ALBERT

Description: Volcanism across the North Tanzanian Divergence Zone (NTD), part of the East African Rift System, occurred episodically from the late Miocene to Recent. Here, we present a summary of previously published K–Ar and 40 Ar/ 39 Ar ages, new 40 Ar/ 39 Ar ages, and geochemical and Sr–Nd–Pb isotopic analyses on samples collected from several volcanoes distributed across the NTD: Burko, Monduli, Tarosero, Ketumbeine, Gelai, Kerimasi and Meru. The locus of volcanism over this period progressed from the southwestern portion of the NTD to the north and east, with a main pulse occurring at about 2.3 Ma, possibly marking the inception of a main rifting event. We model the source of the NTD volcanic rocks as a metasomatized subcontinental lithospheric mantle that includes minor and variable amounts of garnet and amphibole. REE data indicate variations in residual garnet content, consistent with varying depth of melt separation. Radiogenic isotopic data show systematic variations requiring the involvement of up to three components. Two alternative but not exclusive tectonic scenarios are proposed: one requiring the involvement of contributions from recent plume-related fluids, and one explaining the observed geochemical variations by melting of a lithosphere layered by multiple metasomatic events. Supplementary material: Details of analytical methods, operating system and calibration methods, a summary table of the recalculated 40 Ar/ 39 Ar and K–Ar NTD ages, a complete set of detailed release spectra analysis and dating figures, 40 Ar/ 39 Ar incremental heating data and analytical conditions, and examples of NTD calculated fractional crystallization modes are available at http://www.geolsoc.org.uk/SUP18813 .

Description: We used cores and logs from Integrated Ocean Drilling Program (IODP) Expedition 313 to generate biostratigraphic, lithofacies, biofacies, and geochemical data that constrain the ages and paleoenvironments of Pleistocene sequences. We integrate sequence stratigraphy on cores with new seismic stratigraphic data to interpret the Pleistocene history of the Hudson shelf valley and paleoenvironmental and sea-level changes on the inner to middle continental shelf. Improved age control compared to previous studies is provided by integrated calcareous nannofossil biostratigraphy, Sr isotopic stratigraphy, and amino acid racemization. We recognize four upper Pleistocene–Holocene sequences: sequence uP1 is correlated with Marine Isotope Chrons (MIC; "chron" is the correct stratigraphic term for a time unit, not "stage") 7 or 5e, sequence uP2 with MIC 5c, sequence uP3 with MIC 5a, and sequence uP4 with the latest Pleistocene to Holocene (MIC 1–2). However, within our age resolution it is possible that sequences uP2 and uP3 correlate with MIC 4–3c and 3a, respectively, as suggested by previous studies. Lower Pleistocene sequences lP1 and lP2 likely correlate with peak interglacials (e.g., MIC 31 and MIC 45 or 47, respectively). Thus, we suggest that preservation of sequences occurs only during peak eustatic events (e.g., MIC 45 or 47, MIC 31, and MIC 5), unless they are preserved in eroded valleys. The architecture of the Pleistocene deposits at Sites M27 and M29 is one of thin remnants of highstand and transgressive systems tracts, with lowstand deposits only preserved in the thalwegs of incised valleys. Incised valleys at the bases of sequences uP3 (IODP Site M27) and uP2 (IODP Site M29) document more southward courses of the paleo–Hudson valley, compared to the more southeastward course of the MIC 1–2 paleo–Hudson valley. The patchy distribution of Pleistocene sequences beneath the New Jersey inner-middle continental shelf is due to low accommodation during an interval of large eustatic changes; this predicts that sequences in such settings will be discontinuous, patchy, and difficult to correlate, consistent with previous studies in Virginia and North Carolina.

Description: Integrated Ocean Drilling Program Expedition 313 continuously cored and logged latest Eocene to early-middle Miocene sequences at three sites (M27, M28, and M29) on the inner-middle continental shelf offshore New Jersey, providing an opportunity to evaluate the ages, global correlations, and significance of sequence boundaries. We provide a chronology for these sequences using integrated strontium isotopic stratigraphy and biostratigraphy (primarily calcareous nannoplankton, diatoms, and dinocysts [dinoflagellate cysts]). Despite challenges posed by shallow-water sediments, age resolution is typically ±0.5 m.y. and in many sequences is as good as ±0.25 m.y. Three Oligocene sequences were sampled at Site M27 on sequence bottomsets. Fifteen early to early-middle Miocene sequences were dated at Sites M27, M28, and M29 across clinothems in topsets, foresets (where the sequences are thickest), and bottomsets. A few sequences have coarse (~1 m.y.) or little age constraint due to barren zones; we constrain the age estimates of these less well dated sequences by applying the principle of superposition, i.e., sediments above sequence boundaries in any site are younger than the sediments below the sequence boundaries at other sites. Our age control provides constraints on the timing of deposition in the clinothem; sequences on the topsets are generally the youngest in the clinothem, whereas the bottomsets generally are the oldest. The greatest amount of time is represented on foresets, although we have no evidence for a correlative conformity. Our chronology provides a baseline for regional and interregional correlations and sea-level reconstructions: (1) we correlate a major increase in sedimentation rate precisely with the timing of the middle Miocene climate changes associated with the development of a permanent East Antarctic Ice Sheet; and (2) the timing of sequence boundaries matches the deep-sea oxygen isotopic record, implicating glacioeustasy as a major driver for forming sequence boundaries.