The Enigma of Missing Jurassic and Cretaceous Rocks - Episodic Deposition and Unroofing of the UK and Adjacent Continental Shelves During the Mesozoic and Tertiary

Abstract

Jurassic and Cretaceous palaeogeographic reconstructions of the UK are constrained by extensive well and field exposures that provide both lithofacies and biostratigraphic control. However, this information is limited to ‘remnant’ sections preserved after regional erosional events presenting a biased perspective of originally more extensive pre-erosion depositional systems. Stratigraphic clues remain in the preserved section which can be used to develop a more comprehensive understanding of sequence architecture including: isolated outliers of sequences more fully developed in adjacent grabens; facies characteristics of once more laterally extensive depositional systems now limited to scattered erosional remnants; and sediment recycling of pre-tectonic sediments into intra-graben lows. While missing section cannot be studied directly, its effect can be detected by palaeo-thermal indicators such as apatite fission track analyses, sonic velocities and vitrinite reflectance, allowing a much more robust reconstruction than is possible from the preserved section alone. There are several key unroofing events in the Mid-Jurassic, Late Jurassic and Early Cretaceous alternating with periods of subsidence and deposition, culminating with severe uplift and exhumation in the Late Palaeocene, followed by an Oligo-Miocene event. Early Jurassic rocks probably once covered much of the UK region, with shale and carbonates to the south and more clastic dominated facies in the north, prior to Aalenian uplift and unroofing of the mid-North Sea Dome. As this subsided during M-eL Jurassic, it was gradually onlapped by fringing deltaic-paralic and shallow marine depositional systems. With increasing rift-generated topography in later Jurassic time much of this earlier sedimentary cover was stripped off inter-rift platforms and recycled into adjacent grabens. The main axis of rifting jumped westwards to the North Atlantic in the Early Cretaceous when the marginal areas of western Britain, Ireland and adjacent offshore were partially exhumed. As rifting waned, the region subsided and transgressed by shallow marine sands passing up into laterally extensive chalks with marls and shales in the north during the Late Cretaceous. Intra-Tertiary unroofing removed much of the Cretaceous and earlier remnant Jurassic from the western lands and caused a pronounced easterly tilt and progressive pattern of older subcrop to the west leaving the rather ambiguous stratigraphic record of today.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017