GP41F-01
Micropaleontological and Paleomagnetic Characterization of La Ceiba K/T Boundary Section, Central Mexico
We report results of a micropaleontological and magnetostratigraphic study of the La Ceiba section that spans the K/T boundary. La Ceiba is located in central Mexico (20o 19.8' N, 97o 41.0' W) within the Tampico-Mizantla basin. The K/T boundary is marked by a clastic unit of about one-meter thickness intercalated between the carbonate hemipelagic marls of the Cretaceous Mendez Formation and the Paleocene Velasco Formation. The clastic unit can be divided into four sub-units, according to their texture and architectural characteristics following Arenillas et al. (2002). The basal sub-unit is formed by calcareous marls and is rich in shocked quartz and millimeter size spherules with microtektites and bioclasts of shallow water origin. The second sub-unit is formed by medium-grained sandstones, with clasts and quartz fragments, feldspars, metamorphic and calcareous fragments and re-worked benthic and planktic foraminifera. The third sub-unit is composed by a single body of medium- to fine-grained sandstones with tabular geometry. In this sub-unit, cross- and parallel-lamination trough cross-stratification, current ripples and climbing ripples have been observed. The top sub-unit is a tabular body of fine-grained sandstones, showing parallel-lamination and low-angle cross-lamination, with asymmetric ripples and burrow traces to the top. For the paleontologic and paleomagnetic study we collected twenty-five oriented samples across the section. We measured the low-field susceptibility, intensity and direction of the NRM. The vectorial composition and stability of NRM were analyzed by progressive thermal and alternating field demagnetization. Vectorial orthogonal diagrams and vector subtraction and principal component analysis were used to determine the characteristic magnetization and secondary components for each sample. The characteristic NRM negative inclination and southward declination in the K/T clastic sediments indicate a reverse polarity, which is correlated to reverse chron 29r that spans the K/T boundary. Micropaleontology analyses permit identification of six biozones. Two biozones (biozone of Rugoglobigerina scootti and Abathomphalus mayaroensis) correspond to the Maastrichtian. Four biozones (Guembelitria cretácea, Parvularogoglobigerina eugubina, Parasubotina pseudobulloides and Acarina trinidadensis) correspond to the Danian.
GP41F-02
Magnetostratigraphy of Lutetian marine sediments in the External Sierras (Southern Pyrenees)
Magnetostratigraphy has been extensively applied in the southern Pyrenees (Spain). At the moment there are more than 50 km of series most of them in continental and transitional facies and only a few in marine sediments. Seeing that Lutetian age has received very little attention and Ypresian - Lutetian transit in marine series has been rarely studied, in this work we present a composite Lutetian magnetostratigraphic study made of three sections of carbonate platform facies. The composite section has been calibrated with the available biostratigraphic studies (mostly benthonic faunas). The three sections are located in hanging wall of the Southwestern Pyrenean sole thrust (External Sierras). The SIV section (260 m and 105 samples), is located in the southeastern limb of the Balzez anticline, and comprises lower Guara Formation. The BZ profile (600 m and 285 samples) is located in the northwestern flank and cuts a progressive unconformity within the Guara formation. The ISU section (550 m and 237 samples) is located along the Isuela valley immediately south of the well-known Pico del Aguila anticline. In total more than 600 standard cores were sampled every 2 - 3 meters of section. Paleomagnetic analyses were conducted in the University of Burgos paleomagnetic laboratory. Detailed stepwise thermal demagnetization (every 25-50°C) and some AF demagnetizations were able to unravel the NRM components. IRM essays and the thermal demagnetization of three components IRM allowed controlling the magnetic mineralogy. A 2G magnetometer, an ASC TD-SC oven and a M2T-1 pulse magnetizer were utilized. Only one paleomagnetic component has been characterized between 200° - 250° and 450°C - 575°. This component represents a primary record of the Eocene magnetic field because it presents two antipodal directions and passes the fold test (Balzez data). Comparison with the global polarity time scale (GPTS) displays the following correlation; chrons C21 and C20 were unambiguously identified in the SIV section. Chrons C21 to C19 Chrons were identified in Isuela samples as well as in the long BZ section. This data perfectly fits with the expected Lutetian polarity sequence (GPTS). A remarkable result from this study is the lack of coincidence between the base of the Guara Fm. and the base of the Lutetian as it was previously assumed. This study demonstrates that the onset of the Guara platform sedimentation started within the normal polarity of chron 22 during Cuisian (Upper Ypresian) times. These sections, together with future biostratigraphic research, may become in a reference chronologic frame both for the Pyrenean basin and for global studies.
