Basin Clastic Sequence Hierarchies

Chronologically evolving cross-section with 3D perspective and detail of clastic response to varying sea level.
Systems Tract Evolution.

The purpose of this page is to prepare for the chronostratigraphic exercise that follows. The page is based on a movie of a 3D rendered Sedpak simulation. The simulation provides the cross sections of the evolving sedimentary geometries.  Added to the simulation output is a interpretive 3D perspective of the depositional surfaces responding to changes in relative sea level. Without the movie, the information available is overwhelming. The movie is intended to give a better understanding of the depositional settings featured in the simulation. The movie shows how and why sequence stratigraphic interpretative analysis uses the subdividing "surfaces" enveloping discrete sediment body geometries of the sedimentary section.

The movie was built by first reversing the order of deposition by back-stripping the geometries from oldest to youngest and then reassembling the sedimentary strata in order of accumulation. The subdividing surfaces provide the framework to the lithofacies geometry enabling the interpretation and rebuilding of the evolving character of depositional setting. The simulation and so movie track how a hypothetical siliciclastic clastic margin setting would respond to varying rates of shoreline progradation, sea level, and base level change. So the output is expressed as a series of block diagrams with a three dimensional perspective and involves sand and shale deposition.

History of Relative Sea Level
The simulation traces rise in
relative sea level, a lowstand of relative sea level, a transgression of relative sea level, a highstand, a relative fall in sea level, a further rise of relative sea level, and finally highstand of relative sea level, and a fall of relative sea level. The sailing boat provides the viewer with a sense of where the elevation of the sea's surface as a function of time.

Tectonic History
It was assumed that the accommodation available in the basin for the sedimentary fill was the combined product of a history of subsidence and eustasy. Thus, for this reason, all the changes in sea level shown on the diagram are relative changes.

Eustatic 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. 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 "ravinement " 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.
Sunday, July 24, 2016
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