Middle Miocene–Pliocene siliciclastic influx across a carbonate shelf and influence of deltaic sedimentation on shelf construction, Northern Carnarvon Basin, Northwest Shelf of Australia Carla M. Sanchez, * , † Craig S. Fulthorpe‡ and Ronald J. Steel * *Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA †ConocoPhillips, Subsurface Technology, Houston, TX, USA ‡Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA ABSTRACT Middle Miocene to Pliocene siliciclastics of the Bare Formation represent a long-lived (ca. 11 Myr) break in the otherwise carbonate-dominated shelf of the Northern Carnarvon Basin, Northwest Shelf of Australia. The quartz-sandstone interval is correlated with the appearance of spectacular clinoform sets mapped on 3D and dense 2D seismic data. Twenty-seven clinoform sets are inter- preted as delta lobes primarily based on their plan-view morphology (strike-elongate to lobate fea- tures) and their 40–100-m-high clinoform amplitudes. The delta lobes were deposited on outer-shelf to shelf-edge positions, and the older deltas show evidence of a higher degree wave reworking than the younger deltas. Measurements of the along-strike (migration) and down-dip (progradation) movement of these deltas are compared with relative sea-level behaviour inferred from shelf-edge trajectory analysis. Delta lobes exhibit greater lateral shifting during relative sea-level rise, whereas delta lobes are more restricted to dip-oriented fairways during sea-level fall, although no major incised valleys have been identified. Long-term (cumulative) progradation of this delta system and subsequent backstepping correlates with long-term sea-level fall and rise during the late middle and late Miocene. In addition, a long-term northeastward migration trend for these delta lobes was likely a result of localized uplift of an inversion anticline in the Rosemary–Legendre Trend; the growth of this anticline probably steered the fluvial source for the delta system towards the northeast. The Bare Formation siliciclastic influx correlates with other middle Miocene increases in siliciclastic sediment supply worldwide. Global cooling and a shift to more arid conditions, negatively influencing vegeta- tion cover, may have combined with more seasonally variable rainfall to generate the high sediment supply that built the deltas. Retreat of the siliciclastics could correlate with ice-sheet growth in the Northern Hemisphere and/or increase in the Indonesian Throughflow and Leeuwin Current (ca. 1.6 Ma), which might have modified climate regionally. INTRODUCTION Understanding the occurrence and progradation history of late-middle Miocene siliciclastics, which occur atop the pre-existing Miocene carbonate shelf of the North- ern Carnarvon Basin (NCB), Northwest Shelf of Aus- tralia (NWS), is necessary to decipher the Neogene stratigraphic architecture of this margin (Figs 1 and 2). These Neogene siliciclastic sediments have been identi- fied previously as the Bare Formation in at least 13 industry wells and on reflection seismic data (Cathro et al., 2003; Wallace et al., 2003). Based on well data, observations of progradational style and recognition of a partially mounded geometry along a strike-oriented regional 2D seismic profile, Cathro et al. (2003) have interpreted this quartz-sandstone rich succession as a shelf-restricted, clastic wedge that was reworked by a shelf-parallel, northeastward-flowing ocean current that also caused its interpreted along-strike progradation (Cathro, 2002). The documented late-middle Miocene influx of siliciclastic sediments to the NCB raises three ques- tions of broad importance that will be discussed in this study: Correspondence: Carla M. Sanchez, ConocoPhillips, 600 N Dairy Ashford, Houston, Texas, 77079, USA. E-mail: Carla.M. Sanchez@conocophillips.com © 2012 The Authors Basin Research © 2012 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 1 Basin Research (2012) 24, 1–19, doi: 10.1111/j.1365-2117.2012.00546.x EAGE