Chapter 3 Comparisons Between Open-Ocean and Continental Margin Chert Sequences

Abstract

In order to determine the depositional environments of orogenic belt cherts, we compared chert sequences that we studied in Costa Rica, California, and Alaska with cherts recovered by the Deep Sea Drilling Project during Legs 62 and 69. Leg 62 recovered cores in open-ocean environments on Hess Rise and the Mid-Pacific Mountains, and Leg 69 recovered deposits from the south flank of the Costa Rica Rift. Comparisons of lithologic associations, sedimentary structures, chemistry, sedimentation rates, and modes of formation clearly show that chert sequences occurring in orogenic belts are not analogous to cherts that have been drilled in open-ocean environments. the orogenic belt chert sections we studied apparently formed in tectonically produced basins near continental margins. No known cherts of any age drilled in the Pacific Ocean basin resemble the ribbon cherts observed on land. Cherts from the deep sea occur mainly as nodules or lenses in, or above, limestone and chalk, and compose a small percentage of the overall sedimentary sequences. Typical deep-sea open-ocean cherts such as those from Legs 62 and 69, formed by replacement of calcareous or clayey sediments, whereas, bedded ribbon cherts in orogenic belt sequences, such as those from Costa Rica, California, and Alaska, formed by diagenetic recrystallization of alternating contrasting sediment types, such as siliceous ooze and hemipelagic clay, tuff, or sandstone. Many of the chert beds were laid down by turbidity currents. the cherts we studied in orogenic belts are in depositional continuity with greenstone as well as graywacke turbidites. Sections rarely, if at all, have limestones associated with them.