Mixed Clastics and Carbonates

Movie of Two Sided Fill of Basin by Carbonates and Clastics
Two-sided clastic fill of a sedimentary basin
This module uses a movie (click on image above) of a 3D rendered Sedpak simulation of mixed carbonates and clastics. It shows a cross section with evolving sedimentary geometries in a 3D perspective.  Note how the resulting depositional surfaces respond to changes in base level. The interpretive 3D perspective provides a better understanding of the depositional settings featured in this simulation. It supports the contention that a picture is worth a thousand words. The amount of data available in the movie is overwhelming. The short write up below is intended to highlight some of this.

The simulation incorporates the deposition of three sediment types: sand, shale, and carbonate.

Tectonic History
The depositional basin being modeled has no subsidence history on the left margin but to the right the rate of subsidence increases. This rate is rapid enough about a third of the way through the simulation and 2/3rds of the way across the basin to cause a local sea level rise during the first eustatic high stand (HST ) and the following fall or lowstand systems tract (LST). Somewhere about 2/3rds of the way across the basin and halfway through the simulation the right hand side of the basin is uplifted.

Sea Level History
The simulation traces a fall in sea level, lowstand, transgression, highstand, fall, a further rise in sea level and a final highstand.

This can be tracked in the graph to the right of the simulation and the red triangle tracks the position of the sea. This matches the movement of the sea surface in the 3D perspective diagram. The sailing boat is provided to help show the position of the sea's surface.

The percentage of sand and shale initially coming from the left and finally from the right is shown on the graphical display to the left.

The Sedpak simulation varies the relative percentage of these sediments as a function of time and this relative amount can be seen when matched to the position of the sea level (the red triangle).

In Sedpak, as in nature, the carbonates accumulate more rapidly in shallow water and more slowly as the water depth increases.The carbonate rates tend to be higher where the rates of clastic input are low and slow to zero where the clastic inputs are high.

The Simulation
Initial LST: Note that at the beginning of the movie the sea level is falling and at first sediment is coming from incised valleys to form deepwater fans.
In the second half of the LST the sea level fall slows and the sea level position relative to water bottom enables shallow water conditions to exist. At this time shallow water carbonates are able to form and this initiates the progradation of a carbonate margin through the remaining LST while clastics dominate the lagoon to the lea of this margin.

Initial transgression: The LST terminates with an increase in clastic influx. This influx is inferred to cause retardation of carbonate growth in the lagoon, presumably in response to an increase in nutrients. However the carbonates of the margin, now distant from the clastic source, are able to KEEP UP with sea level rise. The base of the TST is marked by a transgressive surface (TS) . As the sea level rise continues the rate of subsidence offshore increases and with the resultant relative rise in sea level the carbonates of the margin are stressed but lag and then KEEP UP while the lagoonal carbonates GIVE UP .
At the same time "ravinemement " reworking of the clastics occurs along the inner edge of the lagoonal shoreline producing an eroded surface. As the sea level rise of the TST continues the rate of subsidence offshore further increases and so the resultant relative rise in sea level is characterized by the carbonates of the offshore margin forming isolated buildups and while landward closer to the left shore where subsidence is lower the carbonates become re-established along the coastline at the mfs.

Initial HST: With the onset of the HST seaward of a deltaic margin the carbonate< margin is turned on but does not become as big a factory as it was during the earlier LST. Offshore the margin continues to develop CATCH UP pinnacles but these don't reach sea level till the next LST.

Final LST: The onset of the LST is accompanied by a forced regression of downward stepping clinoforms along the left coastal margin. These develop a deltaic/chenier coastal system on their surface while offshore the now shallow water carbonate barrier has caught up with dropping sea level.
Seaward of this margin deeper water carbonates accumulate in the flanking basin and to the right of this basin, the basin floor starts to uplift. Clastic input is increases towards the end of the LST and this causes the leeward lagoon to fill with fluvio/deltaic sediments and the carbonate margin is temporarily turned off, and deepwater clastics bypass this margin into the flanking deepwater basin.

Final TST: The onset of the TST is accompanied by the restarting of deposition of carbonate on the left flank of the deepwater basin. Towards the end of the TST clastics start to increase in amount from both the left and right of the basin.

Final HST: Clastics dominate the basin fill and turn of the carbonate factory.

Monday, August 27, 2018
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