Search

Many basins are filled under the influence of tectonics and variable climatically driven processes. Unravelling the relative contributions of these is generally easier in fossiliferous sequences if faunal diversity patterns are considered in conjunction with lithofacies analysis. This approach has been successfully applied to Late Miocene strata of Sicily, where progressive restriction and desiccation within the Sicilian Caltanissetta region occurred under a semi-arid climate during the Messinian Stage. This restriction was partly due to Antarctic cooling which produced Palaeo-Mediterranean sea-level oscillations during the Early Messinian, as a prelude to closure of the Atlantic connections. In addition, regions within this Afro-European plate collision zone were also subjected to syndepositional tectonic deformation associated with progressive perched basin development above foreland (south) verging thrust sheets. The relative impacts of climatic (eustatic) versus tectonic control on sedimentation patterns within these perched basins has been debated at length. However, progress towards resolving this problem by a combined approach involving lithofacies analysis, and faunal phase analysis provides clues for separating the effects of the controlling factors. Faunal phase analysis identifies general faunal associations with minimal emphasis on species diversity. The resulting 'phase' plots for Early Messinian strata appear to be transgression skewed when controlled by eustatic cyclicity, and regression skewed when driven by regional tectonics associated with perched basin tectonics. Keywords: Messinian, Sicily, faunal phase analysis, tectonics, eustasy.

Basinward migration of half-graben footwall margins occurs throughout the active Aegean extensional province and has significant implications for facies distributions and preservation potential in synrift fills worldwide. Recognition of this phenomenon is particularly important for predicting the preservation and distribution of hydrocarbon reservoirs in rift basins. Using an example from the Gediz graben of western Turkey, it is shown that basinward migration of footwall margins exhumes proximal facies of lateral sedimentary systems, severely limiting their preservation potential and providing a ready source of easily erodible material. This in turn leads to the generation of extensive footwall-derived depositional systems, which frequently displace axial drainage systems toward the hanging-wall dip slope.

The California Continental Borderland began to form during late Miocene time. Only the latest Miocene and younger sediments are ponded in the contemporary basins. Upper Miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying ponded clastic sequences that fill the contemporary basins. A seismic facies analysis has defined facies patterns in the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins (Patton, Tanner, Santa Cruz, and San Nicolas), fine-grained mass-flow and hemipelagic sediments derived from the local insular sources dominate the basin-fill sequences. Santa Catalina Basin is a transitional basin filled with fine-grained turbidites and hemipelagic sediments derived from both continental and local insular sources. The nearshore basins (Santa Barbara, Santa Monica, and San Pedro Basins and San Diego Trough) are filled by thick sequences of continental sediments that form prominent deep-sea fans. The borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by detrital-sediment-gravity-flow deposition in the Pliocene and Quaternary. Accumulation rates estimated from Pliocene-Quaternary sediment thickness for each basin show that in the offshore basins, as well as in San Diego Trough, rates have remained fairly constant since the Pliocene. In contrast, the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent-derived sediments. Outer- and intermediate-basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence. Intermediate basins show an increase in mass-wasting deposits (slope facies) with time, which is the result of increasing fine-suspension slope deposition and eventual failure. Figure 1. Geologic framework and basin locations of the California Continental Borderland. Dotted areas are topographic highs; 1: Northern Channel Island Ridge; 2: Patton Ridge; 3: Santa Rosa-San Nicolas Ridge; 4: Nidever-Tanner Ridge; 5: Cortes Ridge; 6: Santa Cruz-Santa Barbara Ridge; 7: San Clemente Ridge; 8: Catalina Ridge; 9: Palos Verdes Ridge. Dashed areas are basins; C: Catalina; LA: Los Angeles; P: Patton; R: Ridge; S: Soledad; SB: Santa Barbara; SC: Santa Cruz; SD: San Diego; SM: Santa Monica; SN: San Nicolas; SP: San Pedro; T: Tanner; V: Ventura. (Geographic terms modified from Vedder and others, 1974; Vedder and Toth, 1976). Sections A-B, C-D, N-S, W-E, and X-Y are illustrated in following figures.

Few detailed-facies analyses of the rift-to-passive margin transition have been undertaken in exhumed orogenic belts. The Unicoi and lower Hampton Formations in the central and southern Appalachians record the transition from late Proterozoic to Early Cambrian rifting to initial opening of the Iapetus Ocean. The lower and middle Unicoi Formation consists of feldspathic sandstones, conglomerates, and basalts. Sandstones and conglomerates were deposited from hyperconcentrated flows and tractional currents in distal alluvial-fan and proximal and medial braid-plain environments. Extrusion of basalts was associated with influx of coarse-grained siliciclastics. The presence of thick alluvial-fan sediments, basalts, and abundant lithic clasts and the feldspathic nature of the sandstones are suggestive of a rift setting. Paleontological and geochronological data indicate that rifting continued into Early Cambrian time. The upper Unicoi Formation is composed predominantly of transgressive, quartzose sandstones which represent the incipient phase of passive margin sedimentation related to a second-order, sea-level rise. Differences in degree of crustal attentuation probably controlled the distribution of sedimentary environments during transgression. On most attenuated crust to the east, initial transgressive facies consist of tidal sand-wave and sand-ridge deposits intercalated with proximal and medial braid-plain deposits. As transgression progressed cratonward, tidal sedimentation was supplanted by tide- and wave-influenced sedimentation characterized by sand-wave complexes, tidal inlets, and longshore-directed bedforms. Drowning at the top of the Unicoi Formation is indicated by outer-shelf black mudstones containing Tabc, Tbc, and Tc beds and gravity-flow deposits at the base of the Hampton Formation. Deepening may have been enhanced by continued movement along listric faults throughout the incipient phase of passive-margin development. Most of the Hampton Formation and the overlying Erwin Formation and Shady Dolomite record progradation of the passive margin which culminated in development of a carbonate-rimmed shelf. The rift-to-passive margin phases of sedimentation in the central Appalachians reflect a continuum from fault-influenced to thermotectonics subsidence.