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A review of techniques for the estimation of magnitude and timing of exhumation in offshore basins

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Earth-Science Reviews 72 (2005) 129 168 www.elsevier.com/locate/earscirev

A review of techniques for the estimation of magnitude and timing of exhumation in offshore basinsD.V. Corcoran a,*, A.G. Dore b a

Statoil Exploration (Ireland) Ltd, Statoil House, 6, Georges Dock, IFSC, Dublin, Ireland b Statoil (UK) Ltd., Swan Gardens, 10, Piccadilly, London W1V OLJ, UK Received 1 July 2004; accepted 13 May 2005

Abstract Exhumation, the removal of overburden resulting from the vertical displacement of rocks from maximum burial depth, occurs at both regional and local scales in offshore sedimentary basins and has important implications for the prospectivity of petroliferous basins. In these basins, issues to be addressed by the petroleum geologist include, the timing of thermal dswitchoff T of source rock units, the compactional and diagenetic constraints imposed by the maximum burial depth of reservoirs (prior to uplift), the physical and mechanical characteristics of cap-rocks during and post-exhumation, the structural evolution of traps and the hydrocarbon emplacement history. Central to addressing these issues is the geoscientists ability to identify exhumation events, estimate their magnitude and deduce their timing. A variety of individual techniques is available to assess the exhumation of sedimentary successions, but generic categorisation indicates that dpointT measurements of rock displacement, in the offshore arena, are made with respect to four frames of reference tectonic, thermal, compactional or stratigraphic. These techniques are critically reviewed in the context of some of the exhumed offshore sedimentary basins peripheral to the Irish landmass. This review confirms that large uncertainty is associated with estimates from individual techniques but that the integration of seismic interpretation and regional stratigraphic data provides valuable constraints on estimates from the more indirect tectonic, thermal and compactional methods. D 2005 Elsevier B.V. All rights reserved.Keywords: exhumation; techniques; magnitude; timing; offshore basins

1. Introduction and definition of exhumation Exhumation describes the removal of overburden material by any means from a basin or other terrane

* Corresponding author. Fax: +353 91 592336. E-mail address: dermot.corcoran@statoil.com (D.V. Corcoran). 0012-8252/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2005.05.003

such that previously buried rocks are exposed at the earths surface (Table 1; Dore et al., 2002a). As a generic term it has been used to describe both the return of deep-seated lower-crustal/mantle rocks to the Earths surface (Ring et al., 1999) and the removal of sediment by erosion from basins at shallow crustal levels. Exhumation is a global process that occurs in a range of geological settings from mountain belts

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D.V. Corcoran, A.G. Dore / Earth-Science Reviews 72 (2005) 129168

Table 1 Exhumation/uplift lexicon summary definitions. The preferred terms used in this paper, net exhumation and gross exhumation are shown in italics at bottom of Table Term Uplift Summary definition Non specific term referring to displacements bopposite to the gravity vector Q (England and Molnar, 1990) Displacement of Earths surface averaged over area N103104 km2 (England and Molnar, 1990) Also duplift of rocksT vertical displacement of rock column (England and Molnar, 1990; Dore et al., 2002a) Present elevation of a marker bed above its maximum burial depth (Riis and Jensen, 1992; Dore and Jensen, 1996) Broad regional uplift of continental interiors driven by thermal, isostatic or intra-plate stress fields (Turner and Williams, 2004) Compressional reactivation of formerly extensional fault systems leading ultimately to extrusion of synrift fill (Cooper and Williams, 1989) Local subareial or submarine removal of material by both mechanical and chemical processes (Ring et al., 1999; Riis, 1996) Loss of mass from both surface and subsurface parts of a drainage basin or regional landscape by all types of weathering, physical and chemical (Leeder, 1999) Descriptive term describing removal of overburden material such that previously buried rocks are exposed (Dore et al., 2002a) Difference between present-day burial depth of a reference unit and its maximum burial depth prior to exhumation (this paper) Magnitude of erosion which must have occurred at a particular unconformity prior to post-exhumation re-burial (this paper) Frame of reference Object displaced and/or reference frame not specified Geoid or mean sea level Geoid or mean sea level Spatial-wavelength Not specified

