GP52A-01 INVITED
Magneto-biostratigraphy of the Vikinghøgda Fm, Central Svalbard and the Geomagnetic Polarity Timescale for the Lower Triassic
A composite magneto-biostratigraphy for the Lower Triassic is constructed, using as a framework the high resolution ammonoid biostratigraphy available for arctic Boreal successions, using new data from Spitsbergen and published data from the Sverdrup basin. The magneto-biostratigraphy through the Vikinghødga Fm has been determined in central Spitsbergen, using the type section in Deltadalen, with additional sampling from an additional section at Milne Edwardsfjellet some 15 km distant. The successions here are horizontally bedded and essentially undeformed, and can be closely related to several nearby sections on the southern flanks of Sassendalen. Combined thermal and alternating field demagnetisation is effective in isolating characteristic Triassic directions, whose mean directions fall close to the European apparent polar wander path for the Lower Triassic. However, the Triassic reversed and normal directions are partially overprinted with a frequently strong, probably recent magnetisation. The remanence appears to be predominantly carried by magnetite, although an unidentified magnetic sulphide is important in some intervals (particularly the Vendomdalen Mbr). The polarity stratigraphy from the Vikinghødga Fm, when integrated with the ammonoid and meager conodont data is similar to that previously determined from the Canadian arctic successions. Together, these allow the construction of a composite boreal magneto-biostratigraphy for the Lower Triassic tied to the boreal ammonoid zonations. In the Vikinghødga Fm the Permian Triassic boundary cannot be accurately located, occurring between 1.7- ~10 m above the base of the Deltadalen Mbr either below or at about the level with an Otoceras boreale fauna. The Late Griesbachian to the early part of the E. romunderi Zone is mostly reverse polarity, with 3 substantive normal polarity intervals. The mid and upper Smithian (lower Olenekian) part of the Vikinghøgda Fm, which has the best ammonoid age control, is dominantly normal polarity, and can be closely correlated to that in the Sverdrup Basin. Integrating data through the Spathian (upper Olenekian) is complicated by poor biostratigraphic age control in the Sverdrup basin, and large changes in sedimentation rate in the Spitsbergen succession. The Spathian appears to be dominantly normal polarity, with evidence of at least 3 major reverse polarity intervals.
GP52A-02 INVITED
Closing the Mid- Paleocene gap: toward a complete astronomically calibrated Paleocene Epoch at Zumaia (Basque Basin, W Pyrenees)
The ~10 Myr long Paleocene Epoch is bounded by two of the most popular and studied chronostratigraphic limits, the Cretaceous/Tertiary (K/T) boundary at its base and the Paleocne/Eocene (P/E) boundary at the top. The Paleocene time scale has relied on an age model for magnetic polarity chrons derived from a cubic-spline fit of marine magnetic anomaly pattern in the South Atlantic to two radiometrically dated calibration points (Cande & Kent, 1992, 1995). These include an age of 65 Ma for the K/T boundary (66 Ma in the CK92 time scale) and a derived age of 55 Ma for the P-E boundary (this age constrained from 40Ar/39Ar dated volcanic ash layers within a clay sequence in Denmark). An age of 65.5±0.3 Ma for the K/T and 55.8±0.2 Ma for the P/E are taken in the most recent time scale GTS2004 (Gradstein et al, 2004) which combines both isotopically (using a 28.02 Ma age for the Fish Canyon Sanidine FCT monitor standard) and astronomically derived ages in the Neogene. However, intercalibration of single crystal sanidine dates of primary ash layers in astronomically dated sections arrives at an astronomically calibrated age of 28.24±0.1 Ma for the FCT standard (Hilgen et al., 2006), which will suggest an ~1% underestimate in current Paleogene ages. Thus, the astronomically tuned chronology for the (hemi)-pelagic basal Paleocene succession at Zumaia (Dinarès-Turell et al, 2003) that arrives an estimated age of ~65.8 Ma for the K/T appears consistent. In that study an ~4 Myr long tuned chronology based on the R7 full numerical solution for the Solar System of Varadi et al. (2003) we presented. However, more recently a second solution has been proposed (La04, Laskar et al., 2004), which differs notably in the Paleocene with respect R7 (offsets between the ~2.25 Myr long-term cycles). The differences arise from the uncertainty due to the chaotic behaviour of the inner planets to some resonant argument that limits an accurate age determination of successive minima in this very long eccentricity cycle. It is interesting to note, however, that in the Paleocene both solutions share one of such nodes of reduced eccentricity amplitude at about 62.2 Ma, which was the feature used as starting point in our tuning at Zumaia. Recent tuning efforts of hyperthermal events within the lower Cenozoic greenhouse climate record documented in Ocean Drilling Program (ODP) core sediments (Lourens et al., 2005) have provided tuned ages for the P/E thermal maximum using both the La04 and R7 astronomical target solutions (~55.270 Ma and ~55.675 Ma respectively) and showed that hyperthermal events correspond to maxima in the ~405-kyr and ~100-kyr eccentricity cycles that postdate prolonged minima in the 2.25-Myr eccentricity cycle. Here, we present integrated magnetostratigraphy and calcareous plankton biostratigraphy for the Mid Paleocene interval at Zumaia and evaluate the lithologic cyclicity using spectral analysis on magnetic susceptibility and CaCO3 proxy records. Considering that a previous study in the uppermost Paleocene interval included the record of chron C25n (Dinarès-Turell et al., 2002), all Paleocene reversals have now been identified at Zumaia, and a complete tuned Paleocene record is possible, rendering Zumaia an exceptional section. Implications for the definition of the Selandian stage, the absolute ages of chron C26r and the Mid-Paleocene Biotic Event (MPBE)) will be also considered.
