Organic geochemistry of lower Cretaceous sediments at DSDP Hole 77-535

At Site 535, the four lithologic units of Cretaceous age are controlled by two types of sedimentologic facies: (1) the massive light-colored limestones or marly limestones in which the total organic carbon (TOC) content is low and the organic matter more or less oxidized and (2) laminated dark facies in which the TOC content is higher and associated with a well-preserved organic matter of Type II origin. Very little typical Type III organic matter occurs in the whole series from late Berriasian to Aptian and Cenomanian. Fluctuations from oxidizing to reducing environments of deposition are proposed to account for the variations in properties of the Type II organic matter between the different facies. Dark laminated layers are good but immature potential source rocks: petroleum potential is often higher than 2 kg HC/t of rock.

Lithology and organic geochemistry at DSDP Hole 76-534

The organic facies of Early and middle Cretaceous sediments drilled at DSDP Site 534 is dominated by terrestrially derived plant remains and charcoal. Marine organic matter is mixed with the terrestrial components, but through much of this period was diluted by the terrestrial material. The supply of terrestrial organic matter was high here because of the nearness of the shore and high runoff promoted by a humid temperate coastal climate. Reducing conditions favored preservation of both marine and terrestrial organic matter, the terrestrial materials having reached the site mostly in turbidity currents or in the slow-moving, near-bottom nepheloid layer. An increase in the abundance of terrestrial organic matter occurred when the sea level dropped in the Valanginian and again in the Aptian-Albian, because rivers dumped more terrigenous elastics into the Basin and marine productivity was lower at these times than when sea level was high. A model is proposed to explain the predominance of reducing conditions in the Valanginian-Aptian, of oxidizing conditions in the late Aptian, and of reducing conditions in the Albian-Cenomanian. The model involves influx of oxygen-poor subsurface waters from the Pacific at times of high or rising sea level (Valanginian-Aptian, and Albian- Cenomanian) and restriction of that influx at times of low sea level (late Aptian). In the absence of a supply of oxygenpoor deep water, the bottom waters of the North Atlantic became oxidizing in the late Aptian, probably in response to development of a Mediterranean type of circulation. The influx of nutrients from the Pacific led to an increase in productivity through time, accounting for an increase in the proportion of marine organic matter from the Valanginian into the Aptian and from the Albian to the Cenomanian. Conditions were dominantly oxidizing through the Middle Jurassic into the Berriasian, with temporary exceptions when bottom waters became reducing, as in the Callovian. Mostly terrestrial and some marine organic matter accumulated during the Callovian reducing episode. When Jurassic bottom waters were oxidizing, only terrestrial organic matter was buried in the sediments, in very small amounts.

Mineralogy and pyrolysis at DSDP Hole 62-463

A 15-meter sequence of early Aptian organic-matter-rich sediments, cored at Deep Sea Drilling Project Site 463 (western Mid-Pacific Mountains) has been submitted for detailed mineralogical studies (XRD, SEM) and organiccarbon characterization. Although intense diagenesis has obscured the sedimentary record of depositional conditions, the history has been tentatively reconstructed. Through sustained volcanic activity and alteration processes on the archipelago, large amounts of silica were released into the sea water, resulting in a "bloom" of radiolarians. Hard parts settled in large amounts, yielding a hypersiliceous sediment; amorphous silica was diagenetically transformed into chalcedony, opal-CT and clinoptilolite through dissolution and recrystallization. Oxidization of part of the radiolarian soft parts (1) depleted the sea water in dissolved oxygen, allowing the burial of organic matter, and (2) generated carbon dioxide which led to dissolution of most of the calcareous tests. Moderate depositional depth and a high sedimentation rate are though to have prevailed during this episode. An immature stage of evolution is assigned to the studied organic matter, which is of two origins: autochthonous marine material, and allochthonous humic compounds and plant debris. Rhythmic sedimentation characterizes the distribution of the organic matter; each sequence shows (1) an upward progressive increase in organic-carbon content, and (2) an upward enrichment in marine organic matter.

Primary composition, alteration, and origin of Cretaceous volcaniclastic rocks at DSDP Site 89-585

An upper Aptian to middle Albian series of volcaniclastic rocks more than 300 m thick was drilled at Site 585 in the East Mariana Basin. On the basis of textural and compositional (bulk-rock chemistry, primary and secondary mineral phases) evidence, the volcaniclastic unit is subdivided into a lower (below 830 m sub-bottom) and an upper (about 670-760 m) sequence; the boundary in the interval between is uncertain owing to lack of samples. The rocks are dominantly former vitric basaltic tuffs and minor lapillistones with lesser amounts of crystals and basaltic lithic clasts. They are mixed with shallow-water carbonate debris (ooids, skeletal debris), and were transported by mass flows to their site of deposition. The lower sequence is mostly plagioclase- and olivine-phyric with lesser amounts of Ti-poor clinopyroxene. Mineralogical and bulk-rock chemical data indicate a tholeiitic composition slightly more enriched than N-MORB (normal mid-ocean ridge basalt). Transport was by debris flows from shallow-water sites, as indicated by admixed ooids. Volcanogenic particles are chiefly moderately vesicular to nonvesicular blocky shards (former sideromelane) and less angular tachylite with quench plagioclase and pyroxene, indicating generation of volcanic clasts predominantly by spalling and breakage of submarine pillow and/or sheet-flow lavas. The upper sequence is mainly clinopyroxene- and olivine-phyric with minor plagioclase. The more Ti-rich clinopyroxene and the bulk-rock analyses show that the moderately alkali basaltic composition throughout is more mafic than the basal tholeiitic sequence. Transport was by turbidity currents. Rounded epiclasts of crystalline basalts are more common than in the lower sequence, and, together with the occurrence of oxidized olivine pseudomorphs and vesicular tachylite, are taken as evidence of derivation from eroded subaerially exposed volcanics. Former sideromelane shards are more vesicular than in the lower sequence; vesicularity exceeds 60 vol.% in some clasts. The dominant clastic process is interpreted to be by shallow-water explosive eruptions. All rocks have undergone low-temperature alteration; the dominant secondary phases are "palagonite," chlorite/smectite mixed minerals, analcite, and chabazite. Smectite, chlorite, and natrolite occur in minor amounts. Phillipsite is recognized as an early alteration product, now replaced by other zeolites. During alteration, the rocks have lost up to 50% of their Ca, compared with a fresh shard and fresh glass inclusions in primary minerals, but have gained much less K, Rb, and Ba than expected, indicating rapid deposition prior to significant seafloor weathering.

Spores, pollen and dinoflagellates in Cretaceous sediments of ODP Hole 129-802A

Lower Cretaceous and Jurassic sediments from Ocean Drilling Program Leg 129 (Sites 800, 801, and 802) and Deep Sea Drilling Project Sites 167, 195, 196, and 463 were analyzed for palynomorphs. In contrast to Atlantic occurrences, all Cretaceous pelagic sediments at these sites in the Pacific are barren of preserved palynomorphs. This absence of palynomorphs appears to be independent of facies, sedimentation rate, paleodepth, and paleolatitude. Except for one sample, the dinocyst-bearing sediments also contain spores and pollen grains. The only palynomorphs observed were in redeposited material having sources near former emergent seamounts. Among the dinoflagellate cysts at Site 802, Dingodinium cerviculum, Odontochitina operculata, Canninginopsis colliveri, and Oligosphaeridium complex are the most important species. Based on the presence of these species and their known biostratigraphic ranges, this basal interval of Site 802 is considered to be Aptian/earliest Albian in age. The lack of dinocysts within the Pacific pelagic sediments may be the result of ubiquitous oxygenated bottom waters throughout the Cretaceous or may indicate that open-marine dinoflagellate populations in this ocean did not produce cysts.

