ALBERT

Description: The first and last appearances of Quaternary planktonic foraminifers in the Great Australian Bight were evaluated using datum levels from magnetostatigraphy, oxygen isotope stratigraphy, and calcareous nannofossil biostratigraphy to determine whether they were synchronous or diachronous with open-ocean biostratigraphic events. The first appearance of Globorotalia truncatulinoides is diachronous at 1.6-1.7 Ma at Site 1127 and 1.1-1.2 at Sites 1129 and 1132, similar to other local appearances in high latitudes. All other datum levels, however, are synchronous with open-ocean events, including the first appearance of Globorotalia hirsuta and the last appearances of Globorotalia tosaensis and pink Globigerinoides ruber in the Indo-Pacific region. A local reappearance of Gt. hirsuta at ~0.12 Ma and the disappearance of Globorotalia crassaformis at ~0.10 Ma were found to be useful for local biostratigraphy. Age control at the bottom of all of the sections is poor at this time, but results suggest that sedimentation recommenced starting at ~1.9 Ma above the regional unconformity that marks the base of seismostratigraphic Sequence 2. Sediment accumulation is distinctly reduced in the lower Pleistocene compared to the upper Pleistocene, perhaps in part because of processes associated with several omission surfaces.

Description: This report presents mineralogic and geochemical data from Ocean Drilling Program Leg 182 Site 1128 in the Great Australian Bight. Clay mineralogy is dominated by mixed-layer illite-smectite, followed by minor amounts of kaolinite and illite, with intervals of pure smectite. Carbonate mineralogy is exclusively low-Mg calcite, except for one interval of dolomite in lower Oligocene sediments. Carbonate increases significantly in upper Eocene sediments, decreases through the lower Oligocene, then increases again in the Neogene. Quartz is present as a minor component that covaries inversely with carbonate. High-resolution sampling associated with Chron 13 normal (early Oligocene) reveals high-frequency (~23 k.y.) fluctuations in clay mineralogy and carbonate abundance and a positive oxygen and carbon isotope excursion (in bulk carbonates) related to Antarctic glaciation.

Description: We report paleomagnetic data for the Lower to Middle Jurassic La Silla and Todos Santos formations of southern Mexico, in west-central Chiapas and the Tehuantepec Isthmus region. Volcanic rocks and red beds of these formations were deposited prior to or during the early stages of Gulf of Mexico opening. Dual-polarity characteristic magnetizations reside primarily in hematite and pass intraformational conglomerate, regional tilt, and reversal tests; they are thus interpreted as primary magnetizations. Our sampling sites are concentrated in three localities; around La Angostura Lake, 17 accepted sites yield a tilt corrected mean of declination (Dec) = 325{degrees}, inclination (Inc) = 4.6{degrees} (k = 11.9, {alpha}95 = 10.8{degrees}); in the Matias Romero region, the mean is Dec = 312.9{degrees}, Inc = 3.2{degrees} (based on only seven sites); and in the Custepec area, Jurassic andesitic dikes intruding rocks of the Permian Chiapas Massif yield a corrected mean of Dec = 335.0{degrees}, Inc = 5.0{degrees} (six sites). The mean directions are discordant with respect to expected North America reference directions, and indicate a counterclockwise rotation of 35{degrees} to 40{degrees}. Inclinations indicate deposition or emplacement at near equatorial paleolatitudes (2.1{degrees}N {+/-} 3.4{degrees}). This paleolatitude estimate is statistically indistinguishable from those previously observed in the La Boca Formation of the Huizachal Group in northeast Mexico. The localities we sampled in southern Mexico are separated by [~]150 km, suggesting that the paleomagnetic signature of these rocks reflects regional-scale rather than local deformation. These Jurassic paleomagnetic directions support a rotational origin for the Gulf of Mexico. The data are also consistent with an Early to Middle Jurassic reconstruction that places the Chiapas Massif offshore the Tamaulipas state in the western Gulf of Mexico, and an Euler rotation pole for the Maya Block for this time period in the eastern gulf. The apparent polar wander path defined by paleomagnetic poles for the Chiapas Massif and Jurassic rocks reported here suggests that relative motion between North America and the Maya Block occurred between Late Permian and Early Jurassic time, during a protracted rifting phase, and then in the Late Jurassic in association with seafloor formation in the Gulf.

Description: Stratigraphic relationships, detrital zircon provenance, U-Pb and 40Ar/39Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early-Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian-Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 {+/-} 3.0 Ma), Early to Middle Jurassic 40Ar/39Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196-161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian-Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.