stage: An Outcrop Example (Late Miocene, New Zealand)

A Late Miocene deep-water (basin floor to slope) siliciclastic succession exposed in north Taranaki, New Zealand, contains several stacked 4th-order basin floor fan depositional cycles, superimposed upon the basal part of a 3rd-order progradational cycle. These 4th-order cycles typically comprise four main facies intervals (upsequence): thick-bedded, massive sandstones; thinbedded sandstones/siltstones; weakly-bedded siltstones; and contorted sandstones/siltstones. Interval boundaries are abruptly gradational, but are sharp between the siltstone and deformed intervals. The main surfaces of discontinuity (mildly erosional to highly scoured) invariably occur at the top of the deformed intervals, and correspond to sequence boundaries.

We infer that the deformed deposits are the product of slope instability and slumping, induced during the falling stage of base level, and that they represent the deep-water correlatives of shelfal forced regressive deposits. Following shelf incision and canyon formation during maximum rate of base-level fall, regressive shoreline sands were captured and reworked downslope, onto a sequence boundary created by the initial sandrich mass flows. Nearby exposures of deformed slope siltstones (with dewatering features) beneath large, sand-infilled channels (feeder conduits) provide up-dip analogues for this model. The thick- and thin-bedded sandstone portions of each fan cycle were deposited just before, and at, lowest base level (lowstand system tract). The weakly bedded siltstone intervals represent late lowstand, transgressive and highstand deposition.

The critical feature of this study is the recognition of deep-water falling stage deposits separated from overlying lowstand fan deposits by a sequence boundary.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas