Kerry Kelts

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the oceans planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Resedimented Facies in Ventura Basin, California, and Model of Longitudinal Transport of Turbidity Currents

An ash bed has been recognized as a chronostratigraphic horizon in the Ventura basin, California, at a level near the Pliocene-Pleistocene contact. Suites of samples just above and below the ash bed were taken for facies analyses from seven outcrop sections and one well. The rock types recognized on the basis of petrographic and microfaunal analyses are (1) tuff, (2) laminated shales, (3) mudstones, (4) cobbly mudstones, (5) sandstones, with graded bedding, (6) sandstones and siltstones, without graded bedding and commonly cross-laminated, and (7) conglomerates with mud matrix. These sediments are believed to be the products of four different kinds of sedimentary agents: (1) catastrophic sediment-gravity flows which deposited muddy conglomerates near the source, (2) turbidity currents which deposited graded sands distant from the source, (3) bottom currents which sorted and reworked turbidites and deposited cross-laminated silty sediments, and (4) surface currents which deposited laminated shales in the anoxic zone, and mudstones where bioturbation wa active. The sediment types are present in four characteristic associations: (1) the conglomerate facies characterized by an interbedded sequence of conglomerates, cobbly mudstones, mudstones, and sandstones; (2) the graded-sand facies characterized by sequences of turbidite sands with graded bedding, separated by laminated-shale intercalations; (3) the thin-bedded sand facies characterized by an interbedded sequence of thin-bedded, laminated, or cross-laminated sandstones, mudstones, and shales; and (4) the mudstone facies characterized by monotonous sequences of mudstones, with rare intercalations of laminated shales and cross-laminated siltstones. The paleogeography, paleobathymetry, paleooceanography, and geometry and trends of the sediment bodies led to the identification of those four associations, or facies, as sediments of (1) submarine canyons and fans, (2) basin trough, (3) basin flanks or continental rise, and (4) basin slopes or escarpments respectively of the Pliocene-Pleistocene Ventura basin. The Ventura facies model emphasizes longitudinal transport and deposition of turbidite sands as potential reservoirs in a basin-trough setting, and is thus different from the submarine-fan model which postulates deposition of fine sediments there.

Trübeströme in Seen: Sauerstoffeintrag durch grundnah eingeschichtetes Flusswasser

Near-bottom current measurements in the main delta areas of the Walensee (Switzerland) and of Lake Constance yielded evidence that sporadic high influxes of clastic suspensions caused by flood stage discharges of tributaries generate turbidity underflows. Turbid suspensions plunge down the delta front as hyperpycnal inflows with speeds up to 130cm/s. These periodic intrusions of well oxygenated river water may influence the oxygen budget of the basal hypolimnion, thus possibly antagonizing eutrophication in marginal cases, supported by the adsorption of phosphate by suspended clay particles.

Controls on Diatomaceous Lithofacies in Obliquely Rifted Marginal Basin: Gulf of California: ABSTRACT

DSDP Leg 64 dissected Quaternary sedimentation patterns in the Guaymas Basin which confirm many similarities but underline some differences with other Neogene circum-Pacific diatomite basins. Tectonic setting in this morphologically complex basin includes broad hemipelagic slopes, fault-controlled outer slope basins and highs, and relatively small transform-bounded, obliquely rifted deeper basins with complex ocean crust. Frequent mass flows are triggered from either muddy delta foreslopes or hemipelagic diatom ooze drape. These accumulate as mud turbidites in the narrow rift zones at rates exceeding 2,000 m/m.y. Interaction of climatic and oceanographic parameters control the intensity of biogenic productivity (ergo, the oxygen budget) producing alternating sequences of aminated and homogenous diatomaceous ooze, generally confined to slope regions (400 m/m.y.). Laminated diatom-ooze also accumulated in deeper basins which were deprived of turbidity flows during limited periods. Sediments in slope areas contain a uniform 4% carbon but CaCO3 (mostly foraminifera) ranges episodically from 2 to 25%. Phosphate occurs as fish-debris-rich laminae or rare, soft, centimeter-size pellets. Diagenetic dissolution of silica is recorded at Site 479 on the slope where finely laminated hard muds occurring below an unconformity at 380 m subbottom are now devoid of frustules, except those cemented in dolomite beds. Paradoxically, porcellanites were not encountered, although traces of clinoptilolite suggest that some silica reactions are presently active. Chert only occurs in proximity to basaltic intrusions. Dolomite precipitation occurs at shallow subbottom depths in zones of high alkalinity and methanogenesis, gradually forming decimeter-thick hard layers by slow vertical accretion. These layers commonly preserve primary fabrics. Petrologic and heavy carbon isotope evidence suggest that ions for dolomite precipitation are mainly derived from interstitial waters. End_of_Article - Last_Page 945------------