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Sedimentology and Ichnology of a Lobe-Deflected Delta, Ferron Sandstone (Turonian), Utah, USA

Abstract

Many deltas are strongly affected by sediment plume deflection, but facies models for such deltas have not been developed. We present a sedimentological and ichnological model for the asymmetric “Notom Delta” of the Ferron Sandstone (Turonian), southeastern Utah, USA. Thirty-two detailed measured sections were taken along a 16-km transect through two continuously exposed, ∼10 m thick allomembers containing delta front, mouth-bar, and distributary channel facies. Azimuths from paleocurrent indicators show southeastward deflection of near-shore currents relative to the inferred N-S shoreline, as well as minor reversal of flow. Two end-member ichnosuites are recognized in delta front sandstones: 1) a stressed suite of low abundance, low diversity, diminutive traces reflecting mobile deposit feeding, living, and locomotion behaviors, and 2) a comparatively unstressed, high abundance, moderate diversity suite with a regular, heterogeneous distribution of deep, vertical, or U-shaped suspension feeding burrows which often thoroughly homogenize the sandstones. Delta environments down-drift of river mouths are thought to be unfavorable for organisms, but here we show that the unstressed ichnosuite exists in discrete units down-drift of contemporaneous mouth-bars and distributary channels, as well as at the tops of abandoned lobes. Some of these units terminate against large growth faults. We postulate that the down-drift side of the delta was colonized by suspension feeders during seasonal reversal of the seaway gyre when mud plumes were swept northward. During normal seaway circulation, very high sedimentation rates and mud-laden, wave-dampened waters down-drift of the river mouths heightened the preservation potential of the pervasively bioturbated facies. Updrift of the river mouths, these bioturbated facies were either not preserved or not developed until the lobe was abandoned. This model provides a framework for predicting the distribution of bioturbation-enhanced porosity and permeability in lobe-deflected deltas.