An early mid-Miocene, strike-parallel shelfal trough and possible karsti®cation in the Northern Carnarvon Basin, northwest Australia Donna L. Cathro a,b,c, * , James A. Austin, Jr. b a Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712-1101, USA b Institute for Geophysics, The University of Texas at Austin, Building 600, 4412 Spicewood Springs Road, Austin, TX 78759-8500, USA c The Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia Received 22 June 2000; accepted 24 April 2001 Abstract As part of a study to investigate relative sea level history from the Tertiary North West Shelf of Australia, we describe a seismically imaged, stratigraphic architectural element not previously associated with prograding carbonate clinoforms, a strike-oriented trough on an early mid-Miocene sequence boundary. This trough, located on a shallowly dipping carbonate ramp, is recognized on both 2-D and 3-D seismic data by truncation, re¯ector interruption and amplitude variations. It is composed of three individual segments, each up to 500 m wide and 60 m deep, with variable cross-sections. Their combined length, orthogonal to the progradation direction, is ,8 km. The trough occurs where there is a slight basinward increase in dip, andafacieschangeontherampfromsand-sizedcalcarenitetoclay-sizedcalcilutite.Ithasbothacross-cuttingandsub-parallel relationshipwithunderlyingPaleogenefaultsreactivatedwithinaMesozoicbasin-formingtrend.Thetrough,doesnotconform to known drainage patterns, or contour current incisions mapped in similar environments elsewhere. We propose that this seismically mapped trough is karst topography that developed as a result of preferential dissolution focused by heterogeneities within the exposed carbonate ramp. Stable isotope analyses conducted on bulk carbonate samples ,2 km updip, suggest a mixed, marine to slightly meteoric origin for associated porewaters. Furthermore, the sequence boundary on which the trough is developed is contemporaneous with recognized subaerial exposure surfaces to the northeast and southwest. The presence of such a karst feature indicates a minimum fall in relative sea level at this location of 80±160 m during the early mid-Miocene. Crown Copyright q 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Carbonate platform; Carnarvon Basin; NW Australia; Seismic interpretation; Shelfal trough; Karsti®cation 1. Introduction Carbonate-dominated Tertiary sediments of the Northern Carnarvon Basin, North West Shelf, Austra- lia, have received relatively little study compared to the more hydrocarbon-prospective Cretaceous and older section, despite one Paleocene discovery Sit et al., 1994). Published Tertiary sequence strati- graphic studies e.g Apthorpe, 1988; Romine et al., 1997; Westphal and Aigner, 1997; Hull et al., 1998; Mutch and Vail, 1999) have been restricted either to wells alone, or to a combination of wells and regional 2-D seismic pro®les spaced at ,10±50km. In contrast, we are interpreting a 3-D seismic volume within 2-D control calibrated by exploration wells Fig. 1) to understand the detailed relationship Marine Geology 178 2001) 157±169 0025-3227/01/$ - see front matter. Crown Copyright q 2001 Published by Elsevier Science B.V. All rights reserved. PII:S0025-322701)00177-3 www.elsevier.com/locate/margeo * Corresponding author. Address: Institute for Geophysics, The University of Texas at Austin, Building 600, 4412 Spicewood Springs Road, Austin, TX 78759-8500, USA. Tel.: 11-512-232- 3234; fax: 11-512-471-8844. E-mail address: dcathro@mail.utexas.edu D.L. Cathro).