The Domeyko basin of northern Chile records Late Triassic-Early Cretaceous mixed carbonate and siliciclastic marine deposition along the western margin of Gondwana. The carbonate and siliciclastic ramp environment is characterised by the deposition of siliciclastic-dominated sediments at times of low accommodation space (lowstand and late highstand systems tracts) and carbonate-dominated sediments during periods of high accommodation space (transgressive and early highstand systems tracts). Along strike variations in siliciclastic sediment supply do not overprint the effects of basinwide changes of relative sea-level. Sequence stratigraphical analysis identifies five second-order sequences resulting from changes in accommodation space and hence, relative sea-level. Within these second-order cycles a higher-frequency cyclicity has also been identified. Each second-order sequence is composed of between 3 and 5 high-frequency sequences which results in a second-order composite sequence, but does not produce the "typical" sequence-sets normally associated with composite sequences. Comparison of the relative sea-level fluctuations interpreted from the Domeyko basin succession with those documented from other similar age South American marginal basins and northern hemisphere basins allows the distinction of regional from global events, and a methodology for potentially differentiating between eustatic and tectonic driving mechanisms. The choice of these basins permits an objective analysis of relative sea-level change using basins of different tectonic setting, on different continental plates. Relative sea-level fall in the late Early Sinemurian, earliest Pliensbachian, earliest Aalenian, Early Callovian, earliest Valanginian, and rises in the earliest Hettangian, earliest and Late Toarcian, Early and Late Bajocian, Late Bathonian and earliest Oxfordian of the Dorneyko basin appear time-equivalent to similar events in other southern and northern hemisphere basins and thus are interpreted to be products of eustatically driven, global sea-level cycles. Relative sea-level falls in the earliest Bathonian, Late Oxfordian, earliest Valanginian and rises in the Late Kimmeridgian are interpreted to be tectonically-driven, continental-scale changes in accommodation space. Although the earliest Valanginian relative sea-level fall can also be seen globally the sequence boundary is interpreted to be tectonically-enhanced by documented regional uplift in Chile and Argentina. The Domeyko basin succession appears to be dominantly controlled by global sea-level fluctuations during the Early-Middle Jurassic, interpreted to have been driven by glacio-eustasy, and by continental-scale fluctuations during the Middle Jurassic to mid-Cretaceous. Mesozoic back-arc basins of western South America record both eustatic and subduction-related relative sea-level fluctuations. To date, the latter have been assumed to be driven chiefly by Pacific plate spreading. However, five tectonically-driven sequence boundaries in the Domeyko and Neuquen basins which have no time correlative expressions in northern hemisphere basins display a close temporal association with five major Gondwanan fragmentation phases. They are: (A) the earliest Bathonian (170 Ma) sequence boundary was driven by the separation of Laurasia and Gondwana; (B) the Late Oxfordian (157 Ma) salinity crisis was driven by the separation of west from east Gondwana; (C) the termination of the marine Domeyko basin was driven by the fragmentation of east Gondwana; CD) the termination of the marine Neuquen Basin (114 Mal was driven by the opening of the South Atlantic; and (E) the major basin inversion associated with the Peruvian tectonic event (100 Mal was driven by the final fragmentation phase within east Gondwana. It is deduced that plate reorganisation associated with the initiation of new oceans during Gondwanan fragmentation resulted in increased coupling along the Andean subduction zone producing regional uplift and thus relative sea-level fall. Responses variously involve: basin barring and Kimmeridgian evaporite production; erosively-based fluvial sandstones directly overlying offshore marine deposits; closure of the Rocas Verdes oceanic marginal basin; sequential termination of marine conditions in the Domeyko and Neuquen back-arc basins; sudden influx of arc-derived alluvial conglomerates resulting from the onset of contractional tectonics; and an incremental jump in the eastward propagation of the Andean volcanic arc. Discrete contractional episodes in the otherwise extensional Andean active margin were thus driven by the incremental spreading phases of the supercontinent. Associated thermal signatures are recorded near the centre of Gondwana by kimberlite emplacement frequency maxima in southern Africa. Plate tectonic theory emphasises the dynamic interaction between adjacent plates, whereby relative rates of motion can be fixed to either the underriding or overriding plate as a reference frame. With reference to the circum-Gondwana subduction zone, I believe that in the past an overemphasis has been placed on the motions of oceanic plates in the Pacific region, while overlooking the role that the thermal evolution of the Pangean supercontinent played in driving circum-Pangean subduction. Thus, careful sequence stratigraphical analysis of active margin sedimentary basins provides a high resolution record, presently under-utilised in detecting, identifying and analysing global tectonic events in time.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:307620 |
| Date | January 1996 |
| Creators | Ardill, John |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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