Journal of Conference Abstracts

Controls on Epeiric Carbonate/Siliciclastic Sedimentation ­ Lessons from the Lower Keuper (Upper Ladinian, German Basin)

Michael Poppelreiter (M.C.Poppelreiter@OPENMAIL.LAN7.NAMSNB.SIMIS.COM)¹ & Thomas Aigner (t.aigner@uni-tuebingen.de)²

¹ Tuinstraat 1A, 9711 VA Groningen, Netherlands

² Sigwartstrasse 10, 72076 Tubingen, Germany

Sedimentation in shallow marine, low accommodation epeiric basins shows distinct contrasts to many modern shelves. Intrinsic characteristics of epeiric basins are low subsidence rates, negligible morphological gradient over vast areas, low accommodation potential and reduced rates of carbonate production. The influence of these key controls on sedimentation style was investigated in the Lower Keuper Formation. The mixed siliciclastic carbonate Lower Keuper originated in the semi-enclosed German basin during the Late Ladinian. The succession consists of storm-dominated, tide-dominated and bioturbated facies. It formed in the bathymetric interval between storm wave base and sea level and was interpreted to represent distal shelf to coastal plain environments.The facies were traced along the depositional gradient. This led to the identification of facies associations. These show an unusual lateral energy zonation, similar to the classic Irwin-Shaw model:1. a seaward low-energy zone close to the wave base, 2. an intermediate high energy zone within the wave base and 3. a landward low energy zone where waves were largely dissipated by friction.

The facies associations are stacked into meter-scale transgressive-regressive cycles, the building blocks of the succession. These show systematic changes in symmetry and composition in up dip direction:

1. symmetric, carbonate-marlstone cycles developed on the distal shelf

2. asymmetric, regression-dominated carbonate-claystone cycles formed on the distal to intermediate shelf

3. asymmetric, regression-dominated carbonate-sandstone cycles were encountered on the intermediate shelf

4. symmetric sandstone-carbonate-sandstone cycles were present on the intermediate to proximal shelf

5. asymmetric, transgression-dominated marlstone-siltstone cycles are typically developed on the proximal shelf to coastal plain.

The different cycles were correlated over a distance of more than 500 km. Within the resulting time-stratigraphic framework, the facies distribution was mapped out in 3 dimensions. This revealed that the facies are sheet-like and extend for several 10's of km's in an aggradational facies architecture. High energy (reservoir prone) siliciclastic and carbonate facies were concentrated in zones of stronger subsidence, few kilometres wide. These tectonically controlled zones caused a 'sediment volume funnelling' into the centre of the basin.

A time related 4-D facies analysis showed a stratigraphically controlled facies differentiation:1. clastic tide-dominated facies formed exclusively during initialtransgression2. clastic bioturbated and thin storm-dominated facies formed duringtransgression3. mud-dominated carbonates formed during peak transgression4. skeletal and oolitic sheets formed exclusively during initial regression5. storm-dominated clastics formed during regression.

In conclusion the specific properties of epeiric basins resulted in an unique epeiric style of shallow marine sedimentation with

1. specific epeiric energy zonation,

2. aggradational facies architecture and

3. sediment volume funneling.

These conditions caused high energy reservoir facies to form in ashoreline detached zone. Reservoir facies extended sheet - like over several 10's of km's and concentrated in zones of stronger differential subsidence.

211