19. MINERALOGY OF FIRM MUDS AND MUDSTONES ASSOCIATED WITH VOLCANOSEDIMENTA RY LAYERS FROM THE TYRRHENIAN SEA, HOLES 650A AND 651A, O.D.P. LEG 107 1

At Holes 650A and 651 A, set respectively in the Marsili Basin and the Vavilov Basin, Pleistocene sediments (turbi­ ditic inputs interbedded with essentially hemipelagic sediments) may show layers of mudrocks with moderate to strong induration. Except in the two samples from Hole 651 A, it seems that zeolite crystallization does not play a role in the induration phenomenon. This latter appears to result from in situ clay authigenesis. Secondary K-Fe beidellite or Fe-Mg beidellite form diagenetic growths and bridges between sedimented particles. Turbidites are rich in volcaniclastics (glass, pumices and other volcanogenic elements) but the induration phenomenon appears to be associated essentially with the occurrence of basaltic detritus. It is proposed that clay authigenesis results from low temperature alteration of basal­ tic fragments issued from Vavilov and probably Marsili seamounts in sediments isolated from seawater by overlying deposits.