Paleomagnetic Reversal Stratigraphy Research Articles

Lithostratigraphy, chronostratigraphy, and sedimentary environments of the middle Miocene Quebrada Honda Basin in southern Bolivia and implications for Andean climate and uplift

The late middle Miocene Quebrada Honda Basin of southern Bolivia has long been studied for its diverse fossils of terrestrial vertebrates and, more recently, for its record of paleoelevation and paleoclimate in the Eastern Cordillera of the Andes Mountains. This paleobiological and paleoenvironmental archive is constrained by high resolution radioisotopic ages linked to paleomagnetic reversal stratigraphy in multiple areas within the basin. Nevertheless, studies of the basin's stratigraphy have been limited and questions remain about how the sedimentary deposits and fossil horizons in different parts of the basin relate to each other. We confirm the identification of three lithostratigraphic units, show that the units extend across the basin, and examine the distribution of sedimentary and pedogenic facies and volcanic tuffs within them. We identify different styles and stages of pedogenic development that correspond to mappable differences in outcrop character. Volcanic tuffs and sedimentary and pedogenic facies exhibit high lateral continuity consistent with accumulation on a low relief basin floor. In contrast, sedimentary and pedogenic features change vertically over multiple scales reflecting secular variations in aridity and stability over seasonal to multimillennial scales. Erosional truncation of lithostratigraphic units and a ~ 700,000-year gap in the ages of superimposed strata delineate an erosional unconformity with >50 m of relief that was cut ~12 million years ago. Breach of the basin margin along a boundary between local structural blocks may account for the development of localized incision during a period when landscapes of the Eastern Cordillera were stable with little deformation or erosion. Regional landscape stability at that time has been attributed to low elevation or a position behind an orographic precipitation barrier. The sedimentary and pedogenic facies, faunal communities, and intraformational unconformity of the Quebrada Honda Basin favor a low elevation setting, which has implications for the timing and mechanisms of uplift of the Bolivian Andes.

Magnetostratigraphic constraints on the fossiliferous Ulantatal sequence in Inner Mongolia, China: Implications for Asian aridification and faunal turnover before the Eocene-Oligocene boundary

The transition from Eocene to Oligocene and its implications in the terrestrial realm has been a focal target for Cenozoic climate and environment research as it is widely considered the most dramatic climatic shift of the past 50 million years. Tibetan Plateau and proximal areas have been of utmost interest since the biogeographic relationships and understanding of the depositional environments in the region have remained unsettled during and after the Eocene-Oligocene transition (EOT). This study derives a first chronostratigraphic framework for Ulantatal, a fossiliferous area in Inner Mongolia, China. Based on paleomagnetic reversal stratigraphy and the constraints of faunal correlations, the time spanned in the strata is between ca. 35 and 27 Ma, thus exposing a long sedimentary succession ranging from the latest Eocene to late Oligocene. The lithological characteristics reveal these extensive fine-grained sediments mainly originate from eolian dust deposition, the onset of which is constrained at the latest Eocene (ca. 34.8 Ma). The presence of post “Mongolian Remodeling” fauna already in the late Eocene of Ulantatal demonstrates unequivocally that the major faunal turnover preceded the Eocene-Oligocene boundary, earlier to what has been recorded from other East Asian localities. The faunal composition predominated by rodents and lagomorphs remains strikingly stable across the Eocene-Oligocene boundary, suggesting the EOT related change in the animal communities was gradual or stepwise rather than abrupt. Moreover, the turnover into this environment dominated by small mammals can be linked with Eocene aridification of Asia, highlighting the dynamic responses of terrestrial systems to changing environment and climate associated with the EOT.

