Mineralogy and Origin of Clay Minerals, Silica and Authigenic Silicates in Leg 14 Sediments

Abstract

Preliminary X-ray, SEM, TEM, and optical mineralogical results are reported to discuss the paragenesis, age distribution, and origin of clay minerals, silica modifications and authigenic silicates found in nine drilling sites of Leg 14 of the Deep Sea Drilling Project, located off northwestern Africa and northeastern South America.Most noncarbonate minerals are diagenetic alteration products of volcanic ashes. Continent-derived clay minerals, such as mica-illite, kaolinite, and chlorite (rare), are frequent only in terrigenous sediments, especially off the Amazon River and off Cape Blanc. Smectites, mainly normal montmorillonite (+greenish ?nontronite, and hectorite-saponite), are the dominant clay minerals in sediments of all ages and sites. Montmorillonite, ultimately derived from the decomposition of volcanic glass, is especially abundant in volcanogenic clays and ashes, nannofossil marl-oozes, hemipelagic siliceous oozes, and brown pelagic clays. A mixed-layer mineral, consisting of regularly interstratified chlorite and trioctahedral montmorillonite, occurs mainly in pelagic clays.Four types of amorphous or microcrystalline silica were observed in siliceous oozes and silicified mudstones and bedded cherts: (a) unaltered, amorphous opal (in siliceous organisms); (b) unidimensionally disordered low-cristobalite (-tridymite), either cryptocrystalline ("opal-cristobalite") or as fibrous "lussatite" needles, slowly crystallizing from closely-packed silica gel spheres (in cherts of intermediate maturity); (c) macrocrystalline fibrous chalcedony; and (d) crystocrystalHne quartz, commonly filling recrystallized radiolarian tests and forming the matrix of homogenized, mature, quartz-rich cherts.Palygorskite, commonly associated with small amounts of sepiolite, is an abundant constituent of volcanogenic white (and brown) clays, carbonaceous clays, some siliceous muds, "white" lussatite cherts, and pelagic clays, all of Latest Cretaceous to Paleogene age. A diagenetic origin from degraded bentonitic clays by precipitation from magnesium-rich solutions with excess silica, derived from the devitrification of silicic ash and/or the dissolution of opaline organisms is suggested. The maximum occurrence of sepiolite is farther from shore than that of palygorskite.Authigenic clinoptilolite, the only zefolite identified in all sites, has a similar age range (minimum age: Middle Miocene; minimum depth of burial: 120 meters) and paragenesis as palygorskite. It has also been formed diagenetically from the devitrification of volcanic glass and palagonite, especially in pelagic and volcanogenic brown clays and black carbonaceous shales, and at least 20 million years after their deposition.High-temperature sanidine, possibly also of authigenic origin by hydrothermal solutions, was found in Late Cretaceous pyroclastic clays and altered vitreous ashes, associated with volcanic glass, palagonite, and montmorillonite.

Silica Diagenesis in Continental Margin Sediments off Northwest Africa

“…Clayey Environment ("bedded porcellanites") "Bedded" porcellanites, derived from clays and claystones with various amounts of terrigenous silt, nannofossils, radiolarians, or diatoms are less abundant in the Leg 41 samples than in previously studied samples from deeper and more pelagic environments Rösch, 1972, 1974). The palygorskite-bearing porcellanite (type BI3) Rösch, 1972, 1974) is relatively common in the greenish hemipelagic silty clays of Site 368 and in the pelagic zeolitic clays of Site 367.…”

“…The analytical procedures for X-ray diffractometry follows the method described in von Rösch (1972, 1974). The semiquantitative percentages of Table 2 were measured with a standard deviation of less than 10% using mainly the "method of transparent specimens."…”

“…The palygorskite-bearing porcellanite (type BI3) Rösch, 1972, 1974) is relatively common in the greenish hemipelagic silty clays of Site 368 and in the pelagic zeolitic clays of Site 367. Transitions into the pelagic (BI2) and hemipelagic (BIi) types (see Table 1) do exist.…”

“…Detailed studies of siliceous ooze and silicified rocks from DSDP sediments have yielded many new insights into silica diagenesis and the origin and formation of porcellanites and cherts (Calvert, 1971;1974;Heath and Moberly, 1971;Wise and Weaver, 1973;1974;Rösch, 1972, 1974;Lancelot, 1973;. Great progress has been made during the past 5 years in the identification of various silica modifications, in the application of the DSDP results to Cretaceous and Tertiary silicified shallowwater sediments on land (e.g., Wise and Weaver, 1973;Hakansson et al, 1974;Aubry, 1975;Murata and Larson, 1975), and in testing of hypothetical diagenetic processes by hydrothermal experiments (e.g., Oehler, 1973;Kastner and Keene, 1975;Flörke et al, 1975).…”

Site 367: Cape Verde Basin

“…The silty clay is characterized by a lack of biogenic carbonate, common quartz, silt, and clay. Fine particles, especially abundant in Core 7, may be palygorskite, identified in sediments of a similar age at Site 140 (von Rad and Rosch, 1972). This subunit has alternating greenish gray (5GY6/1) and black (Nl), diatom-bearing radiolarian clay.…”