Magnetic Polarity Stratigraphy Research Articles

Magnetic polarity stratigraphy of quaternary sediments from Ramganga paleolake, NW Himalaya, India and its paleoenvironmental implications

Paleomagnetic records from the sediments provide the foundation for studying variations in the geomagnetic field across the geological time sacle. Here, we report a new paleomagnetic data from Quaternary sediments of Ramganga Paleolake, NW Himalaya, India, to provide more insight into the global geomagnetic excursions and paleoenvironmental implications. Paleomagnetic and rock magnetic analyses were performed on a 5 m thick vertical profile of unconsolidated sediments from the Ramganga paleolake (29°46′867″N; 79°14′043″E), located in the Lesser Himalaya at a height of 798 m. One hundred fifty oriented samples from 50 sites (10 cm intervals) were obtained from the vertical section. Extrapolating the optically stimulated luminescence (OSL) dates from literature indicates that the entire section was probably deposited between 38 and 0.8 ka. The rock magnetic data shows that the remanence is carried by a low coercivity mineral and a significant amount of a soft magnetic mineral, most likely magnetite. Virtual Geomagnetic Pole (VGP) latitudes were calculated. Plot of the VGP latitudes against the sample locations from the litho-column allows us for the construction of Magnetic Polarity Stratigraphy (MPS) column. The MPS of the Ramganga paleolake has detected three geomagnetic excursions that are correlated with the global events: the Mono Lake Event (∼28.4-25.8 ka), the Gulf of Mexico Event (∼12.5–17 ka), and the Bagwalipokar Excursion Events (15.5-14.7 ka and 8-2.85 ka) respectively. The lower and middle portion of the paleolake represents the low and high energy depositional environment during the sediment deposition due to wet and arid climatic conditions.

First magnetostratigraphic results in the Aïn Beni Mathar-Guefaït Basin, Northern High Plateaus (Morocco): The Pliocene-Pleistocene Dhar Iroumyane composite section

The Aïn Beni Mathar-Guefaït Basin, in the High Plateau Region (Morocco), is dissected by the Za River, the main eastern tributary of the Moulouya, which incises more than 150 m into Plio-Pleistocene sediments. The main goal of the present study is to provide an initial geochronologic framework for such basin infill based on a combination of magnetostratigraphy and electron spin resonance (ESR). The combined results have implications on the age of the paleontological record, the evolution of the Moulouya River, and the activity of the faults that delimit the basin. We have studied sedimentary rocks that are essentially flat-lying and of an alluvial and lacustrine/palustrine origin. An approximately 140 m-thick section has been sampled at an average of 2.5 m per site, allowing to build a local magnetic polarity stratigraphy that includes nine geomagnetic reversals. Although no fold test is available, the presence of dual polarities and rockmagnetic analysis give us confidence that magnetization directions are primary. We then anchored the obtained magnetozones to the Geomagnetic Polarity Timescale (GPTS) using the biostratigraphic data as well as local geological observations. Our proposed magnetostratigraphy-based chronology reveals a Plio-Pleistocene sedimentary infill spanning from Gauss to Olduvai Chrons. The lower, detrital formations mostly fall within the normal Gauss Chron, whereas the upper lacustrine and palustrine carbonates, which are almost widespread to the top of the sedimentary fill, are Olduvai in age. These results provide the first chronological constraints for the basin fill in one of the largest intermontane basins of the High Plateaus. The new magnetostratigraphy also reveals that the major environmental change that triggered a switch from alluvial to lacustrine-palustrine conditions in Northern Maghreb occurred near the Gauss-Matuyama reversal, ca. 2.6 Ma. In addition, it shows that the age of the paleontological site Guefaït-4 is approximately 1 myr older than the ESR ages obtained from the quartz grains. The possible sources for this underestimation are discussed. However, we also acknowledge and discuss other possible chronostratigraphic interpretations of the current data, although less likely.

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

AbstractWe report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ∼44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Internal and External Modulation of Folding Rates With 104 to 105 Year Time Resolutions From Growth Strata, Pico del Aguila, Spain

