Distinguishing allogenic versus autogenic parasequences
Conferences
Overview
Overview
abstract
In their original formulation, parasequences were defined as part of a hierarchy between bedsets and parasequence sets (equivalent to systems tracts) that build allogenic sequences. Parasequence sets and sequences were implied to be allogenic in origin, and bedsets and parasequences implicitly autogenic. In this original formulation, parasequence sets could be retrogradational, aggradational or progradational, but offlapping and downstepping (i.e., forced regressive) patterns were absent. This is very problematic, because in 1D (i.e., core, well log or isolate outcrop sections), it can be hard to know if a classic upward-coarsening facies succession is sample of a simple autogenic parasequence, or a component of an offlapping or downstepping parasequence set. These can be resolved if cross-sectional data are available.
In 1D, simple autogenic marine parasequences will show a Walther’s Law shallowing upward facies succession, capped by a deeper water facies, especially if they are never subaerially exposed. Complex parasequences, in which shoaling upward to sea level is followed by bypass (i.e., offlap, donwstepping) will be capped by a distinct bypass surface or facies that is commonly reworked during subsequent transgression. Attributes of this transgressive facies can include a basal Glossifungites surfaces, as well as a lag deposit, commonly rich in marine fossils, such as shark’s teeth or shells. These compound surfaces may record multiple events including both subaerial exposure and subsequent transgressive reworking as opposed to simple parasequences that lack the distinct lag facies.
