Late Chert Research Articles

Field Classification and Oxygen Isotope Composition of Quartz Cherts from the Monterey Formation, Santa Maria Basin, California

Field, petrographic, and isotopic studies indicate that chertification occurred at different times during the burial history of the Miocene Monterey Formation sediments. Early, intermediate, late, and dolomite-replacement quartz cherts can be identified by differential compaction, cross-cutting relationships, and by precursor lithology. The authors' oxygen isotope analyses largely substantiate their field and petrographic classification of Monterey cherts. Early quartz cherts contain oxygen isotope ratios of {delta}{sup 18}O (SMOW) = 24.5 to 29.4{per thousand}, intermediate cherts = 26.5 to 29.4{per thousand}, late cherts = 24.8 to 25.9{per thousand}, dolomite-replacement cherts = 27.9 to 30.2{per thousand}, and shale-associated cherts = 29.4 to 31.2{per thousand}. Assuming a prograde sequence of chertification, and that {delta}{sup 18}O{sub pore water} remained relatively constant at about 0{per thousand} (SMOW), shale-associated cherts were formed the earliest at 40 to 49C, dolomite-replacement cherts followed at 45 to 56C, normal cherts formed between 48 and 64C, and late cherts formed at 67 to 73C (temperature expression of Matsuhisa et al., 1979). Early quartz cherts apparently inherited their differential compaction features from early-formed opal-CT cherts; their isotopic composition indicates replacement by quartz at a wide range of temperatures, from 48 to 75C. Further synthesis of field and isotopic data will also helpmore » constrain the oxygen isotopic evolution of the Monterey Formation pore waters.« less

Stratigraphy and sedimentology of Jurassic bedded chert overlying ophiolites in the North Apennines, Italy

ABSTRACTIn the North Apennines of Italy, Upper Jurassic bedded chert stratigraphically overlies ophiolitic rocks and is overlain by Lower to Middle Cretaceous pelagic limestone and shale, and Upper Cretaceous flysch. The bedded chert, best exposed in East Liguria and on Elba, is typically 30–80 m thick, but occasionally reaches 150–200 m thickness. It consists of two main alternating lithologïes: siliceous mudstone (SM) and radiolarite (R).Chert sections commonly show characteristic stratigraphic changes. Lower cherts display a striking rhythmic alternation of R and ferruginous SM beds. In middle cherts, SM beds are much less ferruginous and shalier intercalations are locally present. In upper cherts, R beds are less frequent and SM beds are essentially non‐ferruginous.R beds are generally 1–4 cm thick, and consist of 80–90% quartz, 5–15% clays and usually < 1% hematite. They are commonly parallel‐laminated, and rarely size‐graded. In size‐graded beds, large radiolaria are more abundant near the bed base (commonly together with ophiolitic or SM clasts) and small radiolaria more abundant near the bed top. Sorting is poor throughout most R beds. R beds are interpreted as turbidites (cf. Nisbet & Price, 1974). Model calculations suggest that typical settling velocities of radiolaria during redeposition are < 1 cm sec−1, which is low and of restricted range relative to the 1–10 cm sec−1 settling velocities of clastic grains of comparable size range. Radiolaria therefore should have only a limited tendency to grade and sort during deposition from a turbulent current.SM beds are commonly 1–7 cm thick, although much thicker ones occur near the base of sections, and consist mainly of 50–70% quartz, 15–35% clays and 0–15% hematite. Microscopic clay‐silica aggregates and highly corroded remnants of radiolaria are common. SM beds are interpreted as mainly ambient pelagic sediment which accumulated slowly in topographic lows, and which was modified by near‐surface dissolution of biogenic silica. In SM beds which contain two texturally different layers, the lower one is interpreted as the top of the underlying radiolarian turbidite.North Apennine cherts represent the first sediment deposited on oceanic crust formed during the opening of the North Apennine part of the Tethys. The ophiolitic basement had a rugged topography which favoured the redeposition of siliceous sediment. Hematite and local Mn enrichments in SM beds in the lower chert sections represent hydrothermal precipitates inferred to have originated at a spreading axis. During seafloor spreading, accumulation of siliceous sediments progressively reduced the topography. Deposition of ophiolitic detritus within the sediments phased out during early chert sedimentation, and the hydrothermal contribution during early‐middle chert sedimentation. As local basins filled, during late chert sedimentation, radiolarian turbidites became less frequent.The first limestones at the top of chert sections are calcareous ooze turbidites derived from above the CCD and deposited slightly below it. Gradual descent of the CCD to ocean floor depths at the end of the Jurassic (Bosellini & Winterer, 1975) led to the replacement of siliceous by carbonate sedimentation.

Nature and Origin of Early and Late Cherts in the Leadville Limestone, Colorado

Two generations of chert have been observed in the Mississippian Leadville Limestone of west-central Colorado: (1) an early chert inferred to have precipitated from hypersaline marine waters of high pH after those waters seeped into carbonate muds prior to final burial and lithification of the carbonate; and (2) a late chert that appears to have precipitated from ground waters as amorphous silica after initial lithification but prior to or during karst erosion of the formation in Late Mississippian (?) and Early Pennsylvanian time. A third type of microcrystalline quartz, hydrothermal jasperoid, is associated with ore deposits of Laramide age in the Leadville, but it is not discussed here. Conditions in west-central Colorado at the time that the early cherts formed in the Leadville were probably similar to those associated with the formation of early diagenetic chert in modern sediments, and the early chert in the Leadville is believed to have formed in a manner similar to the modern early cherts. The silica of the late chert appears to have come from Paleozoic sandstones below the Leadville. It was carried by slightly acid artesian waters into the formation, probably in concentrations of less than 60 ppm SiO 2 Super-saturation of amorphous silica occurred within the Leadville and probably was obtained by evaporative concentration at the water-air interface during dry seasons. Dissolution of the limestone by the slightly acid waters may account for the removal of the calcite that is replaced by chert.