William R. Normark

East pacific rise: hot springs and geophysical experiments.

Hydrothermal vents jetting out water at 380� � 30�C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Gal�pagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

Comparing Examples of Modern and Ancient Turbidite Systems: Problems and Concepts

A useful comparison of modern and ancient submarine fans can be based only on well-understood and thoroughly mapped systems. In addition, the examples selected for comparison must represent depositional systems similar in such characteristics as type of basin, size of sediment source, physical and temporal scales, and stage of development. Many fan sedimentation models presently in use do not meet these criteria.

Growth Patterns of Deep-Sea Fans

The growth pattern of a deep-sea fan relates events in and around the fan-valleys to the structure and morphology of the open fan. The growth pattern cannot be determined without knowledge of the origin and recent history of the fan-valley system. The mapping of La Jolla and San Lucas deep-sea fans with the deep-towed instrument package developed at Marine Physical Laboratory of the Scripps Institution of Oceanography details the fine-scale morphology, structure, and internal fill of the fan-valleys and suggests the growth patterns of these fans. The La Jolla fan, 20 km west of Scripps Institution, has one meandering fan-valley that extends across the entire fan. Except on the toe of the fan, the deeply incised valley has terraced walls with steeper walls on the outside of meanders. Very low-relief levees border the fan-valley in some localities. The present erosional valley bypasses the partly buried remnants of an older distributary system on the lower fan. The San Lucas fan, off the southern tip of the peninsula of Baja California, shows a depositional lobe of sediment, or suprafan, below the short, leveed fan-valley extending from San Jose Canyon. The suprafan appears as a convex-upward bulge on a radial profile of the fan. The surface of the suprafan has a series of discontinuous depressions up to 55 m deep and 1 km wide. The depressions are generally asymmetric in cross section, commonly have terraced walls, and are underlain by coarse sand and gravel. They are interpreted to be channel remnants. A model for deep-sea fan growth, based on this study, predicts that deposition on a fan will be localized in a suprafan at the end of large, leveed valleys commonly found on, and generally confined to, the upper reaches of deep-sea fans. The suprafan normally is on the midfan and is characterized by numerous smaller distributary channels. Rapid aggradation in the suprafan coupled with migration and meandering of the channels produces a surface marked by isolated depressions or channel remnants. Uniform deposition, producing a symmetrical half-cone morphology, results from the shifting through time of fan-valleys across the area of the fan.

Massive deep-sea sulphide ore deposits discovered on the East Pacific Rise

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise. Although their presence on the deep ocean-floor had been predicted there was no supporting observational evidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land. They contain at least 29% zinc metal and 6% metallic copper. Their discovery will provide a new focus for deep-sea exploration, leading to new assessments of the concentration of metals in the upper layers of the oceanic crust.

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.

Fan Valleys, Channels, and Depositional Lobes on Modern Submarine Fans: Characters for Recognition of Sandy Turbidite Environments

The growth-pattern concept for modern submarine fans has been reviewed and broadened by additional data published or obtained in the last five years. The similarities in morphology, structure, and surficial-sedimentation patterns among modern fans from different geographic and geologic settings support a general growth-pattern model that can be applied to ancient turbidite deposits. Most submarine fans have three recognizable morphologic divisions that are related to distinct facies associations for sandy and coarser turbidites. (1) The large-leveed valley(s) of the upper fan produce wide (1 to 5 km) valley-floor deposits that are the coarsest on the fan and are deposited in meandering or braided, shallow channels within the general confines of the valley. These coarse de osits grade laterally into finer grained and more regularly bedded levee sands and silts. (2) The middle-fan region is recognized as a convex-upward depositional bulge on a radial profile and includes a depositional lobe or suprafan at the terminus of the leveed valley. The coarsening- and thickening-upward sequence of sandy turbidites on the upper suprafan are cut by numerous channels, channel remnants, and isolated depressions, whereas the lower suprafan is relatively free of such features. Suprafan channels are generally less than 1 km across and probably are filled by thinning- and fining-upward sequences. (3) The lower fan division is characteristically free of channel features (and coarse turbidites), is nearly flat-wing or ponded, and, therefore, is indistinguishable morphological y from basin-plain or abyssal-plain settings in many cases. Basin shape and relief and the ultimate size of the fan appear less important than sediment-input parameters, such as the grain-size distribution and rate of sediment supply, in controlling development of the three morphologic divisions of the fan. Specifically, canyon-fed systems common along western North America tend to have a single-leveed valley terminating in a suprafan depositional lobe; some fans, such as the Monterey, have slightly more complex features where more than one canyon is involved in fan development. If the grain-size distribution is weighted toward the silt and clay fractions as in some delta-fed systems, the fans tend to have multiple-leveed valleys on the upper fan (although only one may be active at any given time), to have long valleys crossing much of the fan and to lack (or have poorly developed) suprafan relief.

