Cross-Laminations in Graded Bed Sequences

Abstract

ABSTRACT Thin units of cross-laminated sandy and silty sediments are common in graded bed sequences. The one characteristic common to all is that the thickness of the cross-laminated bed does not exceed 3 inches and rarely exceeds 1 inch. These sediments have been commonly interpreted as turbidity current deposits because of their association with graded beds. The writer offers the following evidence to demonstrate that such cross-laminated sediments in several instances were transported by turbidity currents to deep sea bottom and deposited originally as graded beds but subsequently were reworked and redeposited by the rippling action of deep marine bottom currents: 1. Deep marine bottom currents exist and are occasionally strong enough to ripple deep sea bottom, as indicated by photographs of deep sea bottom. 2. Cross-laminated deep sea sands are, in places, considerably better sorted than graded bedded deep sea sands which were supposedly deposited by turbidity currents. 3. Cross-laminated sands, in places, contain a rich deep sea benthonic fauna, suggesting a rate of deposition slow enough for the establishment of this fauna. 4. Parallel ripples and interference ripples are present on top of some such cross-laminated sediments. 5. The direction of transport shown by cross-lamination is, in places, quite different from that shown by bottom markings which were supposedly scoured by turbidity currents. The question whether turbidity currents could deposit cross-laminated sediments remains unsolved. The writer points out, however, that an indiscriminate assumption of turbidity current deposition of all deep marine sandy sediments has led to confusion, inconsistencies, and controversies. The postulate of bottom current redeposition helps to resolve such a paradox.