AbstractHigh‐resolution cyclostratigraphy in growth strata are used to reconstruct unsteady folding rates at the regional‐scale Pico del Aguila anticline, southern Pyrenees, to evaluate deformation modulation. Magnetic polarity stratigraphy was used to calibrate cyclostratigraphy‐based anhysteretic remanent magnetization intensity variations to establish precessional frequencies in the growth strata record. The astronomically tuning was used to determine sedimentation rates and absolute time. Incremental tilting rates were calculated between selected horizons over ∼6 myr of fold growth. Careful treatment of uncertainties enhances confidence that the results are meaningful and results show significant variability in folding rates over time. The acceleration phase of fold growth was variable, punctuated by a prolonged period of tectonic quiescence, and correlated to sedimentation changes in the wedge‐top basin. Low dipping bedding intrinsically modulated the initial rates of folding for the first 25° of limb tilt, until ∼38.86 Ma. Then, halotectonics in the Paleogene Jaca Basin extrinsically modulated accelerating folding rates for the next 45° of limb tilting, until ∼37.42 Ma. Finally, forelimb‐steepening leading to geometric strain hardening and blunted folding rates for the last 17° of fold tightening and causing a thrust fault to cut the anticline's core. Folding ended at Pico del Aguila ∼35.10 Ma. Calculated folding rates varied between 0° ± 5.5°and 90° ± 19°/myr over 100s kyr time increments. Variations in the folding rate of the Pico del Aguila décollement anticline are attributed to both intrinsic modulation as a result of progressive bedding steepening during folding and extrinsic modulation as a result of variable deltaic sedimentation rates in the wedge‐top basin.

A tale of two Tweefonteins: What physical correlation, geochronology, magnetic polarity stratigraphy, and palynology reveal about the end-Permian terrestrial extinction paradigm in South Africa

Abstract The contact between the Daptocephalus to Lystrosaurus declivis (previously Lystrosaurus) Assemblage Zones (AZs) described from continental deposits of the Karoo Basin was commonly interpreted to represent an extinction crisis associated with the end-Permian mass-extinction event at ca. 251.901 ± 0.024 Ma. This terrestrial extinction model is based on several sections in the Eastern Cape and Free State Provinces of South Africa. Here, new stratigraphic and paleontologic data are presented for the Eastern Cape Province, in geochronologic and magnetostratigraphic context, wherein lithologic and biologic changes are assessed over a physically correlated stratigraphy exceeding 4.5 km in distance. Spatial variation in lithofacies demonstrates the gradational nature of lithostratigraphic boundaries and depositional trends. This pattern is mimicked by the distribution of vertebrates assigned to the Daptocephalus and L. declivis AZs where diagnostic taxa of each co-occur as lateral equivalents in landscapes dominated by a Glossopteris flora. High-precision U-Pb zircon (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) age results indicate maximum Changhsingian depositional dates that can be used as approximate tie points in our stratigraphic framework, which is supported by a magnetic polarity stratigraphy. The coeval nature of diagnostic pre- and post-extinction vertebrate taxa demonstrates that the L. declivis AZ did not replace the Daptocephalus AZ stratigraphically, that a biotic crisis and turnover likely is absent, and a reevaluation is required for the utilization of these biozones here and globally. Based on our data set, we propose a multidisciplinary approach to correlate the classic Upper Permian localities of the Eastern Cape Province with the Free State Province localities, which demonstrates their time-transgressive nature.

A magnetostratigraphic age constraint for the proximal synorogenic conglomerates of the Late Cretaceous Cordilleran foreland basin, northeast Utah, USA

