Brooks B. Ellwood

Magnetosusceptibility event and cyclostratigraphy method applied to marine rocks: detrital input versus carbonate productivity

Magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion. We show here that the magnetic susceptibility signature, found in two forms, resides mainly in paramagnetic and other detrital constituents in most marine rocks. The first form is a short-term, low-magnitude, high-frequency cyclic climate signature that is often useful for regional correlation. The second form is a longer term, higher magnitude, low-frequency signature resulting from transgressive and regressive events that can be used for global correlation. Fluctuations in detrital input, due to eustatic-based erosion, are the primary cause of events. These fluctuations are driven by large-scale processes such as global orogenic cycles. However, variations in carbonate productivity cannot be ruled out when explaining the low-magnitude climate-driven cyclicity also observed in magnetic susceptibility data sets.

The development of antarctic glaciation and the Neogene paleoenvironment of the Maurice Ewing Bank

Abstract A micropaleontologic, magnetostratigraphic, and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late Miocene to Holocene depositional and erosional history of the intermediate-depth Maurice Ewing Bank (MEB) located at the eastern extremity of the Falkland (Malvinas) Plateau, southwest Atlantic Ocean. Sedimentation on the MEB is controlled presently by the position of the Polar Front and strong bottom currents beneath the axis of the Antarctic Circumpolar Current (ACC). Fluctuations in the position of the Polar Front and in the intensity of the ACC are inferred to have largely controlled deposition on the MEB since the initiation of ACC flow over the bank in the Miocene. The sedimentary record of the MEB is used to infer the late Miocene to Holocene oceanographic and climatic conditions in the South Atlantic sector of the Southern Ocean. The MEB suffered major erosion by intense ACC flow during late Miocene time which exposed Cretaceous—Miocene sediment near the surface and shaped the present configuration of the bank. This erosional event is inferred to have occurred during late Miocene to early Pliocene time (∼7.2–4.7 m.y.B.P.) with the major phase of erosion occurring between middle magnetostratigraphic Chron 7 and late Chron 6 (∼7.2–6.2 m.y.B.P.). We suggest that the Miocene sedimentary record of the MEB and other paleoenvironmental evidence from the circum-Antarctic region precludes the existence of a West Antarctic ice sheet or extensive ice shelves along the Antarctic margin prior to the late Miocene. During the late Miocene a West Antarctic ice shelf formed in the former West Antarctic Sea and rapidly thickened until it grounded below sea level to form the West Antarctic ice sheet. Formation of this ice sheet, with floating and partially grounded extensions (ice shelves) in the Ross and Weddell embayments, led to the first major production of true Antarctic Bottom Water (formed in a manner similar to today) which permanently altered global abyssal circulation. Late Miocene changes in oceanic CCD levels and a permanent shift in the oceanic 13 C 12 C composition may be attributed to this major change in abyssal circulation. A decrease in ACC velocity during an early Pliocene amelioration of climate led to resumption of deposition on the bank from ∼4.7 to 3.9 m.y.B.P. Intensification of the ACC during the late Gilbert and early Gauss Chrons again resulted in limited deposition and widespread erosion and/or non-deposition over most of the MEB from ∼4.0 to 3.2 m.y.B.P. ACC velocity apparently began to wane during the middle of the Gauss Chron and allowed widespread deposition on the bank during late Gauss time (2.9–2.48 m.y.B.P.) and over more limited areas during earliest Matuyama time (2.48 to about 2.2 m.y.B.P.). During middle to late Matuyama time bottom current intensity reached its greatest post-Miocene level forming a regional disconformity between sediments of about 2.0 and 1.0 m.y. in age. This increased circumpolar circulation and associated erosion is inferred to have peaked during the late Matuyama Chron (1.2–1.0 m.y.B.P.) and is approximately the same age as the tentatively dated greatest Patagonian Glaciation in nearby southern Argentina. Between ∼1.0 and 0.7 m.y.B.P., the bank was blanketed with a coarse, erosion-resistant layer of ice-rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion. Sedimentation on the MEB during the Bruhnes Chron (720,000 yr B.P.–Present) was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest Pleistocene time.

Flow and emplacement direction determined for selected basaltic bodies using magnetic susceptibility anisotropy measurements

Abstract Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clusters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented. The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid properties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies.

Estimates of flow direction for calc-alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements: Central San Juan Mountains, southwest Colorado

Abstract Flow directions are estimated from the measurement of the magnetic fabric of 106 samples, collected at 18 sites in four welded tuff units in the central San Juan Mountains of southern Colorado. The estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites. Errors in the estimation may arise from topographic variation, rheomorphism (post-emplacement mass flow) within the tuff, and other factors. Magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth. A test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff. This test is positive for the four units examined. Paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest Colorado, during the time (28.2–26.5 Ma) of emplacement.

The Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) Method Used in Geological Correlation of Devonian Rocks from Anti-Atlas Morocco

Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) is used here as a means of establishing sensitive chronostratigraphic markers and chronohorizons useful for high-resolution correlation. MSEC is a composite of the magnetic susceptibility (MS) record of marine strata and the coeval biostratigraphic record and, similar to chronozones, zones based on MSEC data have boundaries that are isochronous. MS, a measure of the concentration of magnetic grains in sediments, proxies for the ratio of lithogenic to biogenic components. Controls on the detrital input of lithogenic material include eustasy and climate, however induced, and sea floor/basin subsidence. The development of an initial MSEC composite reference curve for the Middle-Upper Devonian of the Anti-Atlas region of southern Morocco is presented and discussed in the context of global sea level transgressive and regressive cycles. MSEC trends, composed of increasing MS magnitudes, correlate well with episodes of regression, whereas trends of decreasing MS magnitudes correlate with episodes of transgression.