GP52A-03
New magnetic Polarity Stratigraphy of the Mae Moh Basin in northern Thailand, and Implications for the Age of the First Miocene Hominoids
This magnetostratigraphic study has been conducted on the Miocene Mae Moh basin, in the Lampang province, Northern Thailand. 194 paleomagnetic samples were collected from 65 stratigraphics levels from Na Khaem and Huai Luang formations. The studied sections are mainly composed of clay, sandstone and lignite. Rock magnetic experiments revealed that titanomagnetite, magnetite and hematite are the mains carriers of magnetisation. Samples subjected to progressive thermal or alternating field demagnetization procedures exhibit a low stability and a high stability component, with either normal and reversed polarities. The reversal test is positive and indicates that the characteristic remnant magnetization directions correspond to the primary magnetization directions (McFadden and Mc Elhinny, 1990). The mean direction calculated for section 1 are: incl : 23.24 and decl. : 5.01 and incl. : 31.22 et decl. : 7.01 for section 2. These results don't document any rotation with respect to previous study (Benammi et al., 2002). In total, nine polarity zones (four normal and five reverse), that can be reliably be correlated the geomagnetic polarity time scale developed by Gradstein et al, 2004, are recognized from the studied sections. Based on the biochronological constraints, the magnetostartigraphic column of the Mae Moh formations correlates best with chron C5ACr-C5r.3r, between 14.1and 12 Ma. This correlaton revealed a mean sedimentation rate of approximately 21 cm/ky, and a age of 12.7 et 13.2 for for the fossilferous levels (J5, K1, K2) where the mammals remains were found. The analysis of the elements traces spectrum of two ash levels coming from the basins of Mae Moh and Chiang Muan made it possible to establish a new correlation of the Chiang Muan sequence with the GPTS. This correlation prove that the age of the Chiang Muan sequence would be between 13.1 and 12 Ma, and the fossiliferous levels with hominoid (Khoratpithecus Chiangmuanensis) would be dated between 12.2 and 12.4 Ma for the upper lignite and between 13 and 12.8 My for the lower lignite.
GP52A-04
Magnetostratigraphy of Cave Sediments
Karst and cave fills (karst sediments) are relatively special kinds of geologic materials, as the karst environment favors both the preservation of paleontological remains and their destruction: on one hand, karst is well known for its wealth of paleontological sites, on the other hand, most of cave fills are completely sterile (especially the inner-cave facies). Another specific feature is that karst systems can be „frozen" (halted) and then rejuvenated, often for several times. Reactivation processes may degrade the record into an unreadable form, often mixing karst fill of different ages (collapses, reworking, redepositions). Only the last accumulation phase has been dated in caves in most cases. The fossilisation of the cave (fill by deposits) and rejuvenation (exhumation of the fill) reflect changes in resurgence area. The rejuvenation of the karst process can excavate the previous cave fill completely, which is the most common case resulting from the polycyclicity and dynamics of cave environments. Under favorable settings, fills belonging to more infill phases and separated by distinct hiatuses (unconformities) can occur in one sedimentary profile. Such amalgamation is typical especially in ponor part of the cave. During our previous research we found, that owing to the specific character of karst sediments, the use of paleomagnetic methods can bring surprising result, without regards to the character of the method (correlated- ages not providing direct numerical output). The method can serve as helpful tool to interpret not only the age of cave sediments but also to understand the evolution of karst landscape and tectonic history of the region. To obtain more precise results, it is necessary to combine paleomagnetism with other methods of numerical-, relative- or correlated-dating. The analytical results confirmed that the complete step/field procedure offered by the alternating field and thermal demagnetization methods must be applied. The sampling for magnetostratigraphy analysis must be dense enough. Repeated sampling in some profiles from Slovenia have shown that only dense sampling (i.e. each 2-4 cm) can ensure reliable results. The application of complete analysis only to pilot samples and shortened, selected field/step approach, to other samples did not offer sufficient data set for reliable interpretation. Therefore the dating of cave sediments by the application of palaeomagnetic methods - magnetostratigraphy - represents a highly difficult and sometimes risky task. Case studies from the karst areas of Czech Republic, Slovakia, Hungary, Slovenia, Italy, proved that the correlation of obtained arrangements of normal and reverse polarized magnetozones with standard palaeomagnetic scales can be finish only with difficulties and with a high degree of uncertainty. Such reality can be finely exemplified nearly on all examined logs from Slovenia and some from Slovakia. The correlation of obtained magnetostratigraphic data with standard scales indicate, in most cases, substantially older age of cave filling than expected earlier: e.g., most of Slovenian results indicate even pre-Quaternary ages, in spite of former interpretation to mid-Pleistocene as maximum age expected.
