Slopes and LST

Lowstand Systems Track (LST)
When lowstand systems tracts are compared to highstand systems tracts and transgressive sequences, some of the geometries are downslope. Sea level reached maximum rate of drop, while the rate of subsidence still less than the rate of sedimentation, which initially results in an increased sediment influx seaward since the sediments are being deposited further away from shore along margins on the platform, farther down slope, decreasing the overall sediment availability. The lowstand systems tracts result in a shift of the types of sediments being deposited onto the slopes to an increase in siliclastic input of sediments. Sea level drops also shifts the type of transport further out onto the slope also decreasing the surface area onto which the sediment may be deposited "a smaller area of the shallow sea floor is in contact with the carbonate factory" resulting in retrograding shelves. Slope failure occurs frequently at times of lowstand and as a result debris-flow, grain-flow deposits primarily thickened wedges and fans develop. Additionally, the winnowing and erosion of platform corals, sediments and shallow water sediments exposed at lowstand and are deposit seaward along the slope. Toward the end of the lowstand system initial phase shifts in subsidence and seal transition to a transgressive systems tract (Handford and Loucks1993 AAPG memoir 33)

Click on image below to see a quick time movie of the response of carbonate accumulation to sea level explained above.

Normal carbonate cycle at lowstand of sea level.

Ancient Slopes

Friday, April 05, 2013
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