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province, USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.

Paleocurrent indicators in deep-sea sediment.

The fabric and texture of deep-sea sediments have been used to detect periods of high-velocity bottom-water flow through the Vema Channel in the south-west Atlantic Ocean. In addition, the alignment of the long axis of magnetic grains has been used to indicate the direction of bottom-water flow.

Evidence for a diachronous Late Permian marine crisis from the Canadian Arctic region

A high-resolution chemostratigraphic study of a 24-m-thick section at West Blind Fiord on Ellesmere Island (Canadian Arctic) documents stepwise environmental deterioration in the marine Sverdrup Basin during the late Changhsingian (late Late Permian) as a result of volcanic disturbances to surrounding landmasses. A horizon within the upper Lindstrom Formation (datum A) is characterized by increased Fe-oxyhydroxide fl uxes and weathering intensity as well as modest shifts toward more reducing watermass conditions and higher marine productivity, recording an initial disturbance that washed soils into the marine environment. The contact between chert of the Lindstrom Formation and silty shale of the overlying Blind Fiord Formation, which is 1.6 m higher and ~50 k.y. younger than datum A, records a large increase in detrital sediment fl ux, more strongly enhanced marine productivity, and a regional extinction of siliceous sponges, herein termed the “Arctic extinction event.” The horizon equivalent to the latest Permian mass extinction of Tethyan shallow-marine sections is 5.6 m higher and ~100 k.y. younger than the Arctic extinction event, demonstrating the diachronous nature of the marine biotic and environmental crisis at a global scale; it is associated with intensifi ed anoxia and possible changes in phytoplankton community composition in the study section. Marine environmental deterioration in the Sverdrup Basin, probably triggered by terrestrial ecosystem deterioration and elevated detrital sediment fl uxes, was under way by the early part of the late Changhsingian, well before the onset of main-stage Siberian Traps fl ood basalt volcanism. The event sequence at West Blind Fiord may record the deleterious effects of early-stage explosive silicic eruptions that affected the Boreal region, possibly through deposition of toxic gas and ash within a restricted latitudinal band, while having little impact on marine ecosystems in the peri-equatorial Tethyan region.

Spatial and temporal changes in bottom-water velocity and direction from analysis of particle size and alignment in deep-sea sediment

Abstract We have demonstrated that the size and efficiency-of-alignment of particles in deep-sea sediment can be used to delineate relative velocity and direction of high-velocity bottom water. Our methods have been applied to core-top samples on the east flank of the Vema Channel which is the principal passage for northward flowing Antarctic Bottom Water (AABW) through the Rio Grande Rise in the southwest Atlantic. The size and alignment analyses delineated a zone of high-velocity bottom water below 4150 m which corresponds to the location of AABW as derived from physical oceanographic data. The eastern edge of the AABW was identified as a zone of high sedimentation rate due to the preferential deposition of fine-grained material marginal to the high-velocity flow. Three cores in that zone were used to examine the temporal changes in inferred bottom-water direction and relative velocity using a detailed stratigraphy based on magnetostratigraphy, biostratigraphy, fluctuations in percentage carbonate and oxygen isotopes. The cores were reoriented to the present geographic coordinate system using the stable remanent magnetism. The long-axis alignment of magnetic grains was used to infer bottom-water flow direction. The direction of flow was shown to be consistently northward due to the bathymetric control on the flow and corresponds to the direction determined by a current-meter near the site. The relative velocity of the AABW has fluctuated within the channel but no simple correlation was found with the paleoclimate. The size and alignment analyses were performed on core-top samples from the Amirante Passage in the western Indian Ocean. A zone of high-velocity bottom water was inferred below 4100 m and may correspond to a deep westward boundary current (DWBC) determined from physical oceanographic data. Regions of large sediment waves in the passage correspond to zones of low-velocity flow as inferred by our techniques. The sediment waves may have formed in a shear zone between the high-velocity DWBC and the lower-velocity overlying water mass. Our methods may be routinely applied to the study of deep-sea sediment. The initial results suggest that our geologic parameters correspond to the present benthic physical oceanography. Analysis of older sediments reveals that relative velocity of bottom currents has changed appreciably while the inferred current direction has remained stable due to the bathymetric control at the sites chosen for study. Deep-sea sediment, therefore, may act as a long-term fossil bottom-current meter.

The K–T boundary in Oman: identified using magnetic susceptibility field measurements with geochemical confirmation

Abstract Recognizing distal ejecta marker horizons and correlating these among widely separated sections is typically difficult in the absence of visually distinctive marker beds. Here we propose a magnetic susceptibility (MS) field method to locate such horizons, and explore the K–T boundary interval at Abat, Oman, as a test of the method. A distinctive pattern of high MS values was used to approximately locate the K–T boundary interval in a sequence of platform carbonates, which were then sampled in detail. Whole-rock geochemical enrichments in Ir, V, As, Ni, Co, Zn and Zr and a large negative carbon isotope anomaly confirmed the inferred boundary location. Common microspherules whose chemistry reflects the whole-rock geochemistry are associated with the boundary interval. The association suggests that the microspherules formed during or as a result of the impact. The geochemical record at the Abat locality shows two levels where Ir is high (>1 ppb) suggesting input from two impacts, separated in the section by 1.35 m. The upper level is chosen as the boundary interval based on the high Ir (1.19 ppb), δ13C negative excursion (∼7‰), and distinctive MS pattern.