Abstract

Radiolarites of the Ligurian sequence are rhythmic alternations of generally red chert and shale beds from 1 to 10 cm thick that overlie ophiolite rocks of presumed oceanic crust and underlie pelagic and redeposited limestone. The rhythmic bedding is the product of deposition of radiolarian sand beds and some overlying mud by turbidity currents and possibly other marine bottom currents that alternated with episodes of slow deposition of hemipelagic red clay. Evidence of current deposition of radiolarian sand beds includes: tightly packed Radiolaria; presence of hydraulically-equivalent clasts of red clay and ophiolite debris; beds with grading, laminations, current-ripple cross-beds, and flute and groove casts. Chert beds formed principally by dissolution of the more delicate Radiolaria and reprecipitation of silica cement as either microcrystalline quartz or opal C-T around more resistant Radiolaria during burial diagenesis. Radiolarite has abrupt lateral differences in thickness (from absent to 200 m), which reflects deposition on a highly uneven sea floor. An interpretation of the depth of water during radiolarite deposition depends in part on how one interprets the origin of ophicalcite that locally underlies radiolarite. Ophicalcite consists of clasts of serpentinite and locally gabbro and peridotite in a pink calcite microspar matrix, the whole cut by white vein-filling calcite of younger tectonic origin. Ophicalcite can be interpreted as a breccia formed by total recrystallization, in part, of pelagic carbonate ooze deposited along ocean spreading ridges or as paleocaliche that developed upon ophiolite exposed above sea level. Evidence to support a pedogenic origin of ophicalcite includes textures and structures identical with caliche, silicified serpentinite masses (jasper) whose texture and mineral composition mimic modern soils on serpentinite, presence of a flat ?wave-cut surface beneath radiolarite, upward increase within ophicalcite of both calcite and oxidized serpentinite debris, and absence or thinness of radiolarite on paleotopographic highs. Many unanswered problems exist to both the above hypotheses. The deep-water hypothesis (favored by EFM) is that deposition took place on oceanic crust in water deeper than wave base (>200 m) and probably deeper than 2500 m; the latter depth implies the existence of a CCD during Late Jurassic time to account for the absence of carbonate in radiolarite. The shallow-water hypothesis (favored by RLF) is that deposition took place on previously exposed subsiding oceanic crust in water depths from a few meters to a few hundred meters; the absence of carbonate in radiolarite is attributed to local scarcity of carbonate-producing plankton compared to the overwhelming supply of clay from weathered ophiolite. Both hypotheses involve the interpretation that radiolarian-rich sediment, initially deposited on topographic highs, was transported into adjacent troughs by turbidity and other marine currents; the deeper troughs were floored by pillow lavas.

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