Robert E. Garrison

Carbonate Lithification on the Sea Floor

Indurated Globigerina-rich sediments have been recorded from more than thirty localities on sea floors around the world, between depths of 200 and 3,500 m. Many are stained by iron and manganese minerals or are associated with manganese nodules or crusts, suggesting that such lithification occurs along profiles of sedimentational equilibrium. Samples from the eastern Mediterranean and from off Barbados, here described and illustrated with photomicrographs and electron micrographs, contain high-magnesian calcite and dolomite, and show recrystallization of the micritic matrix, as well as calcitic cavity fillings. We conclude, contrary to widespread opinion, that calcite can be precipitated chemically in seawater, and that carbonate sediments can become lithified on the sea floor.

Geological history of the western North Pacific

A considerable portion of the abyssal floor of the western North Pacific was already receiving pelagic sediment in late Jurassic time. Carbonate sediments were later replaced by abyssal clays as the basin deepened and bottom waters became more aggressive. The resulting facies boundary, which can be recognized on seismic profiles, is broadly transgressive; it ranges in age from mid-Cretaceous in the western Pacific to Oligocene in the central Pacific. Cherts are encountered at and below the major facies boundary and appear to have been formed by postdepositional processes.

Sumeini group, Oman—evolution of a Mesozoic carbonate slope on a South Tethyan continental margin

Abstract Deposition of the mostly Mesozoic Sumeini Group occurred on the slope between the shallow-marine, Arabian carbonate platform and the deep-oceanic Hawasina Basin or South Tethys Sea. These strata record the evolution of the northeast Arabian continental margin from Permian(?) and Triassic rifting to ocean basin closing with the Late Cretaceous obduction of the Semail Ophiolite. Platform margin type and its evolution can be inferred from exposures of these slope sediments at Jebel Sumeini in the Western Oman Mountains. In the Lower Permian base of the sequence, bedded, spicular limestones suggest deep-marine conditions. This is followed by an enigmatic, thick sequence of bedded dolomite. In the Early to Middle Triassic (?), small carbonate submarine fans formed along a distally steepened slope. A subsequent interval of terrigenous, clastic sedimentation was followed by development of a steep escarpment margin during Ladinian to Norian time, with coralgal reefs at the shelf break and a debris apron at the base of the scarp. The Late Triassic is marked by another influx of terrigenous sediment which accompanied widespread emergence of the platform. In the Jurassic, a thick apron of thin-bedded limestone formed on the slope; this apron was cut by channelized gullies that now include blocks of older reefal material and oolitic sands. A well-documented submergence of the platform during the Tithonian and Early Cretaceous led to the deposition of radiolarian cherts on the slope and basin. Upper Lower Cretaceous megabreccias, up to > 200 m thick, suggest an interval of localized slope instability and/or tectonism. Continued deposition of bedded limestone and marl with lesser calcirudite marks the top of stratigraphic sequence at Jebel Sumeini. The Early to Middle Triassic (?) carbonate submarine fan deposits occur as discrete, lenticular sequences of bedded calciturbidite, modified grain flow and debris flow deposits which were derived from mostly slope sources. The Ladinian to Norian base-of-slope debris apron deposits were derived from both slope and shallow-marine sources (including reefs) and appear to form a laterally extensive but narrow belt of thick lenticular to sheet-like beds of debris which accumulated over a long interval of time. Coarse limestones in the Jurassic gullied slope deposits are distinct in that they occur randomly as individual channelized beds in a sequence dominated by thin-bedded lime mudstones. Thick Lower Cretaceous megabreccias are distinguished by their great lateral distribution and general lack of matrix.

Anatomy and origin of carbonate structures in a Miocene cold-seep field

Miocene calcite concretions resembling modern carbonate structures that form at cold seeps are present in fractured opal- CT porcelanites that are interbedded with mudstones in coastal cliffs at Santa Cruz, California. The morphologies of the carbonate structures differ markedly from conventional concretions and are spatially aligned with orthogonal joints in the porcelanites. The structures contain tubular holes that are identical to fluid and gas conduits in modern carbonate seep structures; the orientations of these tubes suggest that fluid and gas flow was both vertical and horizontal, the latter along extensional joints that formed preferentially in the brittle, silica-rich layers that had enhanced bedding- parallel permeability. Petrographic and isotopic characteristics of the carbonate structures indicate that calcite precipitation occurred in a shallow, subseafloor environment in either the zone of microbial sulfate reduction or of methanogenesis, prior to or possibly simultaneously with the silica phase transformation of opal- A in diatom shells to opal-CT.

