578
12.4.4 Summary (Intracontinental Sag Basins)
- Intracontinental sag basins result from large
ernstal depressions which display little extension.
They subsided slowly but non-linearly over long
time periods (mostly 200-600 Ma). The principal
process controlling long-tenn subsidence is either
thermal contraction, eclogitization of the lowermost ernst, and/or intraplate stress.
- Many sag basins show two or several phases of
tectonic and sedimentary evolution (e.g. an initial
"rift cycle" followed by one or two "sag cycles").
- The depositional environments of sag basins are
commonly continental, lacustrine or shallowmarine. Most sag basins experienced drastic
climate changes during their long his tory, reflected by coal, carbonates, evaporites, and tillites (in
some cases).
Chapter 12 Basin Evolution and Sediments
- The thick basin tills (mostly several up to 10 km)
can be subdivided into megasequences (2nd
order sequences) generated by relative sea- or
base-level changes. The megasequences display
prograding and backstepping facies belts and are
bounded by major unconformities. Retined
stratigraphy (including modem sequence and
cyclostratigraphy) allows intercontinental correlations.
- Many intracontinental sag basins contain large
amounts of recoverable hydrocarbons due to
intercalated widely extended blaek shales,
stratigraphie traps, adequate seals and burial
depths, and limited teetonic defonnation.