Plio-Pleistocene Sequence Research Articles

Lithologic Prediction from the Stratal Architecture of Plio-Pleistocene Gulf of Mexico: Are the Eustatic Depositional Systems Tract Models Adequate?

Climatic/eustatic cycles of the Plio-Pleistocene have been defined in the northern Gulf of Mexico and precisely tied to their associated sequences and lithologies by means of graphic correlation. This framework has provided the data necessary for a detailed empirical evaluation of the eustatic depositional systems tract models. The key to this evaluation is a eustatic sea-level curve derived from fossil and isotope data. A curve of this type has been defined for several sequences. Using this eustatic curve the actual lithofacies and position of the various systems tracts were directly compared to those predicted by the models. The evaluation of the data with respect to eustatic sea level yielded conclusions that are significantly different from those predicted by the models. The evaluation of the data with respect to eustatic sea level yielded conclusions that are significantly different from those predicted by the model. The most significant of these differences are: (1) significant amounts of sand were deposited in deep water during transgressive and highstand intervals; (2) the observed vertical succession of eustatic depositional systems tracts within a given sequence are transgressive, highstand, and lowstand, and (3) factors other than eustacy have been the dominant influence on facies distribution within themore » Plio-Pleistocene sequences studied. These results demonstrate that depositional systems tracts and internal facies distribution could not be adequately described by a single model. Therefore, sequence stratigraphic analysis should be empirically based and conducted within the context of the basin, instead of being model driven.« less

Meteoric Diagenesis of Plio-pleistocene Limestones at Enewetak Atoll

ABSTRACT Petrographic, lithologic, mineralogic, and minor-elemental data from Plio-Pleistocene carbonates at Enewetak Atoll indicate that these sediments have been calcitized in the near-surface meteoric diagenetic environment. Shallow (< 100 m), mostly Pleistocene, sediments remain aragonite-rich with a few discrete meter-scale, calcite-rich intervals developed either at Pleistocene soil zones or in thin (< 5 m) paleophreatic lenses. Deeper ( 100 to 180 m), mostly Pliocene, sediments are characterized by pervasive calcitization of aragonite, extensive dissolution with well-developed moldic porosity, and the occurrence of isopachous bladed and pore-filling equant calcite cements. Pervasively-calcitized Pliocene limestones exhibit a narrow range in 18O value (±1 = 0.53), a wide range in 13C value (±1 = 1.63), and low mean Sr (0.02 wt %) and Mg (0.25 wt %) contents. These data are most consistent with diagenetic alteration of Plio-Pleistocene carbonates by meteoric phreatic calcitization, although in some intervals calcitization in the mixed meteoric-marine environment may have also occurred. Petrographic data document the superposition of subaerial exposure surfaces a d paleophreatic lenses in the subsurface of Enewetak. Sr-isotope data and stratigraphic forward modeling are consistent with the multiphase meteoric phreatic calcitization of Pliocene limestones in response to subaerial exposure shortly after sediment deposition and during Pleistocene sea-level lowstands. Quantitative water-rock interaction modeling of whole-rock chemical trends suggests that calcitization occurred in an open diagenetic system at water-rock (W/R) volume ratios of 103 to 105. Proxy indicators of climate (e.g., retention of aragonite at depth and the development of caliche at subaerial exposure surfaces) are consistent with an arid to semi-arid Plio-Pleistocene climate. Moreover, a limited supply of meteoric waters in the vadose environment, despite vertically and temporally extensive paleovadose environments, is consistent with limited calcitization of the Pleistocene sequence. Alternatively, previous estimates of the rates of vadose calcitization may be too high to be applicable at Enewetak. Stratigraphic shortening of the Plio-Pleistocene sequence in response to carbonate dissolution during atoll emergence is not consistent with available data.

