Rhythmic Alternations in Early Pliocene Sediments at Alfredton, New Zealand

Abstract

An interesting rhythmic sequence consisting of massive mudstone and groups of graded beds each about 10 ft thick is exposed near Alfredton, in the southern part of the North Island. During Opoitian time, rotation along a north-east-trending hinge line west of Alfredton caused one side of a fault block to be relatively uplifted and the other depressed, at intervals of several tens of thousands of years, while sedimentation from south-west-flowing turbidity currents was in progress. The sandy fractions of post-faultinff turbidity currents were channelled along the depressed side just to the east of the submarine fault scarp, while on the middle and upper slopes of the tilted block mud was deposited from the turbidity-current clouds. As sedimentation proceeded, graded beds on-lapped eastwards up the slope of the tilted block and across the area where muds had been deposited. Later tilting of the block initiated a new rhythm.