Climatic interpretation of a 1.9 Ma environmental magnetic record of loess deposition and soil formation in the central eastern Pampas of Buenos Aires, Argentina

Much of what we know about Quaternary climate has been learned from sedimentary records from the world’s oceans. With the exception of the extensive studies of the Chinese loess/paleosol sequence and more recent studies of long lake records, there are few long terrestrial climate records, particularly from the southern hemisphere. The loess record of Argentina provides an important opportunity to further our understanding of climate change from a terrestrial environment, but its complexity and discontinuity have led to difficulty in formulating a climatological model of depositional and pedogenic processes. In this study, we present one of the longest and most continuous loess/loessoid records from the central eastern Pampas of Argentina. Our age model is based on optically stimulated luminescent dates and a paleomagnetic reversal stratigraphy and indicates a basal age around 1.9 Ma. Within the age model uncertainties, we characterize the environmental magnetic properties associated with loess deposition and soil formation with respect to wind patterns, moisture availability, and temperature. Major changes in magnetic grain size are linked to a differential northward shift of the subtropical high-pressure cell during glacial periods. We suggest that coarser (finer) magnetic grains correspond to weaker (stronger) glacial periods when the high-pressure cell is located in a more southerly (northerly) position and the source region is more proximal (distal) to our study area. An abrupt increase in the ultrafine-grained magnetic material around 0.9 Ma is related to an increase in moisture transport from the South Atlantic driven by an increase in summer sea surface temperatures at the mid-Pleistocene transition (∼1 Ma). In addition to these grain size variations, there is a relative decrease in the amount of goethite compared to hematite beginning around 0.5 Ma, which has been related to the temperature increase observed after the mid-Brunhes Event (∼450 ka) in the EPICA ice core temperature record. A more detailed comparison to insolation indicates that, for portions of the record, ferrimagnetic minerals are depleted during periods of low insolation. This result suggests that Argentine loess deposition and soil formation follows a model more similar to the Alaskan loess sequences than the Chinese loess sequences. Although further work is needed to validate the models and mechanisms proposed in this study, our record indicates that the mineral magnetic properties of the loess and paleosol deposits record major changes in deposition and soil formation and provide insight into possible mechanisms relating to global and/or hemispheric climate change.

40Ar/ 39Ar dating, paleomagnetism, and tephrochemistry of Pliocene strata of the hominid-bearing Woranso-Mille area, west-central Afar Rift, Ethiopia

40Ar/ 39Ar dating of tuffs and mafic lavas, tephra geochemistry, and paleomagnetic reversal stratigraphy have been used to establish the chronostratigraphy of the Pliocene hominid-bearing fossiliferous succession at Woranso-Mille, a paleontological study area in the western part of the central Afar region of Ethiopia. The succession in the northwestern part of the study area ranges in 40Ar/ 39Ar age from 3.82–3.570 Ma, encompassed by paleomagnetic subchron C2Ar (4.187–3.596 Ma). One of the major tuff units, locally named the Kilaytoli tuff, is correlative on the basis of age and geochemistry to the Lokochot Tuff of the Turkana Basin. A hominid partial skeleton (KSD-VP-1) was found in strata whose precise stratigraphic position and age is still under investigation, but is believed to correspond to the later part of this interval. Woranso-Mille fills a significant gap in the fossil record of northeastern Africa at the time of the lower to middle Pliocene transition, when many extant species lineages of African fauna were established.

Precessional forcing of lacustrine sedimentation in the late Cenozoic Chemeron Basin, Central Kenya Rift, and calibration of the Gauss/Matuyama boundary

The fluviolacustrine sedimentary sequence of the Chemeron Formation exposed in the Barsemoi River drainage, Tugen Hills, Kenya, contains a package of five successive diatomite/fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the Central Kenya Rift. The overwhelming abundance in the diatomite of planktonic species of the genera Aulacoseira and Stephanodiscus, and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive, deep lake systems. A paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40Ar/ 39Ar dating of intercalated tuffs. The sequence spans the interval 3.1–2.35 Ma and bears a detailed record of the Gauss/Matuyama paleomagnetic transition. The 40Ar/ 39Ar age for this boundary of 2.589 ± 0.003 Ma can be adjusted to concordance with the Astronomical Polarity Time Scale (APTS) on the basis of an independent calibration to 2.610 Ma, 29 kyr older than the previous APTS age. The diatomites recur at an orbital precessional interval of 23 kyr and are centered on a 400-kyr eccentricity maximum. It is concluded that these diatomite/fluvial cycles reflect a narrow interval of orbitally forced wet/dry climatic conditions that may be expressed regionally across East Africa. The timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern Africa monsoon, rather than local circulation driven by direct equatorial insolation. This moisture event at 2.7–2.55 Ma, and later East African episodes at 1.9–1.7 and 1.1–0.9 Ma, are approximately coincident with major global climatic and oceanographic events.

