Petrography and Diagenesis of Franciscan Limestones

Abstract

ABSTRACT Lithification of Franciscan limestones did not occur immediately after deposition as demonstrated by the preferred orientation of grains which is a result of compaction. Variable preservation of grains in adjacent layers of Laytonville Limestone are a result of variable degrees and different times of lithification; less compacted layers containing better preserved grains were lithified earlier than the more compacted layers. Recrystallization of the micrite followed burial and compaction of the limestone. All degrees of recrystallization are found in the coccoliths that originally composed at least 50 percent of the micrite. During advanced stages of recrystallization, coccoliths and other grains were transformed to a continuous and uniform calcite mosaic. At this stage the primary po osity of the sediment was removed and the sediment became completely lithified. Stylolites probably formed over a long period of time that started when the limestones were unlithified. Remnants of stylolites are found within the chert and stylolites are common at the margins of chert bodies. Most stylolites are younger than veins. Chert formed only in micritic limestones containing Radiolaria. The silica eventually replaced the original carbonate micrite between the radiolarian tests and the end result of this replacement was transformation of a limestone bed to chert.