John L. LaBrecque

Revised magnetic polarity time scale for Late Cretaceous and Cenozoic time

A revision of the Heirtzler and others magnetic reversal time scale is presented. In addition to incorporating published studies which have increased the resolution and accuracy of their time scale, we have revised the relative lengths of anomalies 4A to 5 and 29 to 34 and have eliminated anomaly 14. We have calibrated the time scale by choosing an age of 3.32 m.y. B.P. for the older reversal boundary of anomaly 2A and 64.9 m.y. B.P. for the older reversal boundary of anomaly 29. The resulting magnetic reversal time scale is in reasonable agreement with the biostratigraphic ages from Deep Sea Drilling Project (DSDP) drill holes.

A plate-kinematic framework for models of Caribbean evolution

Abstract We define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the Caribbean region. Newly determined poles of rotation defining the approximate spreading histories of the central North and the South Atlantic oceans are given. For the late Jurassic-Early Cretaceous anomaly sequence of the central North Atlantic, we have used previously published ∗ definitions of fracture-zone traces and magnetic anomaly picks, redetermining the pole positions and angular rotations for various isochrons on an Evans and Sutherland interactive graphics system. For magnetic anomalies younger than the Cretaceous Quiet Period in both oceans, we (1) used Seasat altimeter data to help define fracture-zone traces, and (2) identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones. By the finite difference method, the relative paleopositions and the relative motion history between North and South America were computed. This analysis defines the size and shape (and the rate at which the size and shape changed) of the interplate region between North and South America since the Middle Jurassic. Thus, a plate-kinematic framework is provided for the larger plates pertaining to the Caribbean region, in which can be derived more detailed scenarios for Gulf of Mexico and Caribbean evolution. North and South America diverged to approximately their present relative positions from Late Triassic? to Early Campanian (about 84 m.y. ago) time. This is the period during which the Gulf of Mexico and a Proto-Caribbean seaway were formed. Since the Campanian, only minor relative motion has occurred; from Early Campanian through to Middle Eocene times. South America diverged only another 200 km, and since the Middle Eocene, minor N-S convergence has occurred. These very minor post-Early Campanian motions have probably been accommodated by imperfect shear and compression along the Atlantic fracture zones to the east of the Lesser Antilles, and along the northern and southern borders of the Caribbean Plate. Accordingly, it is suggested that from Campanian time to the present, the relative motions between the North and South American plates have had only minor effects on the structural development of the Caribbean region. Primarily using the data of Engebretson et al. ∗∗ , the convergence history of Pacific plates with North America was calculated for two points near the western Caribbean. By completing finite difference solutions, the convergence history of the Pacific plates with the Caribbean and South American plates can be approximated. The direction and rate of convergence of the Pacific plates with the Americas may have controlled the style of subduction and possible microplate migration along the North American, South American and western Caribbean boundaries that define the eastern Pacific plate margin.

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.

Magnetic lineations in the Pacific Jurassic quiet zone

Magnetic anomalies of low amplitude (<100 gammas) are present in the Jurassic magnetic quiet zone of the western Pacific Ocean. These small anomalies are lineated and can be correlated among the Phoenix, Hawaiian and Japanese lineation patterns. Thus, they represent seafloor spreading that recorded some sort of magnetic field phenomena prior to magnetic anomaly M25 at 153 m.y. B.P. The most likely possibility is that they represent a series of late Jurassic magnetic field reversals that occurred during a period of anomalously low magnetic field intensity. We propose a time scale of magnetic reversals between 153 and 158 m.y. B.P. to account for these anomalies and suggest that the dipole magnetic field intensity increased by a factor of about four from 160 to 140 m.y. B.P. in the late Jurassic.

Seafloor spreading history of the Agulhas Basin

Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34−31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Abstract Ten years after the USAC ( U.S. – A rgentina– C hile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion. The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.

The isostatic gravity anomaly: key to the evolution of the ocean-continent boundary at passive continental margins

Abstract Isostatic gravity highs bordering the passive continental margins are interpreted as resulting from oceanic basement highs. These basement elevations are relics of the transient phenomenon of a higher ridge axis elevation during early rifting. The steep landward gradient in the isostatic gravity field, generally associated with a magnetic edge effect anomaly, delineates the boundary between oceanic and continental basement.

Late Eocene–Oligocene magnetostratigraphy and biostratigraphy at South Atlantic DSDP Site 522

Upper Eocene to lowest Miocene sediments recovered at Deep Sea Drilling Project (DSDP) Site 522 in the South Atlantic Ocean allow direct calibration of magnetostratigraphy and calcareous plankton biostratigraphy. The results from Site 522 show that the Eocene/Oligocene boundary occurs in the reversed interval of magnetic Chron C13 (= C13R) and that the Oligocene/IMiocene boundary probably occurs in the upper part of Chron C6C.

Late cretaceous—cenozoic magnetostratigraphic and biostratigraphic correlations of the South Atlantic Ocean: DSDP Leg 73

Abstract DSDP Leg 73 sediment cores allow direct calibrations of magnetostratigraphy and biostratigraphy for much of the lates Cretaceous to Cenozoic in the mid-latitude South Atlantic Ocean. A complete record of the Cenozoic was not obtained, however, because strong dissolution, poor core recovery and intense core disturbance have masked the biostratigraphy or magnetostratigraphy over some intervals of all recovered sections. DSDP Leg 73 results show the following correlations: Early/middle Miocene in Chron 16 Oligocene/Miocene within Subchron C6N Eocene/Oligocene within Subchron C13R Middle/late Eocene top of Chron C17 Early/late Paleocene top of Subchron C27N Cretaceous/Tertiary within Subchron C29R

DSDP Leg 73: Contributions to Paleogene stratigraphy in nomenclature, chronology and sedimentation rates

Abstract DSDP Leg 73 was successful in determining magnetostratigraphic—biostratigraphic correlations throughout much of the Paleogene. This paper treats three aspects of the data analysis. The first section treats the development of a chron nomenclature which facilitates the precise correlation of arbitrary events with respect to the geomagnetic polarity history. The second section analyzes the accuracy of radiometric dates for the Paleogene. The conclusion is that despite the recent advances in radiochronology ‘South Atlantic Standard’ remains the most convenient and probably the most reliable chronological standard. The final section studies the correlation in sedimentation rates between the Umbrian and South Atlantic sites. The conclusion is that sedimentation rate changes determined from magnetostratigraphy provide a high-resolution source of paleoenvironmental information. Strong correlations are noted between sites and with respect to other paleo-environmental studies involving oxygen isotope ratios, biogeography and CCD fluctuations within the Paleogene marine sediments.