Cyclic phosphate-rich successions in the upper Cretaceous of Colombia

Abstract Upper Cretaceous neritic to hemipelagic successions from the eastern Colombian Cordillera display frequent and rhythmic intercalations of phosphate-rich sediment. Their accumulation is attributed to a back-arc setting between the Andean arctrench system and the Guayana cratonic shield. In three examined sections near Tausa, Tunja, and Iza (all north of Bogota), respectively, the phosphate-rich sediments occur in 1–15 m thick coarsening-upward series ideally consisting — from the base to the top — of porcelanite, organic-rich claystone, siltstone, sandstone, and a condensed and thoroughly burrowed top bed. Phosphatic particles appear either in thin gravity-flow deposits or in pristine, in-situ occurrences near the base of these successions, intercalated in fine-grained biosiliceous or clay-rich sediment, or in the condensed top bed. The major portion of this coarsening-upward series (porcelanite to sandstone) is considered a shallowing-upward succession and the thin condensed phosphatic top bed a deepening-upward succession. These rhythmic successions are interpreted as parasequences resulting from fourth-order relative sea-level changes. Based upon biostratigraphic age estimates, the time span of formation of these parasequences range between approximately 100,000 and 200,00 yr. The allochthonous phosphate intercalations near the base of the parasequences are derived from condensed phosphatic top beds, which may have been exposed at the sediment-water interface in proximal directions. This suggests that the parasequence boundaries, i.e., marine flooding surfaces, are diachronous and become younger in onshore directions. using the vertical stacking patterns of these parasequences, we distinguish between transgressive and highstand-systems tracts (TST and HST). TSTs are characterized by the dominance of phosphatic sediment, laminated and organic-rich claystone, and laminated porcelanite. This suite of sediments documents high nutrient fluxes and the presence of an oxygen-minimum zone, both probably induced by coastal upwelling. HSTs include laminated to well-bioturbated siliciclastic successions, which may reflect a weakening or basinward shift of upwelling cells and higher levles of bottom-water oxygenation. The dominance of siliciclastics in HSTs is indicative of high detrital fluxes, which outpaced sediment-accomodation rates on the shelf. Upper Campanian ammonoids have been found in three levels of the Lower Plaeners Member of the Guadalupe Formation in the section near Tausa — Nostoceras (Nostoceras) liratum sp.n., Exiteloceras jenneyi (Whitfield, 1887), and Libycoceras sp. E. jenneyi is an important zonal marker in the U.S. Western Interior that is also known from the basal Mount Laurel Sand of Delaware, USA. Its occurrence at Tausa is the first record outside the USA and provides an important datum for intercontinental correlation. The type of Libycoceras sp. encountered in Tausa is also known from the upper Campanian of Peru and Angola. Together with the presence of Andalusiella polymorphia (Malloy, 1972), a dinoflagellate cyst, an age range is given for the formation of the Lower Plaeners Member at Tausa (late Campanian to early Maastrichtian).

Early lithification and hardgrounds in upper Albian and Cenomanian calcarenites, southwest England

Abstract Early diagenetic lithification of calcarenites on the sea floor led to the development of a variety of hardgrounds, intraformational conglomerates, breccias and boulder beds in southwest England during late Albian and Cenomanian time. Cemented nodules commonly developed below the sea floor, and these were avoided by burrowing infauna. In some instances the nodules were exhumed and reworked on the sea floor to form distinctive intraformational conglomerates; exposed nodules frequently became bored and encrusted by organisms and mineralised by glauconite and phosphate. In other cases, sea floor cementation produced true hardgrounds whose upper surfaces were affected by the same processes, but the hardened layers were only a few tens of centimetres thick and were underlain by soft sediment. Fracturing and brecciation of some hardground layers occurred through differential compaction and through undermining and collapse as the result of burrowing and erosion beneath the hardened layer. Reworking of clasts produced in this manner yielded intraformational breccias. Petrographic analysis reveals multiple generations of carbonate cement, commonly beginning with syntaxial overgrowths on echinoderm fragments and “dog tooth” spar on polycrystalline carbonate grains. A progression exists in the Albian-Cenomanian succession of southwest England from relatively simple hardgrounds and intraformational conglomerates low in the sequence up into complex hardgrounds that may record many stages of sediment accretion, cementation, mineralisation and erosion. This progression appears to record increasing water depths and increased sea floor diagenesis.