AbstractReliable ages of proximal conglomerates in the Cordilleran foreland basin that are associated with emplacement and erosion of major thrust sheets are essential for reconstructing the kinematic history of the Sevier fold-thrust belt. Although these conglomerates have been dated by palynology, their absolute ages have been difficult to determine because of their coarse-grained texture and a lack of marine interbeds and tuffaceous deposits. We collected sets of oriented samples from outcrops in northeastern Utah, USA, to construct an overall magnetic polarity stratigraphy that can be correlated to the geomagnetic polarity time scale (GPTS). We sampled fine-grained, hematitic interbeds in the Upper Cretaceous Echo Canyon Conglomerate and Weber Canyon Conglomerate. Common paleomagnetic and rock magnetic analyses were conducted, and several rock magnetic results indicated that the dominant magnetic carriers in these weakly magnetized rocks are hematite and very subordinate magnetite/titanomagnetite/maghemite and goethite. Demagnetization results show that hematitic, fine-grained sandstone to siltstone intervals carry a geologically stable magnetization with directions and polarity consistent with the Late Cretaceous geomagnetic field. A small percentage of samples carry a low laboratory unblocking temperature secondary overprint residing primarily in goethite. Magnetic polarity results indicate that the Echo Canyon Conglomerate is exclusively of normal polarity and that the younger Weber Canyon Conglomerate is of normal polarity in its lowermost part, reverse polarity in the middle, and normal polarity in the upper part of the sequence. The new data indicate that these coarse-grained strata were most likely deposited over the time span of the magnetic polarity Chron (C) 34n to C33r interval and the younger C33r to C33n interval; the former interval includes the Santonian-Campanian stage boundary (ca. 83.4 Ma/83.1 Ma). Palynological data suggest that these rocks span Coniacian-Santonian time (ca. 89–84 Ma); thus, the most parsimonious correlation of the normal polarity magnetozone of the Echo Canyon Conglomerate is with the youngest part of C34n Superchron, which is of ca. 30 Ma duration (ca. 115 Ma to 83.4 Ma/83.1 Ma). The normal polarity magnetozone of the lower part of the younger Weber Canyon Conglomerate likely correlates to the youngest part of C34n, whereas the reverse and normal magnetozone from the middle and upper parts of the Weber Canyon Conglomerate likely correlate to C33r and C33n, respectively. We infer that the Santonian-Campanian boundary resides in the lower Weber Canyon Conglomerate, which implies that deposition of the unit started prior to the C34n/C33r boundary age (ca. 83.4 Ma/83.1 Ma) and continued through the C33r and C33n chrons. Sediment provenance data and growth structures tie the Echo Canyon and Weber Canyon Conglomerates to emplacement of the Crawford thrust sheet. Based on the magnetic polarity data, as constrained by the biostratigraphic age estimates from these synorogenic deposits, we hypothesize that the principal displacement along the Crawford thrust started during the Coniacian (>C34n/C33r boundary age) and continued into the middle Campanian (<C33r/C33n boundary age), from ca. 90–75 Ma, which is nearly 10 Ma longer than previously thought. The new age constraints demonstrate complete temporal overlap between proximal and distal coarse-grained deposits in this part of the Cordilleran foreland basin, coeval with active thrust displacement and rapid hinterland exhumation.

New paleomagnetic pole and magnetostratigraphy of the Cauvery Basin sediments, southern India

Magnetostratigraphy or magnetic polarity stratigraphy (MPS) is constructed with paleomagnetic parameters for the Campanian–Maastrichtian (Upper Cretaceous) onshore sediment sequences of the Cauvery Basin, southern India. Twenty-nine sedimentary outcrops in the vicinity of Ariyalur district of Tamil Nadu, India were studied using AF (5–150 mT) and thermal demagnetization (100–700°C). The observed remanence carrier is attributed to hematite through rock magnetic experiments. The virtual geomagnetic pole (VGP) latitudes were computed using the acquired characteristic remanent magnetization (ChRM) directions. The mean ChRM produce Dm= 338, Im= –38, (α95=23.91°, k = 15.73, N=60). The mean VGP is estimated at 51.33°N, 292.71°E deriving a paleolatitude of 21.3°S. The constructed magnetic polarity stratigraphy (MPS) is correlated with the standard geomagnetic polarity time scale (GPTS). The composite MPS of these sequences comprises of 12 magnetozones (6 normal and 6 reversed events) that are corroborated with Chron C33n to Chron C30n of the GPTS. The derived paleolatitude position from the present study places Indian subcontinent at little shallow southern latitudes indicating moderately higher drift velocities during Upper Cretaceous.

Environmental signals of Pliocene-Pleistocene climatic changes in Central Europe: Insights from the mineral magnetic record of the Heidelberg Basin sedimentary infill (Germany)

Abstract The entrance of Earth's climate into the present icehouse state during a time of rapid temperature decline in the late Pliocene was intensively investigated during the past decade. Even though it is well documented in marine archives, a detailed reconstruction of the Pliocene-Pleistocene climatic evolution of Central Europe is hampered by a general lack of data. The work presented here is based on sedimentary material from drill cores obtained at three sites within the Heidelberg Basin (Germany). The scientific relevance of this unique archive was discovered only in the last decade. The hundreds of metres thick sequences of mainly fluvial sediments record the evolution of the environment and climatic conditions during the late Pliocene and the entire Pleistocene of western Central Europe. In our present study, we implement unpublished mineral magnetic S-ratio data, and new evidence from X-ray analysis into two previously completed studies on the magnetic polarity stratigraphy, and the magnetic mineralogy of the Pliocene to Pleistocene sediments of the Heidelberg Basin. The total set of data enables distinction of environmental and climatic processes, and unveil details on the climatic conditions of continental Europe during this period. We demonstrate the dominance of a warm climate with alternation of humid and dry periods during the Pliocene. Cyclic variations in the groundwater table in the Rhine flood plain resulted in redox fluctuations, which led to the decomposition of the primary detrital mineral assemblage. Authigenic Fe oxides, particularly haematite, formed during dry periods. A rapid transition into cooler and moister conditions occurred at the end of the Pliocene, as indicated by the persistence of Fe sulphides, especially greigite. A high groundwater table and the associated reducing conditions have largely persisted to the present day. We show that the rapid transition from warm to cooler and moister climatic conditions in Central Europe during the final Pliocene is a regional response to the intensification of Northern Hemisphere glaciation (iNHG). This work supplements existing knowledge of the climatic evolution of Central Europe during the Pliocene-Pleistocene transition by data from a region from which little data has been available. A sideglance to climatic archives elsewhere in the Northern Hemisphere (e.g., North Atlantic Ocean, Chinese Loess Plateau, Russian arctic) is used to show the coincidence of the iNHG events in quite different environmental regimes.

