Abstract

Examination of the two novaculite members for additional clues to the origin of novaculite (white, pelletal, spiculitic chert) yielded the discovery of several features: early-formed fractures and breccias, and pockets and fractures filled with quartz sand and silt. Early-formed fractures are chiefly narrow (<4 mm wide) cracks up to several decimeters long which are sub-perpendicular to bedding and which are filled by slightly darker non-fibrous colorless microquartz. Breccias are formed of angular novaculite clasts in a black organic-rich siliceous sediment containing various amounts of pellets, quartz silt, sponge spicules, and locally spherulites of chalcedony or lutecite. Local pockets and veins of brown, very-fine quartz sand up to a few decimeters across are present within the upper member. The contact between the sand and novaculite host rock is crinkly, possibly from solutional enlargement of the pockets prior to filling and partly from later stylolitic solution. Two hypotheses are proposed for these features. One hypothesis is that protonovaculite underwent subaerial exposure, fracturing, development of karst features, and subsequent filling of open fractures partly by windblown sand and, after submergence, partly by dark marine muck. The other hypothesis is that fractures and breccias formed in unevenly lithified sediment, that fractures were filled by indigenous sediment, and that filling took place during continuous subsidence in deep water. Possible origins of fractures include differential compaction, crustal stresses generated by plate motion or vertical motion of basement rocks, and the passage of storm waves.

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