Phosphorus accumulation rates in a Miocene low oxygen basin: The Monterey Formation (Pismo Basin), California

Abstract Phosphorus (P) limits oceanic productivity on long time scales, and therefore determining changes in oceanic P accumulation through time is important for modeling oceanic paleoproductivity. We determined P concentrations and accumulation rates at Shell Beach, California, a section of phosphatic-rich marine sedimentary rocks which includes the widespread Phosphatic and Siliceous Facies of the Monterey Formation. Phosphorus concentrations had wide ranges in the major lithologies found in these facies, with values in phosphatic shales ranging from 0.36 to 3.9 wt%, compared to 0.02 to 0.42 wt% in dolomitic and siliceous strata. Phosphorus accumulation rates (in μmol P cm −2 yr −1 for all values reported here) showed significant variability with both sample lithology and position in the section. Phosphorus accumulation rates were much higher in phosphatic shale strata (1.5–29) than in dolomitic and siliceous strata (0.1–3.1) in both facies. Furthermore, P accumulation rates were generally higher in phosphatic shale strata of the upper Siliceous Facies (3.1–29) compared to the lower Phosphatic Facies (1.5–14). Mean P accumulation rates, calculated using linear sedimentation rate models for each facies and observations of the relative amount of phosphatic shale, dolomite, and siliceous strata, are comparable between the portions of Phosphatic and Siliceous Facies studied. This indicates that the two facies, with their faunal and sedimentological characteristics considered to reflect major oceanic and climatic changes during the middle Miocene, are similar in terms of P burial. Total P accumulation rates are also comparable to other phosphogenic Miocene and modern environments. We modeled variable sedimentation rates to examine the sensitivity of mean P accumulation rates to the linear sedimentation rate assumption. Increasing shale sedimentation rates to values above those estimated by linear models yields higher mean P accumulation rates in both facies, though they remain comparable in terms of net P burial. Modeling also revealed that, for increased shale sedimentation rates in each facies, siliceous sedimentation must be about three times faster than dolomitic strata sedimentation.

A Reservoir-scale Miocene Injectite near Santa Cruz, California

The Yellow Bank creek complex (YBCC) is a large, upper Miocene injectite complex, one of numerous injectites northwest of Santa Cruz, California. The feeder for these injectites is the Santa Margarita Sandstone, a shelfal sandstone unit that is also the reservoir rock in several exhumed oil fields. The impermeable cap rock for these oil fields, the Santa Cruz Mudstone, was breached by sand injectites, some of which reached the sea floor. Located near the edge of one of these oil fields, the YBCC is a dike-sill complex that shows evidence for multiple phases of injection by fluidized sand that was initially gas or water saturated and later possibly oil bearing. Vertical injection of a large sand dike along a fracture was followed by lateral injection of a sill from the dike along bedding planes in the Santa Cruz Mudstone. Flow differentiation during injection of fluidized sand into the sill formed centimeter-scale layering in its lower part. Subsequent emplacement of oil into this sand may have occurred by injection and by seepage that displaced pore water, producing sand masses that became preferentially cemented by dolomite. Some evidence suggests that the injection and cementation occurred at relatively shallow burial depths beneath the sea floor, with the injection resulting from a combination of possible seismic shaking and migration of overpressured fluids from more deeply buried parts of the Santa Margarita Sandstone. A pervasive lamination marked by limonite staining developed following uplift and subaerial exposure of the complex, possibly in a groundwater environment.

Inter- and Intrapillow Limestones of the Olympic Peninsula, Washington

Limestones between and within basalt pillows on the Olympic Peninsula originated in at least three ways. Some were generated when subaqueous pillowed flows and pillow breccias were extruded onto layers of pelagic carbonate sediment, incorporating it as clasts or forcing it into pockets between pillows. Others formed when pillowed basalt intruded carbonate ooze; the resulting rock frequently is a loosely packed pillow lava in which individual pillows float in thermally altered limestone. Another kind of limestone, however, was produced through gradual filling of cavities between and within pillows by later sediments; in most of these, precipitation of void-filling drusy calcite preceded and alternated with clastic sedimentation into the cavities. Crucial to the interpretation of these different limestones is whether they are younger or older than associated pillows. Determination of the age relations is nearly always possible by detailed examination of field and petrographic characteristics.

Petrology and paleogeographic significance of Tertiary nannoplankton-foraminiferal limeston, Guam

Abstract The tertiary stratigraphic column on the island of Guam, in the Mariana Island Arc of the western Pacific, is composed largely of volcanic rocks and limestones of shallow-water reef and bank facies. Small amounts of fine-grained limestones, of Eocene, Oligocene and Miocene age, occur interbedded with the volcanic rocks and reef limestones, and consist chiefly of planktonic foraminiferal tests embedded in a matrix of calcareous nannofossils. Although compositionally these pelagic carbonates are nearly identical to nannoplankton-foraminiferal oozes of the deep-sea floor, their deposition probably occurred at comparatively shallow depths — from a few hundred to perhaps 1000–2000 meters rather than several kilometers. The most favorable paleogeographic settings for pelagic deposition of this kind were probably intra-arc basins and shelf areas within the paleo-Mariana Island Arc during periods of volcanic inactivity. The spatial association of these pelagic limestones with shallow-water reef complexes, or with beds of redeposited, reef-derived coarse skeletal material, may serve to differentiate them from their deep-sea counterparts.