Magnetic polarity stratigraphy, provenance, and paleoclimate analysis of Cenozoic strata in the Qaidam Basin, NE Tibetan Plateau

AbstractThe growth and deformation history of the Qilian-Nan Shan thrust belt bounding the NE Qaidam Basin figures importantly in testing models of Tibetan Plateau uplift during the India-Asia collision. However, debate exists about the onset of uplift and exhumation of the Qilian-Nan Shan, with timing estimates ranging from early Paleocene to late Miocene. Here we report integrated analyses of magnetostratigraphy, anisotropy of magnetic susceptibility, sediment provenance, and paleoclimate (using environmental magnetic parameters) for Cenozoic fluvio-lacustrine strata from the Dahonggou section south of the Qilian-Nan Shan. The results are interpreted to demonstrate an early Miocene (ca. 20 Ma) onset of sediment accumulation in this location, with clastic sediment derived initially from the southern Qimen Tagh highland. The sediment source then switched to the northern part of the Qilian Shan region after ca. 18.5 Ma, consistent with initial uplift and exhumation of the Qilian Shan. Thereafter, two additional provenance shifts reveal progressive southward propagation of deformation in the Qilian-Nan Shan. As a result of this southward growth of Qilian-Nan Shan topography, precipitation increased after ca. 11 Ma at the study site due to orographic interception of moisture from the south. This work improves our understanding of the depositional age, sediment provenance, and paleoclimate history of the Qaidam Basin and reveals a prolonged history of Qilian-Nan Shan deformation and uplift, which may have accelerated during the late Miocene.

Colorado Plateau Coring Project, Phase I (CPCP-I): a continuously cored, globally exportable chronology of Triassic continental environmental change from western North America

Abstract. Phase 1 of the Colorado Plateau Coring Project (CPCP-I) recovered a total of over 850 m of stratigraphically overlapping core from three coreholes at two sites in the Early to Middle and Late Triassic age largely fluvial Moenkopi and Chinle formations in Petrified Forest National Park (PFNP), northeastern Arizona, USA. Coring took place during November and December of 2013 and the project is now in its post-drilling science phase. The CPCP cores have abundant detrital zircon-producing layers (with survey LA-ICP-MS dates selectively resampled for CA-ID-TIMS U-Pb ages ranging in age from at least 210 to 241 Ma), which together with their magnetic polarity stratigraphy demonstrate that a globally exportable timescale can be produced from these continental sequences and in the process show that a prominent gap in the calibrated Phanerozoic record can be filled. The portion of core CPCP-PFNP13-1A for which the polarity stratigraphy has been completed thus far spans ∼215 to 209 Ma of the Late Triassic age, and strongly validates the longer Newark-Hartford Astrochronostratigraphic-calibrated magnetic Polarity Time-Scale (APTS) based on cores recovered in the 1990s during the Newark Basin Coring Project (NBCP). Core recovery was ∼100 % in all holes (Table 1). The coreholes were inclined ∼60–75∘ approximately to the south to ensure azimuthal orientation in the nearly flat-lying bedding, critical to the interpretation of paleomagentic polarity stratigraphy. The two longest of the cores (CPCP-PFNP13-1A and 2B) were CT-scanned in their entirety at the University of Texas High Resolution X-ray CT Facility in Austin, TX, and subsequently along with 2A, all cores were split and processed at the CSDCO/LacCore Facility, in Minneapolis, MN, where they were scanned for physical property logs and imaging. While remaining the property of the Federal Government, the archive half of each core is curated at the NSF-sponsored LacCore Core Repository and the working half is stored at the Rutgers University Core Repository in Piscataway, NJ, where the initial sampling party was held in 2015 with several additional sampling events following. Additional planned study will recover the rest of the polarity stratigraphy of the cores as additional zircon ages, sedimentary structure and paleosol facies analysis, stable isotope geochemistry, and calibrated XRF core scanning are accomplished. Together with strategic outcrop studies in Petrified Forest National Park and environs, these cores will allow the vast amount of surface paleontological and paleoenvironmental information recorded in the continental Triassic of western North America to be confidently placed in a secure context along with important events such as the giant Manicouagan impact at ∼215.5 Ma (Ramezani et al., 2005) and long wavelength astronomical cycles pacing global environmental change and trends in atmospheric gas composition during the dawn of the dinosaurs.

