Volcanism and its Contribution to Mudrock Genesis

Explosive eruptions from volcanoes are recorded in the stratigraphic record throughout the Phanerozoic. The most visible evidence of these eruptions is generally in the form of preserved tephra layers, and they appear to be concentrated in the stratigraphic record at times of active plate collision and concomitant high stands of sea level. The products of volcanic eruptions are lavas, tephra and gases, and whereas low-silica, anhydrous basaltic magmas are usually erupted in the form of lava flows high-silica, hydrous rhyolitic magmas are commonly explosively erupted as plinian and ultraplinian plumes, and associated pyroclastic flows. Fallout tephras may be preserved in ancient sedimentary sequences as tonsteins, bentonites and K-bentonites, but in many cases they also mix with ambient background sedimentation and thus become simply one of the contributing factors to overall sediment composition. The significance of volcanogenic contributions to mudrocks is easily underestimated since their influence requires detailed study of the fine-grained components by petrographic and X-ray diffraction methods. Volcanogenic sediments react with seawater to produce secondary phases such as zeolites and clay minerals. Typical mudrock clay minerals derived by the alteration of tephra include smectite, illite-smectite and kaolinite, and the most diagnostic non-clay minerals include beta-form quartz, euhedral zircon and apatite, clinopyroxene, amphibole, biotite and sanidine. Further modification of these phases by diagenetic and low-grade metamorphic processes further obscures the nature of their origin. Volkanizma ve Kitafl› Oluflumuna Katk›s› Ozet: fiiddetli volkanik patlamalar Fanerozoik boyunca stratigrafik kay›tlarda yer al›rlar. Bu kay›tlar›n en iyi gozlenebilenleri iyi korunmufl tefra tabakalar› olup, yerkabuu carp›flma tektoniinin ve eflzamanl› deniz seviyesi yukselmelerinin younlaflt›¤› evrelerde stratigrafik istifler boyunca s›kca izlenirler. Lav, tefra ve gaz fleklindeki volkanizma urunleri, bazaltik magmadan kaynakland›¤›nda lav ak›nt›lar›, riyolitik magmadan gelifltiinde ise piliniyen ve ultrapiliniyen patlama kolonlar› ve piroklastik ak›nt›lar› meydana getirirler. Atmosferden cokelen tefra, yafll› tortul istiflerde tonfltayn, bentonit ve K-bentonit tabakalar› olarak korunabilirler. Ancak bircok durumda, cokelen tefra, cokelme ortam›na dier jeolojik sureclerle tafl›nan tortullarla kar›flarak, toplam tortul bileflimine katk› koyan etkenlerden sadece birini oluflturabilmektedir. Volkanik kokenli malzemenin kiltafllar›n›n oluflumundaki onemi cou zaman, ince taneli malzemenin petrografik ve X-›fl›n› toz difraktometre yontemleri ile incelenmesi gerektiinden, anlafl›lmamaktad›r. Bu tur malzeme deniz suyu ile tepkimeye girerek zeolit ve kil minerallerine donuflmektedir. Kiltafllar›ndaki tefran›n bozuflmas› ile oluflan tipik kil mineralleri simektit, illit-simektit ve kaolinittir. Tefra kokenli en tipik kil d›fl› mineraller ise beta-kuvars, oz flekilli zirkon ve apatit , klinopiroksen, amfibol, biyotit ve sanidindir. Bu mineral fazlar›n›n diyajenetik ve dufluk dereceli metamorfizma ile daha sonraki deifliklikler ise volkanik kokenlerinin belirlenmesini zorlaflt›rmaktad›r.