Robert D. Jacobi

Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins

Abstract Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km2 in area, at least 600 km3 in volume of translated sediments. No apparent consist...

Peripheral bulge—a causal mechanism for the Lower/Middle Ordovician unconformity along the western margin of the Northern Appalachians

Abstract This report proposes a plate tectonic model that can explain the Early/Middle Ordovician erosional unconformity observed along much of the western margin of the Appalachian orogen. In order for the model to apply, the Taconic allochthons must represent an outer arc (accretionary wedge) and the related subduction zone and Benioff zone must have dipped east (this report reviews the evidence for these assumptions). If these suppositions are correct, then the observed unconformity may have resulted from upwarp along a peripheral bulge (which occurs seaward of present-day oceanic trenches) as the Ordovician continental margin drifted east into the trench. Theoretical calculations show that the amount of uplift experienced by a continental plate over a peripheral bulge is on the order of the amount of uplift observed on the unconformity in Newfoundland. Furthermore, the sequence of events in Taconic times along the western margin of the Appalachian orogen supports the hypothesis that the paleocontinental margin drifted east over a peripheral bulge and on into the trench. The Ordovician shallow-water carbonate bank on the continental margin of the North American plate was uplifted (peripheral bulge) and then rapidly down-dropped to abyssal depths (continental margin entering trench) where it was first covered by flysch and then structurally overlain by the Taconic allochthons (continental margin underthrusting the outer arc). The present western boundary of the maximum relief on the unconformity would delineate the trend and approximate position of the bulge when the craton jammed the subduction zone and ceased convergence with the island arc (in Caradocian times).

Sediment slides on the northwestern continental margin of Africa

Abstract A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km 2 were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.

Sediment slides and turbidity currents on the Laurentian Fan: Sidescan sonar investigations near the epicenter of the 1929 Grand Banks earthquake

The epicenter area of the 1929 Grand Banks earthquake on the continental slope south of Newfoundland has been investigated using Sea MARC I, a deeply towed, midrange sidescan sonar with a 4.5-kHz subbottom profiler. Shallow slides pass downslope into debris flows on the muddy continental slope east of the epicenter. At the head of the Eastern Valley of the Laurentian Fan, west of the epicenter, arcuate slide scars cut undisturbed upper-slope sediment and lead downslope to a lineated erosional seabed. At a water depth of about 1600 m, this erosional seabed passes into extensive fields of 100-m-wavelength gravel waves situated on the broad, irregular valley floor. The gravel waves become better developed downslope and extend at least to water depths of 3000 m. All these morphological features appear fresh on the sidescan sonograms, suggesting that they date from the 1929 earthquake event, and the distribution of slides corresponds to the area of instantaneous cable breaks in 1929. The upper limit of erosion on valley walls suggests that the 1929 turbidity current was less than 300 m thick. Timing of cable breaks downfan suggests that flow velocities were sufficient to rework gravel deposits into large bedforms during waning flow stages over elevated areas of the valley floor. Similar cross-bedded coarse sands and gravels are common in ancient channel deposits.

Sediment waves on the Moroccan continental rise

Abstract A nearly continuous zone of sediment waves is present on the lower continental rise off western Morocco which parallels the regional bathymetric trends. The individual sediment waves within the zone migrate upslope with time and, in general, also trend parallel to the regional bathymetric contours. These observations suggest that geostrophic contour currents are responsible for the formation of sediment waves. Physical oceanographic measurements and sea-floor photographs indicate only a very weak bottom circulation in this region. This suggests either that strong bottom currents are not essential for the formation of sediment waves or that relatively stronger bottom currents flowed along the continental margin of Morocco in the recent past. Turbidity flows may also influence the distribution of these sediment waves.

Bathymetry, Microphysiography and Reflectivity Characteristics of the West African Margin Between Sierra Leone and Mauritania

This report maps and analyzes the bathymetry and seafloor reflectivity characteristics along the northwest African margin between 3° and 23°N. The bathymetric trends strongly suggest that the morphology associated with the mid-ocean ridge (MOR) spreading system strikes north throughout the area mapped (north of ~10°N).

