Figure F23. Thin section photomicrographs of a fragment of dark reddish foraminifer-radiolarian chert/porcellanite taken from the Albian section (Sample 198-1207B-33R-CC, 80-82 cm). There is an abrupt, but irregular, front between silicified (lower) and unsilicified (upper) parts of the sample. Note the many foraminifers filled by tan and colorless (chalcedony/microquartz) cements. The tan cement is interpreted as once having been opal, but it is now birefringent (lower view) recrystallized quartz. Foraminifers are locally partly to wholly replaced by silica. The upper view is under plane-polarized light, whereas the lower view is with nicols crossed.

Figure F23. Thin section photomicrographs of a fragment of dark reddish foraminifer-radiolarian chert/porcellanite taken from the Albian section (Sample 198-1207B-33R-CC, 80-82 cm). There is an abrupt, but irregular, front between silicified (lower) and unsilicified (upper) parts of the sample. Note the many foraminifers filled by tan and colorless (chalcedony/microquartz) cements. The tan cement is interpreted as once having been opal, but it is now birefringent (lower view) recrystallized quartz. Foraminifers are locally partly to wholly replaced by silica. The upper view is under plane-polarized light, whereas the lower view is with nicols crossed.