Geometry and Internal Characteristics of Archaean Submarine Channel Deposits, Pilbara Block, Western Australia

Abstract

ABSTRACT Well-preserved submarine fan deposits of early Archaean age (ca 3.3 Ga) occur in the Pilbara Block of Western Australia. Channel, interchannel, lobe, and basin-plain deposits are recognizable in two facies associations termed, respectively: conglomerate-sandstone-shale and sandstone-shale. Unitary upper-fan channel deposits are recognizable in the conglomerate-sandstone-shale facies association. The channel deposits consist of inverse-to-normal graded, and graded-stratified resedimented conglomerates and sandstones. These facies accumulated respectively in the thalweg and on the adjacent terrace of the upper fan channel and grade laterally into thin-bedded cde turbidites of levee origin. The coarse channel facies are enclosed within interchannel shales, iron-formations, and carbonates. The morphology of midfan channel deposits is strongly influenced by grain-size reflecting different provenances. Where conglomeratic in the west, these deposits are laterally continuous tabular bodies consisting of scour-based, graded-stratified conglomerate-sandstone couplets. Grading predominates but associated horizontal and cross stratification indicates periodic late-stage traction reworking. These midfan sediments are considered to have accumulated in shallow, laterally switching channels on a braided suprafan. Arenaceous midfan channel deposits occur in the southeast and comprise narrow lenses of amalgamated Bouma a or ab sandstones displaying an upward decrease in bed thickness. Sedimentation took place in laterally confined channels within cohesive argillaceous banks. he enclosing interchannel shales contain abundant overbank sandstones often with complete Bouma sequences. Both midfan channel types grade basinwards into upward-coarsening basin plain to lobe deposits. Basin-plain shales and iron-formations are considered to be analogues of Holocene hemipelagic and pelagic sediments.