DEPOSITION AND DIAGENETIC ALTERATION OF MUDDY SEDIMENTS ACROSS AN EARLY ORDOVICIAN CLINOTHEM: THE POWER STEPS FORMATION, NEWFOUNDLAND, CANADA

The specific processes that build muddy clinothems are rarely explicitly identified from the rock record, due in part to the paucity of exposures where all architectural elements of the clinothem are visible. Here we present a detailed study of a fully-exposed muddy clinothem from the Lower Ordovician (Arenigian ~478-472 Ma) Power Steps Formation, Wabana Group, Newfoundland. The Power Steps Formation offers the unique opportunity to study muddy depositional and diagenetic processes on an Early Paleozoic clinoform. This study aims to understand (1) the mechanisms of muddy sediment dispersal on this ancient clinothem; (2) how the component grain origin changes as the clinothem evolves; and (3) how post-depositional changes, particularly burial diagenetic processes, modify the composition of the original starting grain assemblage. Here six mudstone and three sandstone facies record episodic, storm-dominated sedimentation events on the clinothem topset. The episodic nature of these events restricts bioturbation to bed tops, preserving grading relationships and primary sedimentary structures which are imperative to deciphering specific clinothem building processes. Within the Power Steps Formation muddy sediments are inferred as having two distinct origins. The majority of the clay mineral components (primarily illitic material) appear to be detrital in origin, sourced via mechanical weathering from a mafic, non-vegetated hinterland. A significant fraction of the chloritic clay mineral assemblage however, appears to be authigenic in origin - the product of the diagenetic alteration of Fe-rich mafic grains to chlorite. These chloritized grains were most likely delivered to the delta as bedload in the form of coarse-grained lithoclasts, which originally formed the framework component of the deposit.