Optically Stimulated Luminescence dating supports central Arctic Ocean cm‐scale sedimentation rates

This paper presents new results from Optically Stimulated Luminescence (OSL) dating on a sediment core raised from the crest of the Lomonosov Ridge in the central Arctic Ocean. There has been much debate about dating sediment cores from the central Arctic Ocean and by using an independent absolute dating technique we aim to test whether or not relatively fast, cm‐scale/ka, sedimentation rates were typical of Arctic's Pleistocene depositional mode. On the basis of mainly paleomagnetic reversal stratigraphy, many previous studies suggest mm‐scale/ka sedimentation rates. A common feature in these studies is that the first down core paleomagnetic negative inclination is consistently interpreted as the Brunhes/Matuyama boundary at about 780 ka. Our OSL dating results indicate that this assumption is not generally valid, and that the first encountered negative inclination represents younger age excursions within the Brunhes Chron, implying reinterpretation of many published core studies where paleoenvironmental reconstructions have been made for the central Arctic Ocean. Our dating results furthermore corroborates a correlation of the uppermost 2–3 m of the Lomonosov Ridge cores to a well‐dated core located off the Barents‐Kara Sea margin that in turn is correlated to cores in the Fram Strait. Valuable information on the paleoceanographical evolution in the Arctic Ocean from MIS 6 to the Holocene is given through this correlation of records from the central Arctic Ocean to records off the Eurasian continental margin.

40Ar/39Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya

40Ar/39Ar single-crystal laser-fusion dating, K–Ar dating, and paleomagnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6·56–3·8Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5·88–5·72Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5·31±0·03Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined40Ar/39Ar and paleomagnetic data constrain the age of this sequence to 4·63–3·837Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4·48–4·41Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17cm/ka over the 2·7Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses.

Integrated 3-D Geological Modeling of the C1 Sands Reservoir, Maui Field, Offshore New Zealand

Integration of core descriptions with paleontological data and a core-derived, paleomagnetic reversal stratigraphy have been used to provide a new conceptual depositional model for the Eocene C1 Sands reservoir of the giant Maui gas-condensate field. The lowest interval of the C1 Sands was deposited in coastal and lagoonal environments. A subsequent fall in relative sea level gave rise to progradation of channel-fill sands that form a multilateral, multistory sand body. Dipmeter data suggest that the lowermost story of these coalesced channel fills was tidally influenced. The bulk of the overlying C1 Sands was deposited as highstand, regressive shoreline sands that are partitioned by thin, tight transgressive deposits. A lowstand valley was incised into these sediments in the eastern and southern parts of the field. The fill of this valley is interpreted to be of a different age from laterally adjacent shoreline sediments on the basis of the paleomagnetic reversal stratigraphy. This model has been extended from the cored wells by careful calibration and correlation of wireline log response from all wells. Both the landward and seaward pinch-out of the shoreline sands have been delineated by interpretation of amplitude displays derived from the 3-D (three-dimensional) seismic data. The lateral extent of the lowstand valley fill is also defined by the 3-D seismic data. The well and seismic data have been combined, within a sequence stratigraphic framework, to construct a 3-D geological model of the field that is consistent with the dynamic performance of the reservoir and has been used to construct a 3-D reservoir simulation model. In the numerical geological model, sediment body thicknesses are defined on the basis of well data, and their lateral extents are constrained by the 3-D seismic data. The 3-D seismic data volume contains both chronostratigraphic and lithostratigraphic information that has been used to construct the reservoir model.

