Mutti, Emiliano¹
(1) Parma University, Parma, Italy

ABSTRACT: Stratigraphic Architecture and Facies Characteristics of Ancient Delta Systems of Tectonically Active Basins

The detailed stratigraphic and sedimentological analysis of many exposed deltaic successions from orogenic belt basins clearly shows that upbuilding takes place through the stacking of elementary depositional sequences, i.e. m- or dam-thick sandstone-mudstone couplets, each of which records a phase of basinward delta-front sand progradation followed by a phase of abrupt or gradual abandonment and transgression. These elementary depositional sequences (EDSs) are bounded below and above by unconformity surfaces and their correlative conformities. The characteristics of these unconformities, at each location considered, are a function of (1) the original water depth, (2) the rates of subsidence and sealevel fall, and (3) the process(es) governing sand deposition in the delta under consideration. EDSs stack to form larger-scale composite depositional sequences whose boundaries appear to be mainly of tectonic origin. It is herein thought that EDSs are the actual building blocks of sequence stratigraphy and are related to orbitally-driven cyclicity. 
The vast majority of the deltaic successions studied are dominated by facies and facies associations deposited by hyperpycnal flows generated by catastrophic flooding. In particular, most delta-front successions are predominantly made up of tabular and graded sandstone beds commonly containing hummocky cross stratification. These beds can be physically traced landward into either mouth-bar deposits (river-delta systems) or alluvial fan strata (fan-delta systems). Therefore, the stratigraphic architecture and facies characteristics of these flood-dominated delta systems clearly suggest that climate and its cyclic orbitally-driven variations played a major role in sedimentation.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.