Precambrian sequence stratigraphy

Abstract

Sequence stratigraphy studies the change in depositional trends in response to the interplay of accommodation and sediment supply, from the scale of individual depositional systems to entire sedimentary basin-fills. As accommodation is controlled by allogenic mechanisms that operate at basinal to global scales, the change in depositional trends is commonly synchronized among all environments established within a basin, thus providing the basis for the definition of systems tracts and the development of models of facies predictability. All classical sequence stratigraphic models assume the presence of an interior seaway within the basin under analysis and are centered around the direction and types of shoreline shifts, which control the timing of all systems tracts and sequence stratigraphic surfaces. In overfilled basins, dominated by nonmarine sedimentation, the definition of systems tracts is based on changes in fluvial accommodation, as inferred from the shifting balance between the various fluvial architectural elements. The method of sequence stratigraphy requires the application of the same set of core principles irrespective of the age of strata under analysis, from Precambrian to Phanerozoic. The study of Precambrian basins is often hampered by poorer stratal preservation and by a general lack of time control. However, where sedimentary facies are well preserved, the lack of time control may be partially compensated by a good knowledge of facies architecture and relationships, as well as by paleocurrent data. The latter are particularly important to understand the stratigraphic record of tectonically active basins, where abrupt shifts in paleoflow directions allow one to infer tectonic events and map the corresponding sequence-bounding unconformities. Arguably the most important contribution of Precambrian research to sequence stratigraphy is the better understanding of the mechanisms controlling stratigraphic cyclicity in the rock record, and hence of the criteria that should be employed in a system of sequence stratigraphic hierarchy. There is increasing evidence that the tectonic regimes which controlled the formation and evolution of sedimentary basins in the more distant geological past were much more erratic in terms of origin and rates than formerly inferred solely from the study of the Phanerozoic record. In this context, time is largely irrelevant as a parameter in the classification of stratigraphic sequences, and it is rather the stratigraphic record of changes in the tectonic setting that provides the key criteria for the basic subdivision of the rock record into basin-fill successions separated by first-order sequence boundaries. These first-order basin-fill successions are in turn subdivided into second- and lower-order sequences that result from shifts in the balance between accommodation and sedimentation at various scales of observation, irrespective of the time span between two same-order consecutive events. Sequences identified in any particular basin are not expected to correlate to other first- and lower-order sequences of other basins that may be similar in age but may have different timing and duration.