Depositional Cycle Chart for Gulf of Mexico Plio-Pleistocene Strata: Documentation of Regionally Significant Events

Correlations from biostratigraphic checklists of foraminifera and calcareous nannoplankton have been integrated with correlations based on wireline logs and seismic studies to construct a Plio-Pleistocene depositional-cycle chart for the eastern offshore Texas area. The biostratigraphic data defining the sequence boundary and condensed section framework is presented in the form of two cross sections that record the stratigraphic position of all identified faunal discontinuities and pertinent biofacies information. High faunal abundance values define ten regionally significant condensed sections coincident with maximum flooding surfaces. Patterns or rapid change in biofacies distribution and low faunal depositional sequence boundaries. Ages assigned to condensed sections using key index fossils help correlate these faunal discontinuities and result in recognition of seven demonstrable and four probable regionally significant sequence boundaries, bounding ten depositional cycles. These local depositional cycles are compared with the depositional sequences of Beard et al. (1982), Lamb et al. (1987), Haq et al. (1988), Pacht et al (1990), and Wornardt and Vail (1990). The general are agreement for sequence boundaries, by both the above permits regional Plio-Pleistocene sequence stratigraphic analysis with resolution of approximately 100,000 years in the Gulf of Mexico. Tentative correlations of the biostratigraphic framework and depositional cycles with the oxygen isotope more » cycles of Trainor and Williams (1987) suggests that the observed depositional cycles are glacio-eustatic. « less

K-Ar and FT tephrochronology of a plio-pleistocene sequence: The Perrier formations (Massif-Central, France) revisited

K-Ar and FT tephrochronology of a plio-pleistocene sequence: The Perrier formations (Massif-Central, France) revisited

ESR dating of fault movement using various defect centres in quartz; the case in the western South Fossa Magna, Japan

The dates of movements of the Itoigawa-Shizuoka Tectonic Line (I-S T.L.)and Minobu Fault in the western South Fossa Magna have been obtained by ESR dating. Measurements were made on various lattice defects of the OHC, Ge, Al and E′ centres in quartz grains of fault clay and quartz veins intruded at the time of movement. All the dates obtained from these centres in quartz grains from two localities of the I-S T.L. and IWO localities of the Minobu Fault fall in the range of 0.30–0.55 Ma; this suggests that all the centres were annihilated completely. and that the obtained age can be admitted as the actual data of the latest fault movement. Meanwhile, ESR dates of quartz vein and fault clay in other localities (locations 3, 6, 7) are divided into 0.30–0.55 Ma and 1.5 –2.0 Ma, which suggest incomplete resetting. The dates of 0.30–0.55 and 1.5–2.0 Ma might correspond to the clino- unconformities observed in Plio-Pleistocene sequence of this region.

Diagenesis of Plio-Pleistocene Sediments of the Colorado River Delta, Southern California

ABSTRACT Interbedded mudrocks and fine-grained sandstones from three wells in the Plio-Pleistocene Colorado River delta in the Imperial Valley, southern California, were examined by X-ray powder-diffraction methods, chemical analyses, and scanning electron microscopy (SEM) to determine the extent and course of low-temperature metamorphic reactions. This sequence was compared to Tertiary rocks from the U.S. Gulf Coast in an effort to explore the kinetics of low-temperature reactions in natural systems. Mineralogical changes observed are 1) the progressive transformation of mixed-layer illite-smectite to pure illite over the temperature range 70°C to 210°C; 2) the apparent appearance of chlorite as a diagenetic mineral at 180°C to 194°C coincident with the onset of R (symbol) 3 ordering in the illite-smectites; 3) the loss of kaolinite above approximately 210°C; and 4) a progressive decrease in the amount of detrital potassium feldspar over the same temperature interval that brackets the illitization reaction. Comparison of bulk chemical analyses with analyses of the Comparison of Tertiary sequences with this Plio-Pleistocene sequence indicates that kinetics significantly influence the appearance of clay mineral assemblages at temperatures below approximately 175°C. At temperatures above 175°C, temperature, rather than time, appears to govern the reactions.

Age of the Okote Tuff Complex at Koobi Fora, Kenya

The Okote Tuff Complex crops out in the Koobi Fora region next to the north-eastern shores of Lake Turkana, northern Kenya. It occurs in the upper part of the ∼450 m thick, essentially flat lying, Plio-Pleistocene sequence of fluvial, deltaic and lacustrine sediments comprising the Koobi Fora Formation1–3. Many important archaeological sites and hominid fossils, including the Homo erectus finds discussed in the accompanying papers4,5, are stratigraphically close to the level of the Okote Tuff Complex6 or its correlatives, so that its precise numerical age is of great interest. Previously published isotopic ages, mainly by the 40Ar/39Ar dating technique, exhibited a very wide scatter7. Here we report comprehensive K–Ar data showing that pumices within one of the tuffs of the Okote Tuff Complex have an age of 1.64 ±0.03 Myr, which is in the earliest Pleistocene8.

