Calcareous Nannofossil Biostratigraphy, Leg 64

Abstract

To investigate the sedimentology ai\d the spreading history of the Gulf of California, holes were drilled in two different areas of the Gulf (Fig. 1). In most holes, calcareous nannofossils could be used to establish stratigraphy.To date the oldest sediments on continental and oceanic crusts, three sites were drilled off the tip of southern Baja California on an east-west transect from young oceanic crust (Site 474) to continental crust (Sites 475 and 476). A thick Pleistocene and upper Pliocene nannofossil-rich sedimentary sequence overlying basalt flows and sills was penetrated at Site 474. Pleistocene and Pliocene hemipelagic muds were recovered at Sites 475 and 476. At these sites, calcareous nannofossils indicate an earliest Pliocene age (between 5 and 4.6 Ma) for the oldest fossiliferous sediments found above the continental crust and reveal a major hiatus, corresponding to most of late Pliocene time.Five sites were drilled in the Guaymas Basin to investigate the sedimentology, paleoceanography, and biostratigraphy of an active spreading basin and the formation of oceanic crust. Sites 477 and 478 were located in the south rift and Site 481 in the north rift. Over 300 meters of upper Pleistocene diatom oozes and turbidites with abundant and well-preserved coccoliths were recovered at Sites 478 and 481. At Site 477, most of the sedimentary sequence had been hydrothermally altered and only 50 meters of upper Pleistocene sediments were fossiliferous.Sites 479 and 480 were drilled on the Guaymas Basin slope, an oxygen-minimum zone. Calcareous nannofossils were rare and poorly preserved in the hemipelagic, muddy, diatomaceous oozes penetrated at both sites. Because of sampling restrictions, Site 480 could not be studied.

Patterns and Mechanisms of Northeast Pacific Temperature Response to Pliocene Boundary Conditions

“…The Plio‐Pleistocene portion of the core from DSDP 475 is predominantly composed of hemipelagic muds, transitioning to diatomaceous muds in the mid‐to early‐Pliocene section (Curray et al., 1982). Age control for the whole record primarily comes from linear interpolation between 10 biostratigraphic tiepoints, provided in Table S1 in Supporting Information (Alexandrovich, 1989; Aubry, 1982; Bukry & Foster, 1973; Haslett, 2004; Molina Cruz, 1982; Raffi et al., 2006; Young, 1998). Monte Carlo simulated age model iterations suggest that site 475 has an uncertainty of 10–20 ka for each sample (Figure S9 in Supporting Information ; Thirumalai et al., 2016).…”

“…Monte Carlo simulated age model iterations suggest that site 475 has an uncertainty of 10-20 ka for each sample (Figure S9 in Supporting Information S1; Thirumalai et al, 2016). Sedimentation rates for DSDP 475 vary between 4.4 cm/kyr in the mid-to late-Pliocene (e.g., after 3.2 Ma), 3.4 cm/kyr in the early Pleistocene, and then 4.1 cm/kyr in the late Pleistocene (Aubry, 1982).…”

Late Miocene to Pliocene stratigraphic evolution of northeast Carmen Island, Gulf of California: implications for oblique-rifting tectonics

“…Hence, the¯oras are tentatively assigned to calcareous nannofossil zone CN12 of Bukry (1973Bukry ( , 1975. The absence of the early Pliocene forms does not appear to be a preservational problem and they have been routinely reported to be present in lower Pliocene deposits elsewhere within the Gulf of California region (Aubry, 1979;Carren Äo, 1992;McDougall et al, 1999). Finally, extinct species of benthic foraminifera also indicate a late Pliocene age for these sediments, and include Cassidulinella pliocenica and Amphistegina gibbosa.…”