Recognizing magnetostratigraphy in overprinted and altered marine sediments: Challenges and solutions from IODP Site U1437

AbstractCore disturbance, drilling overprints, postdepositional acquisition of remanence, authigenic growth of magnetic iron sulfides, and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments. We address these issues in a sequence of tuffaceous muds and volcaniclastics at International Ocean Discovery Program Site U1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling. Remanence measurements were filtered to remove intervals affected by fluidization, plastic sediment disturbance, and core biscuiting. Drilling overprints are concentrated in the disturbed annulus surrounding intact core material. Bioturbation was limited to a vertical extent of at most 15 cm. Changes in sediment color, stiffness, and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment–water interface. We did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums. Authigenic growth of greigite, in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate, has led to magnetic overprinting. Anomalous polarity artefacts, extending <5 m and occurring within about 20 m below a real polarity transition, appear to be due to a chemical remanence acquired by greigite produced during early diagenesis. Diagenetic magnetic mineral alteration resulted in the progressive loss of fine‐grained magnetite, which enhanced susceptibility to drilling and postdrilling overprints and increased the resistance of these overprints to removal by conventional demagnetization. We recovered the magnetostratigraphic record from many samples with resistant overprints through low‐temperature demagnetization through the Verwey transition.

Magnetic polarity stratigraphy of the Siwalik sequence in Nurpur area, NW Himalaya, India

Magnetic polarity stratigraphy studies were carried out on Jabbar Khad section of Nurpur area, Kangra District, Himachal Pradesh, India. Oriented block samples were collected at 202 sites with 3–5 samples per site from a 2350-m thick continuous sedimentary sequence consisting of sandstone–mudstone alternating beds. Specimens were subjected to thermal demagnetization (100–700∘C) after measuring their natural remanent magnetization (NRM). Palaeolatitudes of virtual geomagnetic pole (VGP) from each site were calculated from the acquired characteristic remanent magnetization (ChRM) directions. The observed remanence carrier could be haematite. Magnetic polarity scale (MPS) was erected. This MPS is correlated with the standard geomagnetic polarity time scale (GPTS) of Lourens et al. (2004). The MPS readily matched from chron C2An.1n to chron C4n.2n, therefore assigning an age 8.20–2.60 Ma to the middle and upper Siwalik sections at Nurpur. The rate of sedimentation in middle Siwalik is 29 mm/yr and upper Siwalik is 59 mm/yr.

Paleoelevation records from lipid biomarkers: Application to the tropical Andes

Research Article| November 01, 2015 Paleoelevation records from lipid biomarkers: Application to the tropical Andes Veronica J. Anderson; Veronica J. Anderson † 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA †E-mail: v.anderson@utexas.edu Search for other works by this author on: GSW Google Scholar Joel E. Saylor; Joel E. Saylor 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Timothy M. Shanahan; Timothy M. Shanahan 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Brian K. Horton Brian K. Horton 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Veronica J. Anderson † 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Joel E. Saylor 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Timothy M. Shanahan 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Brian K. Horton 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA †E-mail: v.anderson@utexas.edu Publisher: Geological Society of America Received: 28 Mar 2014 Revision Received: 11 Jan 2015 Accepted: 15 Mar 2015 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2015 Geological Society of America GSA Bulletin (2015) 127 (11-12): 1604–1616. https://doi.org/10.1130/B31105.1 Article history Received: 28 Mar 2014 Revision Received: 11 Jan 2015 Accepted: 15 Mar 2015 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Veronica J. Anderson, Joel E. Saylor, Timothy M. Shanahan, Brian K. Horton; Paleoelevation records from lipid biomarkers: Application to the tropical Andes. GSA Bulletin 2015;; 127 (11-12): 1604–1616. doi: https://doi.org/10.1130/B31105.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New results from two novel lipid biomarker–based proxies help to constrain the late Cenozoic surface elevation history of the Eastern Cordillera in the tropical northern Andes of Colombia. Previous well-known studies have suggested rapid latest Miocene–Pliocene (6–3 Ma) uplift on the basis of an abrupt shift in pollen species assemblages within sedimentary basin fill of the elevated Bogotá Plateau. From resampling of these original study localities, we provide a revised chronology based on magnetic polarity stratigraphy, and we evaluate paleotemperatures using the MBT′/CBT indices (methylation of branched tetraethers/cyclization of branched tetraethers) and hydrogen isotopic composition (δD) of leaf waxes as two independent proxies of past surface elevation. Reconstructed paleotemperatures from the MBT′/CBT proxy show a more gradual cooling trend from ca. 7.6 Ma to present, consistent with less than 1000 m of elevation gain since latest Miocene–Pliocene time and in agreement with geologic evidence for accelerated shortening and exhumation at this time. The leaf wax isotopic data, on the other hand, lack a systematic trend, potentially due to fractionation changes associated with uplift-induced turnover in floral populations. Such changes could obscure isotopic variations in meteoric water, suggesting that leaf-wax isotopic compositions may not provide a direct proxy for elevation in this particular situation involving uplift in a heterogeneous tropical environment. More promising in this case is the MBT′/CBT proxy, which may offer a suitable alternative to carbonate-based paleoelevation proxies in tropical regions where year-round high precipitation rates prevent formation of soil carbonates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A consistent magnetic polarity stratigraphy of late Neogene to Quaternary fluvial sediments from the Heidelberg Basin (Germany): A new time frame for the Plio–Pleistocene palaeoclimatic evolution of the Rhine Basin

