Aeolian Environments. And Controls on Sedimentation. John Luchok, Kyle Balling, Cristopher Alvarez

Transcription

1 Aeolian Environments And Controls on Sedimentation John Luchok, Kyle Balling, Cristopher Alvarez

2 The Aeolian Environment Aeolian Processes - geologic activity with regards to wind Desert Environments (Hyper-Arid, Arid, Semi-Arid) Sub-environments (Aeolian dunes, Inter-dunes, Sand sheets, salt flats/playas) Factors: Availability of sediment, size of sediment, wind velocity (direction of) Aeolian dunes develop in desert systems where there is an abundance of sand-grade material available for sediment transport, and where to transport capacity of the wind is sufficient to move that material. (Mountney, 2004)

3 Aeolian Bedforms Left: wind ripples (Death Valley National Park). Right: Mesquite Sand Dunes (Death Valley National Park).

4 Video of Saltation and Transport (Death Valley N.P.)

5 Types of Bedforms Aeolian Dunes: Develop from the interaction of wind and sand-grade material. Migrate downwind due to sediment movement over the stoss side avalanching over the lee sides, decelerating and falling out of suspension. Smaller grains present at bottom, coarser at top. Interdunes: Form the flat areas between dunes, and depending on water content from corresponding environment, can be damp or dry. Top: Badwater Basin salt flat. Bottom: Sand sheet, Death Valley N.P. Sandsheets: Develop in areas where there is insufficient sediment supply to build aeolian dunes, or where transportable sediment is restricted, either because of grain size or a high water table. Desert plant life can occur here. Salt Flats: Occur in flat areas dominated by evaporite precipitation. Most often former lakes that were evaporated and left behind large salt deposits.

6 Types of Environments Hyper-arid setting: Extremely low water table (if any). Small playa lakes and brine pools present. Lots of sand for aeolian dunes, sand sheets. Inactive alluvial fans from scarce flash flooding. Arid Setting: Flash floods restart alluvial fan lobes. Ephemeral fluvial system active which replenish playa lakes. Lower supply of sand due to damp surface. Smaller dunes present. Semi-arid Setting: Ephemeral fluvial system now causes the formation of a delta at the lake entry which causes expansion. Flash floods create active alluvial fans, creating bajadas. Sand sheet now damp; controlled by water table. Remaining even smaller dunes are covered with vegetation and actively migrate but do not accumulate. Figure from Mountney, 2004

7 Accumulation and preservation Accumulation occurs when: positive net sediment budget: upwind sediment influx exceeds downwind outflux upgrowth of accumulation surface Preservation: Bed-form climb in aeolian system is at a low angle (usually < 1 ). Consequently, less than 10% of the original bed form is preserved in the rock record, usually only lee-face features are preserved, and the preserved set is only a record of the processes in the basal region. Bounding Surfaces: an erosional surface within or between sets of cross-strata Partial Erosion (Reactivation) of the lee face due to flow changes Migration of superimposed dunes over the lee face of a larger bed

8 The Cretaceous Etjo Formation 200m thick eolian-dominated succession Huab region, northwestern Namibia Overlying Etendeka flood basalts preserved bedforms with heights of up to 100m and wavelengths of up to 1.3Km. Figure from Mountney, 1999

9 Interpretation Transition of a transverse draa without slipface, but with superimposed bedforms into a transverse draa with slipface and no superimposed bedforms Figure from Mountney, 1999

10 Death Valley N.P. Grainfall (Death Valley N.P.) Settling of saltation load onto slipface Grainflow Avalanching down slipface Coarser sediment collapses on its own weight Inversely graded Loosely packed Finer sediment is carried by wind and settles Sediment accumulates until it becomes too steep and grainflow occurs.

11 Preservation of aeolian erg systems by flood basalts

12 Case Study: Namiba -The Huab basin of Namibia has thick aeolian erg systems interlayered with pahoehoe volcanic rock -Located within the Etendeka group, of the Karoo Supergroup -Late Jurassic to Early Cretaceous age

13 Controls on preservation -In order for erg systems as well as larger bedforms like barchan dunes to be preserved, there needs to be rapid, catastrophic burial that manages to also be non-destructive -These types of events that happen on a scale big enough to bury large dunes seldom occur in nature -Besides flood basalts, another possible event that can result in preservation is a rapidly transgressing sea that results in the flooding of aeolian dunes

14 Preserved features -Bedforms 100m thick -Geometries similar to that of modern day Baracan dunes -Very few modification/ deformation features -Implies that sand was very delicately encased in basalt -It was previously believed that these features could not be preserved in such a pristine manner, however the evidence found in this sequence of rock suggests otherwise

15 Preserved Features

16 -Deposition of quartz latite flows marks the termination of aeolian sand deposits Volcano-sedimentary evolution -Volcanism is spilled into an erg system -Lava fills interdune area, channeling lava flow -Large volumes of sand reworked during a period of less volcanic activity, creating secondary dune systems -Topographic height of dunes reduced, resulting in more interaction and preservation -Wind patterns highly variable in record, could be a result of either changing albedo of sand, or possibly regional tectonic activity reworking the topography and ultimately changing the prevailing wind direction

17 Sources Jerram, D.A., Mountney, N.P., Howell, J.A., Long, D. and Stollhofen, H Death of a Sand Sea: An Active Erg Systematically Buried by the Etendeka Flood Basalts of NW Namibia. Journal of the Geological Society, London, 157, Mountney, N.P The sedimentary signature of deserts and their response to environmental change, Geology Today, 20, Mountney, N.P., Howell, J.A., Flint, S.S. and Jerram, D.A., Relating aeolian bounding surface geometries to the bed forms that generated them: Etjo Sandstone Formation, Cretaceous, Namibia. Geology, 27,