GP52A-05
The First Boreal-Tethyan Correlation of the Jurassic-Cretaceous Boundary Interval by the Magnetostratigraphy
Climatic zoning as well as some peculiarities of paleoceanography, existed during the Late Jurassic and Early Cretaceous in the Northern Hemisphere, lead to formation of the significant differences in the taxonomic structure of south and north marine and terrestrial biota and to restriction of two main superrealms: southern Tethys- Panthalassa and northern Panboreal. For the purpose of liquidation of this gap authors undertook joint bio- and magnetostratigraphic studies of the Jurassic-Cretaceous boundary beds, situated at the Nordvik peninsula, northern Siberia (GACR 205-07-1365). Key interval for paleomagnetic sampling of the section, with a thickness of 27 m, has been determined by using biostratigraphy from top part of Middle Volgian (Epivirgatites variabilis Zone) until lowermost Ryazanian (Hectoroceras kochi Zone). As results of these investigations well-founded version of magnetostratigraphic correlation of J/K boundary beds has been supposed for the first time for those distant regions as Southern Europe and Northern Siberia. The Nordvik section is contain of magnetozones ranging from M20n to M17r and two narrow reverse subzones M20n.1r "Kysuca Subzone" in M20n, and M19n.1r "Brodno Subzone" in M19n. Their positions are within normal magnetozones, one inside of lower N-magnetozone and the second in the youngest part of the middle N-magnetozone. In the previously studies analogous two narrow reverse subzones ("Kysuca" and "Brodno") were detected in M20n and M19n and precisely defined in the Brodno (West Slovakia), Puerto Escaño (South Spain), Bosso Valley (Central Italy) and are well correlated with the M- sequence of marine magnetic anomalies. The determination of both reverse Subzones enabled the distinction of individual polarity zones in the Nordvik section with chrons M20 and M19. Identical to analogy of the Kysuca Subzone detected in normal magnetozone M20n is only 7cm thick. According to the Brodno profile, it is situated above the middle of normal magnetozone M20n. Another reverse subzone called the Brodno Subzone, detected in the upper part of normal magnetozone M19n, is 17cm thick. New magnetostratigraphic data obtained by us from Boreal (Arctic) section were compared with the analogous materials from the J/K boundary beds of the Tethyan character. The research showed that the Boreal J/K boundary is much younger than the Tethyan - both boundaries lay in different paleomagnetic zones.