This work presents the results of a magnetostratigraphic survey performed on 1150m of core material from three sites within the Heidelberg Basin. The cores intersect one of the thickest continuous accumulations of Plio–Pleistocene fluvial sediments in western Central Europe. The resultant magnetic polarity stratigraphy includes every Quaternary polarity chron, thereby providing constant age constraint down to the Gauss–Matuyama Boundary (2.58Ma). Older deposits cannot be unequivocally dated; instead, various age–depth models are discussed. We base our chronostratigraphic interpretation of the successions tentatively on three assumptions. A) The accommodation was almost constant over time. B) Hiatuses in the duration of subchrons (on the order of 0.2Myr) may occur, and the actual step-like age–depth relationship is best depicted as a smooth curve with almost constant slope. C) Long chrons and subchrons have a higher preservation potential than shorter polarity intervals.The stratigraphic scenarios with the highest probability – based upon our three assumptions – lead to minimum ages of >5.235Ma and >4.187Ma for the oldest parts of the Viernheim and Heidelberg cores, respectively. Consequently, this study provides the first consistent magnetic polarity stratigraphy for quasi-continuous sequences of late Neogene to Quaternary fluvial sediments in the Rhine Basin and generally in western central Europe. This methodologically independent chronostratigraphy supplies an urgently required temporal model for on-going tectonic and sedimentological studies and the reconstruction of the palaeoclimate since the Pliocene in this part of Europe.

Magnetic polarity of Upper Triassic sediments of the Germanic Basin in Poland

Palaeomagnetic results are presented for 205 samples of cores from the Ksiąz Wielkopolski IG-2, Woźniki K1 and Patoka 1 wells, drilled in the Polish part of Germanic Basin. The magnetic polarity stratigraphy is based on the inclination of the characteristic remanent magnetization, isolated in 60% of the total samples and found to be in general agreement with the expected Late Triassic inclination at the sampling sites. A total of 22 magnetozones from the integration of the three records correspond to about 25% of the published polarity zones for the Upper Triassic sediments that were combined in the worldwide composite polarity-time scale. The magne- tic polarity pattern, defined for the Schilfsanstein, fits very well with the one defined in the Tethys area for the upper part of the Julian sub-stage. According to the magnetostratigraphic data, the uppermost part of the Upper Gypsum Beds (equivalent to the Ozimek Member of the redefined Grabowa Formation) and the lowermost part of the Patoka Member, containing the Krasiejow bone-breccia horizon, can be correlated with the latest Tuvalian (~228.5 Ma) or with the middle part of Lacian (~225 Ma). However, if the “Long-Tuvalian” option for the Late Triassic Time Scale is taken into consideration, the parts of these substages mentioned above should be correlated with ~221.5 Ma and ~218.5 Ma, respectively.