Occurrence of tethyan radiolarian species in early Cretaceous sediments of ODP Hole 123-765C in the Northeast Indian Ocean (Fig. 4)

During ODP Leg 123, abundant and well-preserved Neocomian radiolarians were recovered at Site 765 (Argo Abyssal Plain) and Site 766 (lower Exmouth Plateau). Assemblages are characterized by the numerical dominance of a small number of non-tethyan forms and by the scarcity of tethyan taxa. Remarkable contrasts exist between radiolarian assemblages extracted from claystones of Site 765 and reexamined DSDP Site 261, and faunas recovered from radiolarian sand layers, only found at Site 765. Clay faunas are unusual in their low diversity of apparently ecologically tolerant (or solution resistant?), ubiquist species, whereas sand faunas are dominated by non-tethyan taxa. Comparisons with Sites 766 and 261, as well as sedimentological observations, lead to the conclusion that this faunal contrast resulted from a difference in provenance, rather than from hydraulic sorting or selective dissolution. The ranges of 27 tethyan taxa from Site 765 were compared to the tethyan radiolarian zonation by Jud ( 1992 ) by means of the Unitary Associations Method. This calculation allows to directly date the Site 765 assemblages and to estimate the amount of truncation of ranges for tethyan taxa. Over 70% of the already few tethyan species of Site 765, have truncated ranges during the Valanginian-Hauterivian. Radiolarian assemblages recovered from claystones at Sites 765 and 261 in the Argo Basin apparently reflect restricted oceanic conditions during the latest Jurassic-Barremian. Neither sedimentary facies nor faunal associations bear any resemblance to what we know from typical tethyan sequences. We conclude that the Argo Basin was paleoceanographically separated from the Tethys during the Late Jurassic and part of the Early Cretaceous by its position at higher paleolatitudes and/or by enclosing land masses. Assemblages recovered from radiolarian sand layers are dominated by non-tethyan species that are interpreted as circumantarctic. Their first appearance in the late Berriasian-early Valanginian predates the oceanization of the Indo-Australian breakup (M11, late Valanginian), but coincides with a sharp increase in margin-derived pelagic turbidites. The Indo-Australian rift zone and the adjacent margins must have been submerged deeply enough to allow an intermittent influx of circumantarctic cold water into the Argo Basin, creating increased bottom current activity. Cold-water radiolarians carried into the Argo Basin upwelled along the margin, died, and accumulated in radiolarite layers due to winnowing by bottom currents. High rates of faunal change and the sharp increase of bottom current activity are thought to be synchronous with possible pronounced late Berriasian-early Valanginian lowstands in sea level. Hypothetically, both phenomena might have been caused by a tendency to glaciation on the Antarctic-Australian continent, which was for the first time isolated from the rest of Gondwana by oceanic seaways as a result of Jurassic-Early Cretaceous sea-floor spreading. The absence of most typical tethyan radiolarian species during the Valanginian-Hauterivian is interpreted as reflecting a time of strong influx of circumantarctic cold water following oceanization (M 11) and rapid spreading between Southeast India and West Australia. The reappearance and gradual abundance/diversity increase of tethyan taxa, along with the still dominant circumantarctic species are thought to result from overall more equitable climatic conditions during the Barremian-early Aptian and from the establishment of an oceanic connection with the Tethys Ocean during the early Aptian.