GP52A-06
Paleomagnetism of the Pringle Falls Polarity Episode (Pringle Falls, Oregon): A Revisited Study
We have studied a total of 827 samples drilled from five widely spaced profiles sampled along the Deschutes River Oregon. The five profiles sampled recorded a high-resolution paleomagnetic record of the Pringle Falls magnetic polarity episode (ca. 218±10 ka) and are characterized by diatomaceous lacustrine sediments. We have conducted paleomagnetic and rock magnetic studies in order to investigate the reproducibility of the paleomagnetic signal throughout the 5 km of the sampling of the five profiles. We conducted low-field vs. susceptibility analysis to determine the magnetic carriers of the sediments and we found that the main magnetic carrier is pure magnetite (Curie point 575°C). The magnetic grain size indicated SD-MD magnetite. The demagnetization of the sediments was done by means of alternating field methods and the determination of the mean directions by principal component analyses. The level of detail of the paleo-signal of these five records is highly consistent since they are characterized by rapidly deposited sediments (greater than 10 cm/kyr) that provide detailed representation of field behavior during the excursion. The VGP paths are highly internally consistent and are defined by a clockwise loop traveling from high northern latitudes over the eastern part of North America and the North Atlantic to South America and then to high southern latitudes and that return to high northern latitudes through the Pacific and over Kamchatka. This last clockwise looping is characteristic of other recently found excursions like the Iceland basin excursion (IBE, 188 ka). The published age of the Pringle falls excursion (ca. 218±10 ka) and the most recent radiometric ages (weighted mean 211 ± 11 ka, Singer et al., 2005) indicate that the dominance of such VGP paths (i.e. clockwise looping) of the Pringle Falls, the IBE, Jamaica and other excursion of the same age show that the excursional paleofield had a relatively simple geometric characteristic. A corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths, a conclusion implying that a common mechanism of the generation of the paleofield is involved.
GP52A-07
Magnetostratigraphy of the Lower Cretaceous San Marcos Formation, Coahuila, Mexico
Continental hematitic strata of the Lower Cretaceous San Marcos Formation, in central Coahuila, were deposited on and along the margins of the Coahuila Island, a prominent paleogeographic element in NE Mexico. It contains the stratigraphic record of activity along the San Marcos fault, the southern margin of the Sabinas basin. It is overlain by marine limestones of the Cupido Formation, and the contact is transitional. It rests on ammonite bearing strata of Tithonian age, but the contact is not well exposed. We sampled the San Marcos Formation at localities in Potrero Colorado and Valle de San Marcos, for a total of 34 paleomagnetic sites in three sections (one site=one bed). The beds dip gently to the north to northwest at all three sections.The characteristic magnetization is a dual-polarity, high coercivity, and high unblocking temperature (>650 °C), northwest directed moderately positive component, overprinted by a north directed magnetization of lower stability. Occasionally, the reverse polarity magnetization (south-southwest to southeast directed) is only revealed by demagnetization trajectories. Dual polarity magnetizations within two sites suggest that the remanence acquisition process is of long duration relative to the duration of reversals. A composite section defines 9 magnetozones, and it is characterized by reversed zones of short duration relative to normal intervals. We thus correlate the magnetic polarity sequence of the San Marcos Formation with the M5 to M0 chron sequence of the Barremian to early Aptian GPTS. Site means are relatively well grouped. Tilt corrected means for each of the sections samples vary from discordant (191.7°, -54.9°; k=40.5, α95=9.6°; 7 accepted sites) at the base of the section to concordant (333.6°, 58.3°; k=28.3, α95=12.6°; 7 accepted sites) at the top of the section. The discordance at the base of the section is interpreted in terms of a small clockwise rotation related to activity along the San Marcos fault.
GP52A-08
An Early to Middle Miocene Magnetostratigraphy From the Ebro Basin (NE Spain)
The Ebro Basin represents the last stage of evolution of the south-pyrenean foreland, which formed since the late Cretaceous as a result of northwards subduction and collision between the Iberian and the European plates. The basin evolved into a land-locked configuration by the latest Eocene when continued compression led to closing of its NW marine gateway. Filling of the basin continued from late Eocene to the late Middle Miocene, leading to the accumulation of a thick sequence of alluvial and lacustrine sediments. In the middle-to-late Miocene the basin opened towards the Mediterranean and river incision cut through the complete Eocene to Miocene sedimentary succession. Nearly undeformed Early to Middle Miocene units crop out extensively in the central parts of the basin and have delivered a long and continuous magnetostratigraphic record. Considering the time resolution achievable with magnetostratigraphy, the numerous studies on the sedimentary sequences of the Ebro Basin provide compelling evidence for stratigraphic completeness and relatively steady sedimentation over the Oligocene-Miocene time interval. The perfect match with the geomagnetic polarity time scale provides a robust and high resolution chronology for the late stages of basin infill, allowing cyclostratigraphic analysis to be made as well as correlation with the various records of regional to global climate change. Noticeably, the Early- Middle Miocene boundary corresponds in the central Ebro Basin with a remarkable and sharp transition from a marly-gypsiferous to a carbonate unit. It represents a significant environmental change from a dry period dominated by a hipersaline water body to a wetter period with rapid installation of an areally extensive carbonate lake. Magnetostratigraphic data indicates that this period of lacustrine expansion is simultaneously recorded in other basins of the Iberian Plate and corresponds to the climatic optimum of the Langhian stage that preceeded the middle Miocene global cooling.