Surface uplift Crustal uplift

Long Short

Net uplifta

Ground level or seabed

Short

Epeirogeny

Geoid

Long

Inversion

Pre-extensional regional elevation

Short and/or long

Erosion

Fixed subsurface coordinates

Short

Denudation

Fixed internal reference axes within bedrock

Long

Exhumation

Ground level

Short and/or long

Net exhumation

Gross exhumation b

Tectonic, thermal, compactional, stratigraphic (relative to seabed, ground-level, or stratigraphic marker) Thermal, compactional, stratigraphic (relative to seabed, ground-level, or stratigraphic marker)

Short

Short

a Other terms used in the literature which are synonymous with Net Uplift are: Apparent uplift (Scherbaum, 1982; Bulat and Stoker, 1987); Apparent exhumation (Hillis, 1995a,b); Negative burial anomaly (Japsen, 1998; Japsen et al., 2002); Net exhumation (this paper). b Term Gross exhumation (this paper) is synonymous with: Total exhumation or exhumation at time of denudation (Hillis, 1995a,b); Lost cover (Cope, 1997); Removed section (Bray et al., 1992; Green et al., 2002).

(such as the major collisional orogens of the Caledonides, Appalachians/Hercynides, Alpine, Andean and Himalayan belts) to offshore basins, and at a variety of scales ranging from the c. 80 km of exhumation that has been estimated for the Western Gneiss Region of the Norwegian Caledonides (where eclogites, once deeply buried within the lithosphere, are now at outcrop) (Milnes et al., 1997), to the more modest levels (generally b 5 km) of exhumation which have been documented in the non-orogenic Late Palaeozoic to Cenozoic basins of the North Atlantic Margin (Dore et

al., 2002a,b). Whereas thermochronology, geomorphology, structural geology and petrology, are the main tools used to estimate the timing and magnitude of exhumation in orogenic settings an alternative toolkit is appropriate for the assessment of exhumation in offshore sedimentary basins. This review draws heavily upon the wealth of studies dedicated to exhumation in the petroliferous basins of NW Europe but is primarily focused on a critique of the techniques that have been used to quantify exhumation in the basins peripheral to the Irish landmass.

D.V. Corcoran, A.G. Dore / Earth-Science Reviews 72 (2005) 129168

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A wide variety of techniques are now available to help quantify the magnitude and timing of exhumation in offshore sedimentary basins (Skagen, 1992; Dore et al., 2002a). Methodologies employed in the offshore arena generally utilize local dpointT measurements of rock displacement with respect to tectonic (e.g subsidence curves), thermal (e.g vitrinite reflectance, fission track analysis), compactional (e.g core porosity, sonic velocity) or stratigraphic (e.g section correlation, seismic section subcrop analysis) frames of reference. Some additional techniques such as geomorphological analysis (Lidmar-Bergstrom and Naslund, 2002) and cosmogenic nuclide interpretation (Stroevevn et al., 2002) have been used to assess the exhumation of onshore sedimentary successions, but have no direct application in offshore basins although they can provide useful constraints on the interpretation of exhumation via onshoreoffshore correlations (Japsen et al., 2002). Mass balance computation of eroded and deposited sediment volumes (Jones et al., 2001, 2002) can illustrate general points about the exhumation/burial history of a basin (and hence about its prospectivity) but is less applicable to bpointQ measurements, for example at a well location or prospect. In all cases, large uncertainty is associated with estimates from individual techniques, as confirmed by the discrepancies between estimates of exhumation derived from different methods at a given location (Skagen, 1992; Nyland et al., 1992; Hillis, 1995a,b; Cope, 1997; Japsen and Bidstrup, 1999; Ware and Turner, 2002). The aim of this review is to critically assess the available techniques for the assessment of exhumation in offshore sedimentary basins. These techniques are illustrated and appraised by reference to basins in NW Europe, in particular the exhumed Irish sedimentary basins. One issue with respect to the exhumation of rock columns is the broad lexicon of terms (uplift, surface uplift, crustal uplift, uplift of rocks, inversion, exhumation, erosion, denudation) that has developed in this field of study (Table 1) (for discussion see England and Molnar, 1990; Dore et al., 2002a). Some of these terms have lacked formal definition and have been used interchangeably, often resulting in confusion, in part because of the different dscaleT of focus of various research groups. As highlighted by England and Molnar (1990) ba displacement is only defined when both the object displaced and the frame of

reference are specifiedQ. This study is primarily concerned with the local (at the scale of hydrocarbon traps offshore Ireland, normally less than 300 km2) or dpointT vertical displacement of rocks, from maximum burial depth, relative to the present-day seabed (offshore basins) or ground level

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