Distribution of Cretaceous redeposited benthic foraminifers at DSDP Site 89-585

Cretaceous benthic foraminifers from Site 585 in the East Mariana Basin, western Pacific Ocean, provide an environmental and tectonic history of the Basin and the surrounding seamounts. Age diagnostic species (from a fauna of 155 benthic species identified) range from late Aptian to Maestrichtian in age. Displaced species in sediments derived from the tops and flanks of nearby seamounts were deposited sporadically on the Basin floor well below the carbonate compensation depth (CCD) at abyssal depths of 5000 to 6000 m. These depths, characterized by an indigenous assemblage of benthic foraminifers, recrystallized radiolarians, fish debris, and sponge spicules, existed in the Mariana Basin from late Aptian to the present. Early Albian and older edifice-building volcanism had reached the photic zone with associated shallow-water bank or reef environments. By middle Albian, the dominant source areas subsided to outer-neritic to upper-bathyal depths. Major volcanic activity ceased and fine-grained sediments were deposited by distal turbidites, although intermittent volcanism and the influx of rare neritic material continued until the late Albian. By the Cenomanian to Turonian, upper- to middle-bathyal depths were reached by the dominant source areas, and the sediments recovered from this interval include organic carbon-rich layers. Rare benthic foraminifers from the Coniacian-Santonian interval indicate a continuation of dominantly middle-bathyal source areas. A change in sedimentation during the Campanian-Maestrichtian from older zeolitic claystone to abundant chert in the Campanian, and nannofossil chalk and claystone in the Maestrichtian resulted from migration of the site beneath the equatorial productive zone due to northwestward plate motion. The appearance of rare middle-neritic and upper-bathyal species in the Maestrichtian interval associated with volcanogenic debris gives evidence of the remobilization and downslope transport of pelagic deposits due to thermally induced uplift. Episodic redeposition of shallow-water material during the Aptian-Albian was produced by edifice-building volcanism perhaps combined with eustatic lowering of sea level. The Cenomanian-Turonian pulse coincided with a low global sea-level stand as does the transported material during the Coniacian-Santonian. The Maestrichtian pulse was caused by renewed midplate volcanism that extended over a large area of the central Pacific.

Composition of Lower Cretaceous sandstones of the Galicia margin

The composition of 31 samples of Lower Cretaceous (Valanginian to Aptian) sandstone from ODP Sites 638 through 641 was analyzed using the Gazzi-Dickinson point-counting method. The results show that the source of the Valanginian to Hauterivian sand was a continental block, dominated by granitic and/or high-grade-metamorphic rocks. Although these petrologic results do not allow discrimination between various potential continental block provinces, they suggest, in conjunction with seismic profiles and regional considerations, that the source was the Galicia margin or western Iberia. In contrast, the Barremian and Aptian sand is dominated by carbonate grains that were derived from a carbonate platform, probably on Galicia Bank.

Uppermost Jurassic to lower Cretaceous benthic foraminifera from ODP Hole 123-765C

Benthic foraminifers were studied in 99 samples collected from the lower 200 m of Hole 765C. The studied section ranges from the Tithonian to Aptian, and benthic foraminifers can be subdivided into five assemblages on the basis of faunal diversity and stratigraphic ranges of distinctive species. Compared with deep-water assemblages from Atlantic DSDP sites and Poland, assemblages from the Argo Abyssal Plain display a higher diversity of agglutinated forms, which comprise the autochthonous assemblages. Assemblages at the base of Hole 765C are wholly composed of agglutinated forms, reflecting deposition beneath the carbonate compensation depth (CCD). Most calcareous benthic species are found in turbidite layers, and the presence of an upper Valanginian Praedorothia praehauteriviana Assemblage may indicate deposition at or just below the CCD. The P. praehauteriviana Assemblage from Hole 765C is the temporal equivalent of similar assemblages from DSDP Holes 534A, 416A, 370, 105, and 101 in the Atlantic Ocean and Hole 306 in the Pacific Ocean. Stratigraphic ranges of cosmopolitan agglutinated species at Site 765 generally overlap with their reported ranges in the Atlantic and in the bathyal flysch sequences of the Carpathians; however, several species from Hole 765C have not been previously reported from Uppermost Jurassic to Lower Cretaceous abyssal sediments.

Spores and pollen at DSDP Holes 36-330 and 71-511

Samples from Upper Jurassic and Lower Cretaceous sediments at Site 511 were analyzed for palynomorphs. Three palynological assemblages were identified: Upper Jurassic (presumably Tithonian), Neocomian-Aptian, and lower Albian. These were correlated to synchronous assemblages in Argentina, South Africa, and Australia and were compared with Jurassic palynoassemblages from Hole 330, Leg 36.