GP41F-03
High-Resolution Magnetostratigraphic and Rock Magnetic Cyclo-stratigraphic Study of the Eocene Arguis Formation, Spanish Pyrenees
In order to provide a high-resolution chronostratigraphic record, an 800 meter section of the Arguis Formation, an Eocene marine flysch deposited in the Jaca Basin of the Spanish Pyrenees was sampled for magnetostratigraphy and rock magnetic cyclo-stratigraphy. Oriented cores were collected to refine a previously determined magnetostratigraphy. Their paleomagnetic analysis shows that the oldest 600 meters of the Arguis Formation spans chrons C18n.1n to C17n.2n or about two million years. For 300 m of the section 395 un- oriented hand samples were collected for rock magnetic cyclo-stratigraphy at a 75 cm sampling interval (about every 4 kyr) in order to resolve the precessional index. Anhysteretic remanent magnetization (ARM) was measured for each sample to construct a ferromagnetic mineral concentration time series. Spectral analysis of the ARM time series, calibrated by the magnetostratigraphically-determined chron boundaries, shows periodicities consistent with astronomical forcing at Milankovitch frequencies. Decompaction of the time series using a newly developed rock magnetic technique removed the effects of differential compaction and improved the resolution of Milankovitch power. The time series was bandpass filtered and tuned to the precessional index using the La2004 orbital model. The precessional tuning improves spectral power at all orbital frequencies. Primary detrital magnetite controls the ARM signal with minor contributions from secondary authigenic pyrrhotite. The ferromagnetic mineral concentrations are most likely caused by climate-driven source area, runoff or aridity variations. They provide a high resolution (precessional scale) chronostratigraphic record that allows a detailed deformation rate study of a nearby growth fold.
GP41F-04
Intercalibration of Boreal and Tethyan Middle Triassic biostratigraphies: magnetostratigraphy from central Spitsbergen, Arctic Norway
Providing a high resolution intercalibration of high latitude and low latitude biostratigraphies potentially provides an important key in understanding Triassic climatic events at a global scale along with their relative timing. Here we present information which allows us to achieve this using an integrated magneto-biostratigraphic study of the Middle Triassic Botneheia Fm at Milne Edwardsfjellet, central Spitsbergen. The Botneheia Fm is a succession of mostly dark grey to black shales, more intensely laminated in its upper parts. Palaeomagnetic information was determined by combined thermal and alternating field demagnetization. The palaeomagnetic information from the Botneheia Fm are quite strongly contaminated by an overlapping present-day like magnetization, giving rise to mostly great-circle like trends to apparent Triassic-like magnetizations. The ammonoid biostratigraphic data suggests the Botneheia Fm at Milne Edwardsfjellet probably continues without significant break from the underlying Lower Triassic (Vikinghogda Fm). This is confirmed by the magnetostratigraphy, which indicates how to best equate Boreal- Tethyan successions at the Lower to Middle Triassic boundary. In spite of limited biostratigraphic constraints in the upper part of the Botneheia Fm, and insufficient magnetostratigraphic sampling density, over the condensed late Anisian-early Ladinian parts of the succession, the data does allow a clear magnetostratigraphic match to more intensely sampled magneto-biostratigraphies in Tethyan sections. This demonstrates the relationship between the disparate boreal and Tethyan biostratigraphies. A substantial hiatus characterizes the Ladinian- Carnian boundary (junction of Botneheia and Tschermakfjellet formations) in central Spitsbergen, so latest Ladinian magnetozones are absent.