An Integrated Approach to the Study of Turbidite Systems

Meaningful comparisons of modern and ancient turbidite systems must include consideration of the physical and temporal scales of the deposits as well as the limitations presented by the widely varying techniques used to map and describe the deposits. To facilitate such comparisons, we describe five basic elements of turbidite systems that, with appropriate types of field observations, can be recognized in both ancient and modern systems (i.e., in marine geologic, outcrop, or seismic-stratigraphie studies). The primary elements discussed are (1) major erosional features (other than channels), (2) channels, (3) overbank deposits, (4) lobes, and (5) channel-lobe-transition deposits. The determination of time-equivalent elements within any given turbidite system is necessary for deciphering depositional processes and sequence stratigraphy as well as for comparison with different systems to develop reliable, predictive sedimentation models for both modern and ancient submarine fans and other types of turbidite systems. This chapter is primarily intended to help stratigraphers and explorationists avoid being misled by the application of existing models. These models generally have not adequately taken into account either the complex interaction of the many factors that control turbi-dite deposition or the different stages of growth that many types of turbidite systems undergo with time during their evolution.

Diverse basalt types from Loihi seamount, Hawaii

Loihi seamount is the southeasternmost active volcano in the Hawaiian-Emperor volcanic chain. The seamount is considered representative of the early phase of Hawaiian volcanism because of its youth, small size, and location near the melting anomaly. Seventeen dredge stations recovered transitional basalt, alkalic basalt, and basanite, in addition to the expected tholeiitic basalt. Four flows of alkalic basalt contain common small dunite xenoliths. The recovered samples have thin palagonite rinds and almost no manganese on the glassy surfaces; we estimate that the lavas are less than about 4,000 yr old, and many are less than 1,000 yr old. Loihi seamount is apparently in a transitional growth phase between the early eruption of alkalic lavas and the commonly observed (subaerial) tholeiitic eruptive phase, previously thought to dominate Hawaiian volcanism from inception until the postcaldera collapse, alkalic stage.

Sandy fans--from Amazon to Hueneme and beyond

Most submarine fans are supplied with both sand and mud, but these become segregated during transport, typically with the sand becoming concentrated in channels and channel-termination lobes. New data from high-resolution seismic reflection surveys and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) wells from a variety of fans allow a synthesis of the architecture of those submarine fans that have important sand deposits. By analyzing architectural elements, we can better understand issues important for petroleum geology, such as the reservoir properties of the sand bodies and their lateral continuity and vertical connectivity. Our analysis of fan architecture is based principally on the Amazon and Hueneme fans, generally perceived to be classic examples of muddy and sandy systems, respectively. We recognize depositional elements, for example, channel deposits, levees, and lobes, from seismic reflection data and document sediment character in different elements from DSDP/ODP drill cores. We show the utility for petroleum geology of evaluating sandy and muddy elements rather than characterizing entire fans as sand rich or mud rich. We suggest that fan classification should include evaluation of source-sediment volumes and grain size, as well as the probable processes of turbidity-current initiation, because these factors control the character of fan elements and their response to changes in sea level, sediment supply, and autocyclic changes in channel pattern. Basin morphology, controlled by tectonics, influences overall geometry, as well as the balance between aggradation and progradation.

Highstand fans in the California borderland: the overlooked deep-water depositional systems

Contrary to widely used sequence-stratigraphic models, lowstand fans are only part of the turbidite depositional record; our analysis reveals that a comparable volume of coarse-grained sediment has been deposited in California borderland deep-water basins regardless of sea level. Sedimentation rates and periods of active sediment transport have been determined for deep-water canyon-channel systems contributing to the southeastern Gulf of Santa Catalina and San Diego Trough since 40 ka using an extensive grid of high-resolution and deep-penetration seismic-reflection data. A regional seismic-reflection horizon (40 ka) has been correlated across the study area using radiocarbon age dates from the Mohole borehole and U.S. Geological Survey piston cores. This study focused on the submarine fans fed by the Oceanside, Carlsbad, and La Jolla Canyons, all of which head within the length of the Ocean-side littoral cell. The Oceanside Canyon–channel system was active from 45 to 13 ka, and the Carlsbad system was active from 50 (or earlier) to 10 ka. The La Jolla system was active over two periods, from 50 (or earlier) to 40 ka, and from 13 ka to the present. One or more of these canyon-channel systems have been active regardless of sea level. During sea-level fluctuation, shelf width between the canyon head and the littoral zone is the primary control on canyon-channel system activity. Highstand fan deposition occurs when a majority of the sediment within the Oceanside littoral cell is intercepted by one of the canyon heads, currently La Jolla Canyon. Since 40 ka, the sedimentation rate on the La Jolla highstand fan has been >2 times the combined rates on the Oceanside and Carlsbad lowstand fans.