Basement faults and seismicity in the Appalachian Basin of New York State

Abstract Landsat lineaments identified by Earth Satellite Corporation (EARTHSAT, 1997) can be groundtruthed across the Appalachian Basin of New York State (NYS). Both fracture intensification domains (FIDs) and faults are observed in outcrop along the lineaments. Confirmation of deep structure associated with the surface structure is provided by both well log analyses and seismic reflection data (primarily proprietary). Additional faults are proposed by comparing the lineament locations with gravity and magnetic data. The result is a web of basement faults that crisscross New York State. By overlaying epicenter locations on the fault/lineament maps, it is possible to observe the spatial correlation between seismic events and the faults. Every seismic event in the Appalachian Basin portion of NYS lies on or near a known or suspected fault. It thus appears that not only are there more faults than previously suspected in NYS, but also, many of these faults are seismically active.

Boninites: characteristics and tectonic constraints, northeastern Appalachians

Abstract Boninites are high Mg andesites that are thought to form in suprasubduction zone tectonic environments as primary melts from refractory mantle. Boninites provide a potential constraint on tectonic models for ancient terranes that contain boninites because the only unequivocal tectonic setting in which “modern” boninites have been recognized is a fore-arc setting. Tectonic models for “modern” boninite genesis include subduction initiation (“infant arc”), fore-arc spreading, and the forearc side of intra-arc rifting (spreading). These models can be differentiated by the relative age of the boninites and to a lesser degree, geochemistry. The distinctive geochemistry of boninites promotes their recognition in ancient terranes. As detailed in this report, several mafic terranes in the northeastern Appalachians contain boninites; these terranes were situated on both sides of Iapetus. The characteristics of these boninites can be used to constrain tectonic models of the evolution of the northeastern Appalachians. On the Laurentian side of Iapetus, “infant arc” boninites were not produced ubiquitously during the Cambrian subduction initiation, unless sampling problems or minimum age dates obscure a more widespread boninite “infant arc”. The Cambrian subduction initiation on the Laurentian side was probably characterized by both “infant arc” boninitic arc construction (perhaps the >496 Ma Hawley Formation and the >488 Ma Betts Cove Ophiolite) and “normal” arc construction (Mt. Orford). This duality is consistent with the suggestion that the pre-collisional geometry of the Laurentian margin was complex. The Bay of Islands Complex and Thetford Mines ophiolite boninites are likely associated with forearc/intra-arc spreading during the protracted evolution of the Cambrian arc system. The relatively young boninites in the Bronson Hill Arc suggest that the Taconic continuous eastward subduction tectonic model is less tenable than other models. On the Gondwana side of Iapetus, the Tea Arm boninites of the Exploits Group stratigraphically rest on arc and MORB volcanics. This stratigraphy, and the relatively young age of the boninites (486 Ma), compared to assumed subduction initiation age (>513 Ma), suggest that the boninites may be more consistent with fore-arc spreading/intra-arc spreading. However, an “infant arc” model cannot be dismissed, and is commonly proposed for the nearby boninites in the Wild Bight Group.

Formation of regional cross-fold joints in the northern Appalachian Plateau

Abstract Fold-axis-parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn-orogenic cross-fold joints. Such a mechanism can be coupled with other joint-propagation models, providing a widely applicable resolution to the enigmatic origin of regional systematic joints. The fold-axis-parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross-fold joints and related deformational fabrics in the central and northern Appalachian Plateau. Modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross-fold joints on the convex side of the tectonic boundary. Simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross-fold joint patterns in the central and northern Appalachian Plateau. Modeling also displays a cratonward decrease in both the tangential stress and the tangential strain. Such a decrease is consistent with the deformation styles observed in the central and northern Appalachian Plateau.

Tectonic and eustatic signals in the sequence stratigraphy of the Upper Devonian Canadaway Group, New York state

We have refined the late Frasnian to early Famennian relative sea level curve based on detailed stratigraphic data from more than 1200 outcrops in the Appalachian basin of western New York. This curve is constructed from considerations of lithologies, bedforms, and ichnofacies. We document 3 sequences and 48 parasequences in the Canadaway Group, part of the marine component of the Catskill delta complex. Several apparent lowstands and one transgression are localized along the syndepositionally active Clarendon-Linden fault system; we infer that these systems tracts are forced, and result from interaction between fault-block motion and eustatic sea level changes. This detailed study thus demonstrates that sea level curves inferred from local foreland basins may have a stronger tectonic signal than formerly perceived.