--> Abstract: Not-always-elevated, Not-so-passive Continental Margins, by Paul Green, Ian Duddy, Peter Japsen, Johan M. Bonow, and James A. Chalmers; #90130 (2011)

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Not-always-elevated, Not-so-passive Continental Margins

Paul Green1, Ian Duddy1, Peter Japsen2, Johan M. Bonow2, and James A. Chalmers2
1Geotrack International, Brunswick West, VIC, Australia.
2GEUS, Copenhagen, Denmark.

Low temperature thermochronology techniques (AFTA, apatite (U-Th)/He dating) are commonly employed to study uplift of elevated passive continental margins (EPCMs). Such studies traditionally focus on basement terrains lacking sedimentary cover, with results commonly presented in terms of continuous cooling/exhumation, assuming that margins represent long-lived highs having undergone continual emergence. However, studies of EPCMs where sedimentary cover is preserved reveal more complex histories, involving multiple episodes of burial and subsequent exhumation. The presence of sedimentary cover allows use of complementary techniques such as vitrinite reflectance and sonic velocity to constrain the burial and exhumation history. Consistent results from multiple, independently calibrated techniques provide confident interpretations.

Our studies in West and East Greenland and NE Brazil have shown that these margins underwent subsidence and burial following rifting/separation, and episodic uplift began much later. We believe that similar histories are common to all passive margins, with absence of sedimentary cover reflecting erosional removal, rather than non-deposition. Such histories cannot be explained by current tectonic theories, in which margins (dominated by basement) conventionally represent areas of progressive emergence and erosion, while offshore sedimentary basins undergo progressive subsidence and burial.

Integration of independent constraints from geological information and the landscape is essential in order to define episodes of re-burial (heating), since thermochronology can reveal only cooling events. Rather than using thermochronology to define landscape development assuming permanent uplift, as is commonly attempted, definition of key landscape elements (regional erosion surfaces and generations of valley incision) provides a framework within which more reliable constraints can be extracted from thermochronology data.

The histories of episodic burial and exhumation revealed by such investigations have major implications for hydrocarbon exploration at passive margins. Episodic exhumation of margins leads to a very different distribution of reservoir horizons offshore. Erosional episodes identified onshore may also affect the offshore region, and tracing the ultimate sediment sink is not straightforward. Planation surfaces onshore often correlate with unconformities offshore, which may be very low angle, due to the regional nature of uplift and erosion, and their significance is often missed. This can lead to errors in assessing former burial depths in the absence of direct constraints (AFTA, VR etc), with consequences to estimation of maturity levels at depth. Only by integrating thermochronological data with geological data and evidence from the landscape can the detailed evolution of EPCMs be determined.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.