Physical modelling of climatic control on multistage fan-delta progradation
Final Report Abstract
The conventional sequence stratigraphy predicts that delivery of sand to deep basins occurs during falling and lowstand conditions of relative sea level. Only under exceptional climatic (or tectonic) conditions shelf-deltas prograde during sea-level highstand. However, sediment delivery and sea-level variations can both be regulated by orbitally (Milankovitch) induced climate changes. The exact location on earth, size of the sediment source area (catchment) and the overall setting of a basin, determine the nature of feedback processes between glaciations and sediment supply. Two end members exist, which describe how sediment discharge can either be maximum (out-of-phase, OP) or minimum (in-phase, IP) during sealevel lowstands (e.g. full glacial climates). To test the implications of this mechanism for the development of shelf-edge deltas we performed physical experiments in the Eurotank flume facility. As climate proxy we used a Milankovitch-type curve that mimics 1) glacioeustatic base-level changes and 2) supply variations. The scale-independency of this approach makes the results of this experiment relevant to most 4th - 5th order depositional cycles. Throughout both the numerical and physical experiments, a gradual linear subsidence was applied in order to warrant sufficient accommodation. During flume experiments, the delta clinoform geometry was measured using high resolution photogrammetry and internal delta architecture was studied after the experiment using lacquer peel sections. The behavior (internal build-up) of the delta clinoforms in response to accommodation changes can be effectively illustrated by the shoreline trajectory. The angle of this trajectory line is measured relative to horizontal and as such provides a measure for the ratio between vertical (aggradation) and horizontal (progradation) motion of the delta. Different shoreline trajectory classes were assigned, which we call stratotypes and which objectively describe the depositional behavior. For both the IP and OP experiments, the clinoform behavior seem directly relatable to "eustatic" base level changes. Main progradation systems develop during eustatic falls and/or base-level lowstands in general. Flooding of the delta topset beds occurs during main base-level rise. The difference between the experiments resides in the number of flooding events, which is higher in the OP experiment where eustatic effects are augmented due to low supply. In the IP experiment, however, both transgress and highstands deposits are thicker developed. In both experiments erosional unconformities occur in response to eustatic fall, however, the exact ages differ between the two settings. These results have important implications for sequence stratigraphy, while it can explain many of the odds in the even worid. The strength of the IP/OP principles does not reside in the ability to tell supply-eustasy phase relations from the rock record, but gives the two endmember possibilities for hierarchical stacking of depositional sequences. Future research and flume experiments should of course focus on other scenarios than the here presented end-members, which serve to critically assess shelf-edge deltas in outcrop or seismic profile. Moreover, our models show the two extremes of what mankind may expect if the feared global (glacioeustatic) sea-level rise does occur. Forecasts are exclusively based on sealevel effects caused by delectation, but neglect concurrent climatic effects on sediment delivery. The damping effect of marine flooding and the ability of sediment accumulation to keep pace with sea-level rise as expected for the IP situation, would moderate the doom scenario, whereas the OP situation would instigate the present fear. Unquestionably, assessing the complex phase relationship between glacioeustatics and sediment supply would refine the basis for our present faith in the causes of global warming.
Publications
- G. Orbital forcing and dual control on accommodation in delta systems - a sequence-stratigraphic approach. 25th IAS Meeting of Sedimentology, 4-7 September 2007, Patras, Greece
Ten Veen, J.H, and Postma, G.
- (2008). Flume experiments on delta-architecture in response to in- and out phase changes in supply and sea-level. 26th IAS Meeting of Sedimentology, 1-3 September 2008, Bochum, Germany
Mikes, D., Ten Veen, J.H., Postma, P., Hahn, S., Van Dijk, M. and Gademan, M.
- (2008). Shelf-edge delta architecture resulting from in- and out phase changes in supply and sea-level in ice-house periods. 26th IAS Meeting of Sedimentology, 1-3 September 2008, Bochum, Germany
Ten Veen, J.H, Mikes, D., Postma, G. and R.J. Steel
- (2008). Shelf-edge delta architecture resulting from in- and out phase changes in supply and sea-level in ice-house periods. Clinoform Sedimentary Deposits: The processes producing them and the stratigraphy defining them, SEPM Fieldconference, 15-18 August 2008, Rock Springs, Wyoming
Ten Veen, J.H, Mikes, D., Postma, G. and R.J. Steel
