Primary scalar bedding features of the Siluro‐Devonian sediments of the Seymour district, Victoria

Abstract

Quantitative studies of the variation of the arenite‐lutite ratio, and of bedding thickness for outcrops across the Siluro‐Devonian sediments in the Seymour East Syncline, indicate these significant features: (i) Bunches of massive, thick‐ and thin‐bedded arenites (referred to as members), many containing cobble conglomerate strata, suddenly come in over massively bedded, graded and ungraded lutite. (ii) The arenite members show graded bedding features which are mostly rhythmic, being especially aperiodically interrupted at their bases, and becoming more regular and pulsating in their upper cycles. (iii) Parallel cumulative frequency curves for the bedding of coarse and fine lithologies in these cycles suggest a common origin, viz. their introduction by turbidity currents induced by slumping of coarse sediment into probably deep, standing water. (iv) The siltstones into which the arenite members are emplaced are themselves often graded. Elsewhere, massive, ungraded siltstone resembles the matrices of the cobble conglomerates in the base units of the coarse members. (v) The greywacke laminations within the siltstones are considered statistically, and it is suggested that their occurrence is accidental. The sandy intercalations are the expressions of the distal portions of stronger turbidites which have penetrated beyond the normal range into the typical pelagic or lutaceous environment.