Lower Cretaceous radiolarians from the northwestern Australian margin

During Leg 123, abundant and well-preserved Neocomian radiolarians were recovered at Site 765 (Argo Abyssal Plain) and Site 766 (lower Exmouth Plateau). The assemblages are characterized by a scarcity or absence of Tethyan taxa. The Berriasian-early Aptian radiolarian record recovered at Site 765 is unique in its density of well-preserved samples and in its faunal contents. Remarkable contrasts exist between radiolarian assemblages extracted from claystones of Site 765 and reexamined DSDP Site 261, and faunas recovered from radiolarian sand layers of Site 765. Clay faunas are unusual in their low diversity of apparently ecologically tolerant species, whereas sand faunas are dominated by non-Tethyan species that have never been reported before. Comparisons with Sites 766 and 261, as well as sedimentological observations, lead to the conclusion that this faunal contrast results from a difference in provenance, rather than from hydraulic sorting. Biostratigraphic dating proved difficult principally because of the paucity or even absence of (Tethyan) species used in published zonations. In addition, published zonations are contradictory and do not reflect total ranges of species. Radiolarian assemblages recovered from claystones at Sites 765 and 261 in the Argo Basin reflect restricted oceanic conditions for the latest Jurassic to Barremian time period. Neither the sedimentary facies nor the faunal associations bear any resemblance to sediment and radiolarian facies observed in typical Tethyan sequences. I conclude that the Argo Basin was paleoceanographically separated from Tethys during the Late Jurassic and part of the Early Cretaceous by its position at a higher paleolatitude and by enclosing landmasses, i.e., northeastern India and the Shillong Block, which were adjacent to the northwestern Australian margin before the opening. Assemblages recovered from radiolarian sand layers are dominated by non-Tethyan species that are interpreted as circumantarctic. Their sudden appearance in the late Berriasian/early Valanginian pre-dates the oceanization of the Indo-Australian break-up (Ml 1, late Valanginian) by about 5 m.y., but coincides with a sharp increase in margin-derived pelagic turbidites. The Indo-Australian rift zone and its adjacent margins probably were submerged deeply enough to allow an intermittent "spillover" of circumantarctic cold water into the Argo Basin, creating increased bottom current activity. Circumantarctic cold-water radiolarians transported into the Argo Basin upwelled along the margin and died en masse. Concomitant winnowing by bottom currents led to their accumulation in distinct radiolarite layers. High rates of faunal change and the sharp increase of bottom current activity are thought to be synchronous with the two pronounced late Berriasian-early Valanginian lowstands in sea level. Hypothetically, both phenomena might have been caused by a glaciation on the Antarctic-Australian continent, which was for the first time isolated from the rest of Gondwana by oceanic seaways as a result of Jurassic and Early Cretaceous seafloor spreading. The absence of typical Tethyan radiolarian species during the late Valanginian to late Hauterivian period is interpreted as reflecting a time of strong influx of circumantarctic cold water following oceanization (Mil) and rapid spreading between southeast India and western Australia. The reappearance and gradual increase in abundance and diversity of Tethyan forms along with the still dominant circumantarctic species are thought to result from overall more equitable climatic conditions during the Barremian and early Aptian and may have resulted from the establishment of an oceanic connection with the Tethys Ocean during the early Aptian.

Lower Cretaceous dinoflagellate cysts of Exmouth Plateau, Nortwest Australia

Cores from Leg 122, Sites 762 and 763, were sampled at intervals of one sample per 1.5-m section in the Lower Cretaceous sequences. More than 400 samples were studied, most of which contained dinoflagellate cysts, spores, pollen, and various types of palynoclasts. From the entire palynomorph assemblage mainly dinoflagellate cysts were studied to give a stratigraphic outline for the Lower Cretaceous. Stratigraphic units were interpreted in terms of zones in use for the Jurassic and Cretaceous of Australia. At both sites a condensed Valanginian to Aptian sequence and an expanded middle to late Berriasian sequence containing a rich microplankton assemblage were recovered. Sites 762 and 763 can be correlated with each other and with the wells Eendracht-1 and Vinck-1.