High Precision Absolute Age Determinations Within the Plio-Pleistocene in Gulf of Mexico: ABSTRACT

Absolute ages can be determined from Plio-Pleistocene sequences in the Gulf Coast using an 18O/16O analytical approach. The technique is based on the fact that glacial/interglacial cycles possess uniquely shaped fingerprints. The character of the isotopic record in the Gulf of Mexico is exaggerated because the Gulf has periodically received extreme riverine discharges of meltwater products from the Laurentide ice sheet which covered large portions of North America during the Plio-Pleistocene. The 18O record thus allows age determinations with a precision of ±20,000 to ±50,000 years in the Plio-Pleistocene. Close sampling via sidewall cores is required for initial calibration in a given basin. Sections ranging from middle shelf o bathyal depths can be reliably zoned, independent of regional or local biostratigraphic and lithostratigraphic zonations. Once an absolute age framework is established, stratigraphic correlations, sedimentation rates, and structural effects on sediment distribution can be determined. The technique has been developed using holes drilled from the MV Eureka (part of an early Shell Oil offshore study in the Gulf) as well as strategically placed piston cores and DSDP sites. Both benthic and planktonic foraminiferal shell carbonate should be suitable for these age determinations. A similar Plio-Pleistocene zonation can be established worldwide based on analysis of DSDP material. Preliminary data suggest that a similar zonation may be established throughout the Tertiary and perhaps the Mesozo c. Effects of diagenesis are unknown. End_of_Article - Last_Page 540------------

Correlation of Plio-Pleistocene sequences in the northern Lake Turkana Basin: a summary of evidence and issues

Correlation of Plio-Pleistocene sequences in the northern Lake Turkana Basin: a summary of evidence and issues

Geological and palaeontological background of Hadar hominid site, Afar, Ethiopia

The hominid-bearing Plio–Pleistocene sequence of the west central Afar sedimentary basin in Hadar contains 120–140 m of fossiliferous lacustrine and lake margin sediments. The Hadar sequence is described and divided into four members. The mammalian fauna indicates an age of ∼ 3.0 Myr which is confirmed under the Hadar formation by K–Ar determinations on a basalt.

Marine and Terrestrial Pliocene and Pleistocene Deposits of Florida: ABSTRACT

For the first time fossiliferous marine, estuarine, and terrestrial deposits of peninsular Florida are correlated and are related to topographic terraces. Synthesis of all data reliably proves a middle to late Pliocene End_Page 2162------------------------------ stand of sea level at 120-140 ft altitude, an Aftonian stand at 90-100 ft, a Yarmouthian stand at 40-70 ft, and a Sangamonian stand at about 25 ft. The classic exposures of the Fort Thompson and Caloosahatchee Formations on the Caloosahatchee River are the best sections for correlation. Cooke's four marine units in the Plio-Pleistocene sequence are confirmed by detailed tracing of fossiliferous beds, fresh-water marl, and caliche crust related to unconformities. Faunas in the four marine beds show distinct biostratigraphic and paleoecological trends. The ratio of extinct to extant molluscan species indicates a Pliocene age for unit 1 (the lowest marine bed). The presence of Pleistocene land vertebrate remains and the ratio of extinct to extant molluscan species indicate a Pleistocene age for units 2, 3, and 4. For mapping purposes, the lower two units, 1 and 2, are included in the Caloosahatchee Formation. Units 3 and 4 are inclu ed in the Fort Thompson Formation. The following bathymetric interpretations were made on the basis of molluscan and foraminiferal faunal assemblages: unit 1--maximum depth 15-20 fm; unit 2--maximum depth 10-15 fm; unit 3--maximum depth 5-10 fm; and unit 4--less than 5 fm. End_of_Article - Last_Page 2163------------

Pliocene and pleistocene plant microfossils from drillholes near Frankton, New Zealand

Summary Plant microfossil samples from drillholes near Frankton, through some 700 ft of pumiceous and non-pumiceous sand, silt, clay and lignitic beds, range in age from Waitotaran (Upper Pliocene) to Upper Pleistocene or younger. The Plio-Pleistocene (Waitotaran-Nukumaruan) boundary is clearly shown where a sub-tropical Waitotaran vegetation is replaced by a Lower Nukumaruan vegetation, indicating a climate cooler than that of the present day. A brief comparison is made with similar Plio-Pleistocene sequences elsewhere in New Zealand.