The purpose of this module is to prepare for the execution of
the chronostratigraphic exercise that follows. It does this
by providing an understanding of how sedimentary geometries
respond to changes in base level. The points made in this module
are related to the attached 3D movie that tracks geologic cross
sections with evolving sedimentary geometries while also providing
an interpretive 3D perspective of the depositional settings
modeled in the chronostratigraphic
exercise. The steps followed in this exercise are also tracked
in a movie of the chronostratigraphic simulation tied to this
exercise. The short write up below highlights some of the details
of this simulation that might be missed by the initial viewer.
This simulation tracks how a hypothetical siliciclastic clastic margin setting would respond to varying rates of shoreline sediment input, and base level change. The output is expressed as a series of block diagrams with a three dimensional perspective and involves sand and shale deposition.
Relative Sea Level History
The simulation traces, in order, a progressive rise in relative sea level,
a lowstand, a transgression, a highstand, a fall, a further rise, a highstand and a fall. The sailing boat provides the viewer with a sense of where the position of the sea's surface lies.
Tectonic History
It was assumed that the accommodation available in the basin for the sedimentary fill was the combined product of the subsidence and eustatic history. Thus, for this reason, all the changes in sea level shown on the diagram are relative changes.
Eustatic History
As indicated above the eustatic history and tectonic history combine to form the accommodation for the basin and so all the changes in sea level are relative changes and tied to this combination.
Sedimentation
The relative percentages of sand and shale supplied from the continental interior to the left are reflected in the thicknesses and lateral extent in the sediment in the cross section. The graphical simulation varies the relative percentage of these sediments as a function of time. Rates are shown to be high during relative sea level highs, low during relative sea level transgressions and higher at the relative sea level lowstands.
The
Simulation
Initial
HST
Note that the beginning of the movie is punctuated by a relative sea level rise. Concurrently sediment from fluvial systems supplies the prograding regressive clastic coastal system while longshore drift transports the sediments along the shoreline. As might be expected this HST coastal system is composed of a complex of laterally migrating deltas and linear parallel beach (chenier)ridges that build out 'en-echelon' to form a series of ridges and swales but no back barrier lagoons
.
SMST
The above HST terminates with a small fall in sea level and a short forced regression.
TST
The onset of the TTS there is reworking of the previous shoreline sediments to form a line of barrier islands backed by lagoons. Associated with the reworking of the previous shore would be the development of transgressive surface. This minor "ravinement" reworking of the clastics involves the outer edge of the earlier shoreline. The sea level rise continues to the mfs.
Next
HST
The onset of this HST is marked by deltaic and chenier ridge dominated coastal margin that progrades rapidly seaward. Note that the relative sea level rise is accompanied by sediment from fluvial systems supplying this prograding regressive clastic coastal system. As with the earlier HST this coastal system is composed of laterally migrating deltas and linear parallel beach (chenier) ridges that build out 'en-echelon', forming ridges and swales but no back barrier lagoons.
FFST
The onset of the FFST is accompanied by the development of an
incised
valley system and the development of lowstand fans. Towards the end of the FFST clastic sediment supplied from the eroding interior is trapped in the incised valleys and the lowstand fan deprived of sediment ends its development.
LST
This late LST is represented by a still stand in sea level and accompanied by estuarine
incised valley system fill supplied from the eroding interior and an onlapping of shelf margin slope. This slope is supplied by sediment that bypassed the incised valley, is transported along the coast by longshore drift, and then downslope as the coastal sediments over-steepen. Clastic input fills the incised valleys towards the end of the LST.
Next
TST
The onset of the TST is accompanied by the formation of a transgressive surface and the reworking of the earlier sedimentary surface and the formation a line of barrier islands backed by lagoons and bay head fluvio/deltaic sediments. Essentially these barrier islands and back barrier lagoons that are reworked by the transgressive sea can be considered as an expression of ravinement. The TST terminates with a maximum
flooding surface (mfs).
Final HST
As with earlier HST's the onset of this HST is marked by deltaic and chenier coastal margin that progrades rapidly seaward. Note that the relative sea level rise is accompanied by sediment from fluvial systems supplying this prograding regressive clastic coastal system. As with the earlier HST this coastal system is composed of laterally migrating deltas and linear parallel beach (chenier) ridges that build out en echelon.
Final SMST
A short fall and rise mark the development of a forced regression and an SMST.