Integration of Sedimentological, Paleontological, Petrophysical, Paleomagnetic, and 3-D Seismic Data to Construct a 3-D Geological Model of the C Sands Reservoir, Maui Field, Offshore New Zealand

Integration of core descriptions with paleontological data and a core-derived paleomagnetic reversal stratigraphy has been used to interpret a conceptual depositional model for the Eocene C sands reservoir of the Maui gas-condensate field. The C1 sands were deposited as highstand, regressive shoreline sands that are partitioned by thin, tight transgressive deposits. Lowstand valleys were incised into these sediments in the eastern part of the field. On the basis of the paleomagnetic reversal stratigraphy, the fill of these valleys is demonstrably of a different age to laterally equivalent shoreline sediments. Dipmeter data indicate that the early fill of these valleys predominantly comprises flood-tidal sediments, deposited as relative sea level began to rise. This model has been extended from the cored wells by careful calibration and correlation of wireline logs from all wells. Both the landward and seaward pinch-out of the shoreline sands have been traced from amplitude displays of the three-dimensional (3-D) seismic data. The lateral extent of the lowstand valley fills is also defined by the 3-D seismic data. The well and seismic data have been combined within a sequence stratigraphic framework to construct 3-D geological models of the field that are consistent with the dynamic performance of the reservoir more » and will be used to construct a 3-D reservoir simulation model. « less

New Age Determinations for the Eocene-Oligocene Boundary Sediments in the Fayum Depression, Northern Egypt

The Fayum Depression of northern Egypt contains the richest collection of fossil vertebrates known from the Afro-Arabian Paleogene. This fauna has been collected for over a century, and yet the sediments containing the fossil localities have remained undated. Paleomagnetic sampling was carried out through the fossil-bearing Jebel Qatrani Formation and the underlying Dir Abu Lifa Member of the Qasr el Sagha Foundation to determine a correlation between the paleomagnetic reversal stratigraphy and the Geomagnetic Reversal Time Scale (GRTS). A new date of $23.64 \pm 0.035 Ma$ was determined for the overlying Widan el Faras Basalt by the laser incremental-heating $^{40}Ar/^{39}Ar$ dating method. The results of this study indicate that the majority of the vertebrate fossil localities occur in Chrons C13-C15r, late Eocene to early Oligocene in age. These age determinations provide a minimum date for the origin of higher primates.

The age of the Fayum primates as determined by paleomagnetic reversal stratigraphy

The age of the Fayum primates as determined by paleomagnetic reversal stratigraphy

Significance of Great Lateral Extent of Thin Units in Newark Supergroup (Lower Mesozoic, Eastern North America): ABSTRACT

The lacustrine sediments of the Newark Supergroup accumulated in rift valleys developed during the early phases of the fragmentation of Pangea. Analogy with modern rift valleys has led us to expect abrupt facies transitions and syndepositional structural complexity. However, in contrast to their supposed modern analogs, many thin (< 6 m) organic-rich lacustrine units within the Newark can be traced over large areas (+2,000 km2) with only gradual lateral changes in thickness and lithology. Lateral correlation is afforded by: (1) detailed ecostratigraphic correlation of fossils in lacustrine cycles, (2) key marker beds such as earthquake-induced fluidized beds, (3) detailed paleomagnetic reversal stratigraphy (by others), and (4) matching of microlaminae and turbidites. These observations suggest that the lakes which produced the organic-rich units were very large (+2,000 km2) and deep (+100 m) while at their maximum extent. The continuity of thin beds across the major intrabasin faults and the gradual lateral change in thickness over the basins show that these faults were not active during the deposition of the units and that the size of the subsiding blocks of these Mesozoic basins was much larger than is currently the case in the east African rifts. Simple analogy between Newark and African rift systems in structure and facies is not justified and obscures their real and important differences. End_of_Article - Last_Page 1444------------

Four rhyolitic tuff marker beds, Lower Pliocene, Wairarapa, New Zealand

Abstract Spooner Tuff, Sedd el Bahr Tuff, Missing Tuff and Hikawera Tuff are very thin water-laid tephras that mark time planes in Opoitian (lower Pliocene) sediments in Wairarapa, New Zealand, and have been dated by paleomagnetic reversal stratigraphy at 2.82, 2.87, 2.97 and 3.29 Myr respectively. The tephras are the only marker beds in a thick section of marine mudstone, and reveal a simple geological structure that could not otherwise be determined.