Depositional Character, Distribution and Significance of Hybrid Event Beds: Springar Formation, NW Vøring Basin, Norwegian Sea

Abstract

Hybrid event beds, comprising clay-poor and overlying clay-rich sandstone facies in a single deposit, have been increasingly documented in the marginal settings of otherwise turbidite dominated systems. Their occurrence implies individual flows underwent spatial and/or temporal variation in behavior from non-cohesive (clay-poor) to cohesive (clay-rich), due to clay-enrichment within flows. Hybrid event beds are abundant in Maastrichtian-aged sandstones of the Springar Formation, penetrated by five wells distributed along a downstream transect (140 km) in the NW Vøring Basin, Norwegian Sea. From proximal to distal wells, sandstones decrease in total thickness, degree of amalgamation, sand-to-mud ratio and grain size. Stratigraphic and spatial bed-type distributions, as well as facies variation within hybrid event beds, are used to infer the proximal to distal variation in flows that emplaced hybrid event beds. Hybrid event beds dominate in distal wells, whereas clay-poor beds deposited from non-cohesive flows are most common proximally. Similar bed-type variation can occur stratigraphically. Hybrid event beds were emplaced by flows with longitudinal heterogeneity in their near-bed behavior, changing from non-cohesive at the front to increasingly cohesive at the rear. The dominant type of clay-poor sandstone at the base of hybrid event beds changes distally, from stratified to non-stratified, suggesting that the front of near-bed flow remained non-cohesive and changed from turbulence-suppressed to turbulent, presumably as declining sediment concentration permitted bed traction. Thus, flows emplacing hybrid event beds can undergo spatial and temporal variation in flow behavior, with discrete styles of change occurring in the rear (non-cohesive to cohesive) and front (turbulence-suppressed to turbulent) regions of the flow. Such variation in the concentration and turbulent state in the front of these flows has not previously been documented. This research highlights the complex nature of sediment gravity flows emplacing hybrid event beds and aids understanding of the distribution of their deposits, which influences the distribution of reservoir quality in deep-water systems.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016