Sedimentary and Gravity-Slide Emplacement of Serpentinite

摘要: Large deposits of serpentinite in alpine-type orogenic areas have been formed by sedimentary processes ranging from the detrital accumulation bedded sandstone and shale to emplacement chaotic breccias (olistostromes) gigantic slide blocks. Known occurrences are listed, eight circum-Pacific, Caribbean, Mediterranean described detail. Sedimentary serpentinites range age early Paleozoic Quaternary, although most Cretaceous or Tertiary. Most were deposited eugeosynclinal environments, geosynclinal cycle. Individual thickness a few centimeters nearly 3 km, several extend laterally for tens kilometers. Graded bedding is common, many contain marine fossils. Serpentinite dominant rock constituent, clasts foreign alpine ultramafic assemblage rare. Chemical analyses often show that these rocks slightly more silica alumina than do nondetrital serpentinites, due contamination aluminosilicate minerals quartz during deposition. This nine other criteria potentially useful recognition serpentinites. Several features suggest very rapidly submarine landslides, mudflows, turbidity currents. The sources this debris postulated be upward-migrating protrusions which penetrate seafloor Earth's surface upslope eventual depositional sites. Sedimentary much abundant commonly thought, masses presently regarded as igneous intrusions tectonic may fact coeval with, instead younger than, their enclosing metasedimentary rocks. In sequences such Franciscan Formation, some elongate bodies now sills beds detritus whose masked post-depositional shearing; isolated exotic A origin can explain persistent perplexing characteristics serpentinites: conformity with rocks, grossly planar shapes, absence metamorphism along contacts.