ABSTRACT: Implications of a Regional Tectonic Control on the Intra-Aalenian ("Mid-Cimmerian") Sequence Boundary for Exxon's Sea Level Chart

PARTINGTON, MARK, BP Exploration, Glasgow, Scotland, and JOHN R. UNDERHILL, University of Edinburgh, Edinburgh, Scotland

Mapping of stratigraphic relations in the Middle Jurassic of northwestern Europe suggests that part of Exxon's current sea level chart is still some way off being a valid standard with wide-ranging predictive capabilities. Sections from the North Sea and exposures in Greenland, Britain, and Continental Europe confirm the existence of a significant intra-Aalenian unconformity (the "Mid-Cimmerian Event"), which coincides with one of Exxon's more significant first-order megasequence boundaries (separating the Absaroka and Zuni megacycles). This event affects a broad area with a radius of over 1500 km, centered upon the North Sea rift triple junction. Correlation with localities on its margins demonstrates only minor stratigraphic gaps allowing the event to be dated at the boundary betwee the Opalinum and Murchisonae ammonite zones (Scissum and Haugi subzones). Progressive truncation beneath the unconformity and its subsequent onlap suggest that it formed in response to regional, long-lived uplift. Although recent work on igneous products in the triple junction rules out a Hawaiian-type hot plume, temporal and spatial relations suggest that the uplift and subsidence histories are compatible with the initiation of a "warm" plume followed by Callovian volcanism and eventual Kimmeridgian rifting. Since the Scissum/Haugi sequence boundary is based only on two sections lying within the radius of uplift, it seems likely that this event simply reflects a tectonically driven, regional fall in sea level rather than a significant change in global eustacy. As such, it seems impract cal to use sections spaced more closely than potential plume-top dimensions, particularly in regions experiencing contemporaneous rifting.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)