Punctuated shortening and subsidence in the Altiplano Plateau of southern Peru: Implications for early Andean mountain building

Sedimentologic, provenance, geochronologic, and magnetostratigraphic results from clastic nonmarine deposits in the northern Altiplano Plateau of southern Peru (14–15°S) demonstrate late Eocene–Oligocene (37–26 Ma) accumulation of the >4-km-thick San Jerónimo (Puno) Group within a retroarc foreland basin related to early Andean shortening and crustal thickening. Punctuated Oligocene (29–26 Ma) displacement along deep-seated contractional structures, as revealed by growth stratal relationships, abruptly partitioned this regional flexural basin and established the structural boundaries of the smaller intermontane Ayaviri Basin, which continued to evolve in a hinterland setting during late Oligocene–Miocene shortening. This brief episode of shortening along the Altiplano–Eastern Cordillera boundary is correlated with exceptionally rapid sediment accumulation (>1100–1800 m/m.y.), tightly constrained to 30–28 Ma on the basis of U-Pb geochronology and magnetic polarity stratigraphy. Provenance data from detrital zircon U-Pb age populations and sandstone compositions indicate derivation from a complex belt of Paleogene shortening and probable basin inversion in the Western Cordillera that was subsequently overprinted by Andean arc magmatism. This early Andean zone is interpreted as the along-strike continuation of the better-exposed Marañon fold-thrust belt to the north (5–13°S) and a proposed belt of shortening to the south along the Chilean Precordillera and Western Cordillera of Bolivia and northern Argentina (17–25°S). Subsequent focusing of late Oligocene shortening along the Eastern Cordillera–Altiplano boundary may have been linked to shallowing of the subducting slab and potential reactivation of crustal anisotropies.

Late Devonian carbonate magnetostratigraphy from the Oscar and Horse Spring Ranges, Lennard Shelf, Canning Basin, Western Australia

The Late Devonian was a time of major evolutionary change encompassing the fifth largest mass extinction, the Frasnian–Famennian event. In order to establish a chronological framework for global correlation before, during, and following the Frasnian–Famennian mass extinction, we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the Upper Devonian carbonate reef complexes of the Lennard Shelf, in the Canning Basin, Western Australia. Magnetostratigraphy from these rocks provides the first high-resolution definition of the Late Devonian magnetic polarity timescale. A 581-m-reference section and an 82-m overlapping section through the marginal slope facies (Napier Formation) of the Oscar Range as well as a 117-m section at Horse Spring (Virgin Hills Formation) were sampled at decimeter to meter scale for magnetostratigraphy. Conodont biostratigraphy was used to correlate both sections, and link magnetostratigraphic polarity zones to a globally established biostratigraphy. A stable, Characteristic Remanent Magnetization (ChRM) with dual polarities (NE, shallowly upward and SW, shallowly downward) is recovered from ∼60% of all samples, with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics. These directions define a geomagnetic pole at 49.5°S/285.8°E and α95=2.4 (n=501), placing the Canning Basin at 9.9°S during the Late Devonian, consistent with carbonate reef development at this time. A conservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections, not including possible cryptochrons and short duration magnetozones. Field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the Oscar Range. A strong correlation was found between magnetic polarity stratigraphies of the Oscar Range and Horse Spring sections, and we correlate 12 magnetostratigraphic packages. The relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was ∼4.5× faster in the middle slope deposits at Oscar Range than in the more distal, lower slope Horse Spring deposits for the middle Frasnian through Famennian. The magnetic field during the Late Devonian underwent a relatively high reversal frequency with good potential for regional and global correlation, and should prove useful in deciphering a high-resolution chronostratigraphy across the Lennard Shelf to enable higher confidence examination of reef development across a major biotic crisis.

Early to middle Eocene magneto-biochronology of the southwest Pacific Ocean and climate influence on sedimentation: Insights from the Mead Stream section, New Zealand

The Mead Stream section (South Island, New Zealand) consists of a 650-m-thick series of continuous, well-exposed strata deposited on a South Pacific continental slope from the Late Cretaceous to the middle Eocene. We examined the uppermost Paleocene–middle Eocene part of the section, which consists of ∼360 m of limestone and marl, for detailed magnetic polarity stratigraphy and calcareous nannofossil and foraminifera biostratigraphy. Magneto-biostratigraphic data indicate that the section straddles magnetic polarity chrons from C24r to C18n, calcareous nannofossil zones from NP9a to NP17 (CNP11–CNE15, following a recently revised Paleogene zonation), and from the Waipawan to the Bortonian New Zealand stages (i.e., from the base of the Ypresian to the Bartonian international stages). The Mead Stream section thus encompasses 17 m.y. (56–39 Ma) of southwest Pacific Ocean history. The ages of calcareous nannofossil biohorizons are consistent with low- to midlatitude data from the literature, indicating that during the early–middle Eocene, the low- to midlatitude calcareous nannofossil domain extended at least to ∼50°S–55°S in the South Pacific. Correlation of the magnetic polarity stratigraphy from the Mead Stream section with the geomagnetic polarity time scale allows us to derive sediment accumulation rates (SAR), which range between 8 and 44 m/m.y. Comparing the SAR with paleotemperature proxy records, we found that two intervals of increased SAR occurred during the early Eocene climatic optimum (52–50 Ma) and during the transient warming event peaking with the middle Eocene climatic optimum (40.5 Ma). This correlation indicates that, at Mead Stream, the climate evolution of the early–middle Eocene is recorded in a sedimentation pattern whereby, on a million-year time scale, warmer climate promoted continental weathering, transportation, and accumulation of terrigenous sediments.

