Palaeoceanography of Mesozoic ribbon radiolarites

Abstract

Mesozoic ribbon radiolarites have no parallel in rocks cored from extant ocean basins; their mineralogy is comparable but their repetitively bedded aspect is unique. The peculiarities of the chert-argillite couplet we relate primarily to variations in surface-water productivity on a tens-of-thousands-of-years scale, to a lesser extent to redeposition from turbidity and particularly bottom currents, and diagenesis, the latter being accentuated by the pressure-temperature gradients imposed during major tectonic events: ribbon cherts are unique to orogenic belts. We suggest that these peculiar siliceous rocks were formed, as deep-water deposits (traces of sulphate minerals notwithstanding), in small basins of various types; arc-related regions (e.g. the Great Valley Sequence and, possibly, the Franciscan of California) and embryonic oceans dominated by transform faulting such as those of the western Tethys. Given that diatoms have replaced radiolarians in processing most of the World Ocean's dissolved silica and that some Tertiary-Recent diatomites were deposited at comparable rates (expressed as g/cm 2 10 3 years) to Mesozoic ribbon radiolarites it is instructive to discover that certain present-day sites of diatomaceous sedimentation (e.g. Sea of Okhotsk, Bering Sea, Japan Sea and Gulf of California) provide tectonic analogues for Mesozoic siliceous basins. In these settings, namely arc-related or transform-dominated, the first significant depositional interface probably lay at a depth of 3–3.5 km which was below the Late Jurassic (pre-Tithonian) calcite compensation depth, favouring sedimentation of an entirely siliceous record: thus most Jurassic ophiolites show a basalt-chert contact. The Cretaceous radiolarites of Cyprus and the Middle East which also rest on basalt probably result, however, from local carbonate dissolution in lava-hollows by debouching low-pH hydrothermal fluids; they are not necessarily comparable with their Jurassic counterparts. The prevalence of mid to Late Jurassic radiolarites in the Californian and Tethyan areas is attributed to heightened fertility and elevated CCD in small ocean basins rather than any factor favouring global radiolarian productivity at this time. Depth and sea-floor topography also controlled facies developments on the continental margins of the western Tethys, shallower-water regions being characterized by pelagic-carbonate sediments. Mesozoic ribbon cherts thus provide a tantalizing record of local palaeoceanographic parameters, palaeotectonic regimes and sedimentary environments but they tell us little about the behaviour of the World Ocean during Jurassic and Cretaceous time.