Radiolarian Cherts, Pelagic Limestones and Igneous Rocks in Eugeosynclinal Assemblages

Among the many unresolved problems concerning eugeosynclinal rocks, none has proved more durable than the controversy over the role of submarine volcanism in the genesis of the thin-bedded cherts and pelagic limestones that commonly occur interbedded with, and above, pillow lavas and volcaniclastic rocks. The composition of the pelagic rocks, their relationships to eugeosynclinal igneous rocks, and data from modern ocean basins all argue against a direct connection between volcanism and formation of pelagic sediments, such as volcanically induced chemical precipitation or plankton blooms. Nonetheless, the persistent joint occurrence of bedded cherts and ophiolites, particularly in the Tethyan region, remains a puzzle. The palaeobathymetric interpretation of bedded radiolarian cherts is equivocal because the calcite compensation depth (CCD) has apparently fluctuated in time and space. But the appearance of the calcareous microplankton (coccolithophorids, planktonic Foraminifera) in mid to late Mesozoic time marked a significant change in the distribution of biogenic pelagic sediments. Whereas, prior to mid Mesozoic time, pelagic sedimentation in regions of high plankton productivity was largely radiolarian ooze regardless of water depth, radiolarian oozes most commonly have accumulated at abyssal depths below the CCD from the Cretaceous onward, and pelagic calcareous oozes of organic origin have been deposited in oceanic settings since the Jurassic. Measurements of vesicle size and abundance in eugeo-synclinal pillow basalts may provide an independent palaeobathymetric evaluation; but ancient water depths obtained in this manner should be applied to the associated pelagic sediments with some reserve because the sea floor may have changed elevation substantially following basalt emplacement but prior to sedimentation. Sediment ponding, disruption of sediment bodies by volcanic erosion and intrusion on the sea floor, and tectonic shearing, all of which induce lenticularity, determine the configuration of pelagic sedimentary bodies in eugeosynclinal assemblages. Large submarine volcanic structures like ridges and seamounts strongly influence the regional distribution of pelagic facies, for example, by elevating local areas of the sea floor above the CCD. Varied depositional-tectonic situations can be recognized in eugeosynclinal assemblages by determination of the geometry and size of pelagic sedimentary bodies, their relations to contiguous igneous rocks, and the vertical succession of sedimentary and igneous facies.