WELL PRESERVED ‘ANTIDUNE’ AND BACKSET BEDS ASSOCIATED WITH SLUMPING ON THE FORESETS OF A GRAVELLY GILBERT-STYLE FAN DELTA, NEW ZEALAND

Quaternary Gilbert-style fan deltas crop out for over 5 km along the Conway Coast, New Zealand. Individual foreset beds are normally graded, inversely graded or massive and change rapidly both laterally and in the down transport direction. The foreset deposits at the Conway Coast exhibit a variety of depositional processes including traction carpets, grain flows, turbidity currents, and slumping.

Fieldwork shows that slumping occurred on foreset beds at a variety of locations along the fan delta regardless of water depth. Behind areas of slumping there are imbrication fabrics in pebbles and cobbles that show steeply dipping clasts of approximately 80° relative to the dip of the foreset bed. This contrasts with expected clast imbrication dips of +/-25° from deposition by traction carpet under subaqueous flow conditions. The origin of the steeply dipping fabric was first thought to represent coarse-grained fluid escape structures. However, further fieldwork shows that the steeply dipping imbrication fabric is not fines depleted, develops behind slump blocks, and progrades upslope. This ‘backstacking’ fabric is inferred to be an example of coarse-grained ‘antidunes’ that form as standing waves due to flow obstruction by the slump blocks. Antidunes can be seen to grade upslope into horizontal clasts. In some locations they are truncated by well-developed backset beds that represent the infilling of depressions on the foreset slope left after flow conditions favorable for antidune formation ceased. Some backset beds develop directly behind slump blocks without the presence of antidunes and are similar to deposits described in the Crati Basin (Collela 1987).

Backset beds and antidunes in coarse-grained fan delta deposits have rarely been discussed in the literature. Although previous studies describe slump deposits and backset beds this appears to be the first report of an organized coarse-grained ‘antidune’ fabric forming behind slump blocks.