Basin physiography and tectonic influence on sequence architecture and stacking pattern:Pleistocene succession of the Canoa Basin(central Ecuador)

摘要

Facies,shell bed features,and sequence stratigraphic framework for the shallow-marine Pleistocene upper Canoa and Tablazo Formations are presented,based on outcrop data from the southern coast of Cabo San Lorenzo,Ecuador.Sediments of this succession exhibit a distinct cyclic pattern,consisting of a stack of eight depositional sequences(cyclothems)likely developed under the main control of orbitally induced sea-level changes.As a rule,within the studied interval an idealized cyclothem is composed of a transgressive systems tract(TST)and a highstand systems tract(HST),whereas deposits attributable to the lowstand and falling-stage systems tracts are not present.Transgressive lithosomes may be denned by estuarine deposits interposed between the sequence boundary and the ravinement surface(back-barrier wedge)and by upward fining shoreface to inner-shelf facies successions above the ravinement(backstepping shelf wedge).Separated by an expanded siliciclastic core,hiatal shell concentrations occur at the base(onlap shell beds)and the top(backlap shell beds)of the transgressive shelf wedges,and some occur at the base of highstand systems tracts(down-lap shell beds).On the basis of sedimentary facies,geometry,taphonomy,and paleoecol-ogy of shell beds,and the nature of the transition between siliciclastic and mollusk-bear-ing sediments,cyclothems were classified into two main types that show dependence upon paleoshoreline morphological configuration:sheltered(in the upper Canoa Formation)and exposed(in the Tablazo Formation).Notwithstanding the different synsedi-mentary tectonic and climatic regimes,the Ecuadorian cyclothems share basic patterns of condensation and facies assemblages with other roughly coeval cyclothemic successions around the world.This suggests that(1)hiatal shell bed development is not just a temperate-latitude phenomenon;(2)a global process,such as glacio-eustatic sea-level change,is the primary mechanism of control for the general architecture of sequences;and(3)specific paleogeographic settings play an important role by determining the tapho-nomic and paleoecologic characteristics of key shell beds,the nature of their contacts with the encasing sediments,and the type of the component set of facies.At a multicycle time scale,tectonics influenced the long-term trend of the relative sea-level changes and consequently the large-scale stratigraphic organization.Owing to the continued tectonic uplift of the area,successive high-frequency depositional sequences are nested to form a longer-order falling-stage sequence set.