Foreland Arc Basins
Forearc and backarc basins occur within convergent continental margins, so-called “activemargins”, a term which emphasizes the importance of tectonism in controlling stratigraphic architectures. Several recent studies of arc-related basins have examined the basin fills from the perspective of sequence stratigraphy, and in many cases major differences with the stratigraphic styles of extensional and rifted margins, and even with foreland basins, have become apparent. There is little clear evidence for the existence of cycles caused by 106-year eustatic sea-level cycles. This contrasts with the record of 104–5-year cyclicity, including that of glacioeustatic origin which is locally prominent in arcrelated basins and has been mapped and documented in detail, for example, within the Japanese islands, and North Island New Zealand. Several studies of arc-related basins have been carried out in Nicaragua and Costa Rica. In general, they demonstrate the importance of convergent tectonism as a dominant control in the development of stratigraphic architecture. An example of a sequence stratigraphic framework of Miocene age in a forearc basin in Nicaragua was provided by Kolb and Schmidt. Fan deltas developed during episodes of sea-level fall, as pyroclastic and alluvial deposits spread across an exposed shelf. Coastlines retrogressed far inland during periods of sea-level rise. Correlations of these and other sections in Central America with the Exxon global cycle chart seem forced and unconvincing. Biostratigraphic evidence for the correlations is extremely limited. In most cases, it is evident that folding, faulting, tilting and tectonic uplift and subsidence are the major sedimentary controls. In some cases, volcanic control of the sediment supply is the critical factor. These tectonic and sediment-supply considerations are of paramount local importance. A Jurassic-Cretaceous forearc basin succession in the Antarctic Peninsula is dominated by major facies changes at intervals of several millions of years, suggesting a control by “third-order” tectonism. One major, basin-wide stratigraphic event, an abrupt shallowing, followed by a transgression, gave rise to a unit named the Jupiter Glacier Member, consisting of deepening-upward shelf deposits. Correlation with a major eustatic low near the Berriasian-Valanginian boundary indicated. However, this seems somewhat fortuitous. It is the only such correlation that can be proposed for this basin. The possibility of tectonic control of this particular event is unlikely that many eustatically-controlled sea-level fluctuations will be recognized from forearc basins, unless it is possible to demonstrate that there were prolonged periods of tectonic quiescence. A stratigraphic synthesis of the Cenozoic deposits of also resulted in tentative (and rather tenuous) correlations of some stratigraphic events. The evidence indicates that subduction and strike-slip deformation had a dominant effect on sedimentation patterns in this area. During the late Miocene (Messinian) plate-tectonic events led to the isolation and desiccation of the entire Mediterranean basin, an event that is clearly recorded in Cyprus, at a time when global sea levels underwent several fluctuations.
Backarc Basins
The tectonic evolution of backarc basins may be similar to that of extensional margins, especially along the interior, cratonic flanks of the basin. Legarreta and the Neuquén Basin, a backarc basin flanking the Andes in Argentina, underwent an “exponential thermo-mechanical subsidence” pattern, following an early Mesozoic thermal event. Sediment supply conditions along the cratonic flank of a backarc basin a real so likely to be comparable to those of extensional margins. For this reason these basins may show stratigraphic patterns comparable to those on Atlantictype margins, including the presence of major carbonate suites, relatively mature clastics, and a sequence architecture containing evidence of cyclicity with 106–107-year episodicities. Two studies of Andean basins confirm that this is the case describes a sequence stratigraphy that correlate while Hallam developed his own regional sea-level curve that contains transgressive-regressive cycles with 106–107-year frequencies.
