Sedimentological and Architectural Characteristics of a Rift Initiation Package; Middle - Upper Jurassic, North Sea

Andsbjerg, Jan

In the Danish Central Graben (DCG) the M. Jurassic pre-rift succession consists of laterally extensive, fluvial and lacustrine deposits. In most of the DCG the syn-rift succession is erosively overlain by a package of estuarine sediments that form the lower part of the rift initiation succession. The erosion surface is the pre-rift/syn-rift boundary. In the study area the rift initiation package is subdivided into three tiers, each the result of a rotational event at the nearby main boundary fault.

The lower tier (tier 1) consists mainly of stacked estuarine sandstone capped by a thick coal bed. The estuary deposits developed as a response to the first rotational movements of the rift initiation phase. With the basin floor close to sea level even a limited rotation caused incision into the uplifted parts of the hanging-wall slope, and infilling by estuarine deposits. The coal bed at the top of tier 1 shows, that a low gradient coastal plain was re-established after the initial rotational event.

The two upper tiers (2 and 3) both consist of a lower wedge of paralic deposits thickening up-dip, and an upper wedge of mainly shoreface deposits thickening down-dip. Renewed rotational movement and associated flooding terminated peat growth in the coastal plain swamps that are represented by the thick coal beds separating tiers 1 and 2. Transgression caused onlap of paralic deposits on the hanging-wall dip slope, forming the westward thickening paralic wedge. When transgression stopped, remaining accommodation in the paralic wedge was rapidly filled and sediments shed further seawards, where they formed the eastward thickening shallow marine wedge. A thin but extensive coal bed on top of the shallow marine wedge shows that a low energy coastal plain was re-established, before renewed rotation caused a repetition of the events that led to the development of tier 2. Tier 3 is essentially developed as tier 2.

Deposition of the rift initiation package was terminated by marine flooding and shift to fully marine conditions and deposition of mainly marine mudstone. This change was associated with acceleration of the rotational movements and the transition from rift initiation to rift climax.

Coeval rift initiation packages in the Viking Graben show similar developments. Outside the rift basin in neighbouring Danish-Norwegian Basin contemporaneous successions exhibit the same facies tracts but not the characteristic architecture of the rift related successions.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013