Magnetic polarity stratigraphy and palynostratigraphy of the Mississippian-Pennsylvanian boundary interval in eastern North America and the age of the beginning of the Kiaman

Magnetic polarity data from the Maritimes Basin in eastern Canada reveal patterns of magnetic reversal through ∼17 m.y. of late Mississippian and early Pennsylvanian time. We collected samples from three sections on Cape Breton Island, a fourth that extends a section studied previously, and a fifth from a deep cored well on Prince Edward Island. We compared the polarity data from these sections to previous results from lower parts of the Joggins section of eastern Canada. All sections are constrained by new palynological data and collectively span the late Mississippian, Brigantian, Pendleian, and much of the Arnsbergian substages. A major unconformity at the Pennsylvanian-Mississippian boundary makes correlation difficult. The late Mississippian Arnsbergian substage is dominated by normal polarity, whereas the Pendleian and Brigantian are characterized by normal and reversed polarity intervals in about equal proportions. In the United States, the Mississippian-Pennsylvanian boundary is a major hiatus that juxtaposes strata of early Pennsylvanian age on Mississippian strata ranging in age from Brigantian to mid- to late Arnsbergian. Data for the Mauch Chunk Formation in the United States provide polarity reversal patterns for correlative sequences in the Central Appalachian Basin. Field mapping and magnetic polarity data show closely comparable Mississippian-Pennsylvanian relationships in the Maritimes Basin more than 1500 km northeast from the U.S. sections. The eustatic event that coincides with the Mississippian-Pennsylvanian boundary unconformity is late Arnsbergian to Yeadonian in both the central Appalachians and Maritime Basins. Basal Pennsylvanian strata of Yeadonian to Langsettian age on Cape Breton Island reveal normal paleomagnetic polarity through several hundred meters of section. These strata are overlain by a thick section showing only reversed polarity. We believe that this polarity boundary is the base of the Kiaman. An early Langsettian position (ca. 318 Ma) for the base-Kiaman superchron is indicated by these new data. In eastern Nova Scotia and New Brunswick, the sedimentary rocks just above the unconformity are of reversed polarity. It is clear that the sedimentation in the two areas is affected by the unconformity, and no magnetic or palynological correlation is possible. Above the unconformity, only saetosa zone sediments are present.

Oligocene–Miocene relative (geomagnetic) paleointensity correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090)

Late Oligocene to Early Miocene relative paleointensity (RPI) proxies can be correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090). Age models are constrained by magnetic polarity stratigraphy through correlation to a common geomagnetic polarity timescale. The RPI records do not contain significant power at specific (orbital) frequencies, and hence there is no significant coherency between RPI proxies and the normalizers used to construct the proxies, although orbital power is present in some normalizers. There is no obvious control on RPI proxies from mean sedimentation rate within polarity chrons, magnetic grain size proxies or magnetic concentration parameters. The salient test is whether the equatorial Pacific records can be correlated one to another, and to the records from the South Atlantic. All records are dominated by RPI minima at polarity reversals, as expected, although the comparison within polarity chrons is compelling enough to conclude that the intensity of the Earthʼs axial dipole is being recorded. This is supported by the fit of RPI data from Sites U1334 and 1218 after correlation of the two sites using diverse core-scanning data, rather than polarity reversals alone. We do not see a consistent relationship between polarity-chron duration and mean RPI, and no consistent skewness (“saw-tooth” pattern) for RPI within polarity chrons. Stacks of RPI records for 17.5–26.5 Ma include long-term changes in RPI on Myr timescales that are superimposed on RPI minima associated with polarity reversals, and shorter-term variations in RPI with an apparent pacing of ∼50 kyr. The equatorial Pacific to South Atlantic correlations indicate that RPI can be used as a (global) stratigraphic tool in pre-Quaternary sediments with typical pelagic sedimentation rates of a few cm/kyr.