Gilen Bernaola

Late Cretaceous–Paleocene formation of the proto–Zagros foreland basin, Lurestan Province, SW Iran

Late Cretaceous emplacement of ophiolitic-radiolaritic thrust sheets over the Arabian passive margin was the first manifestation of the protracted closure of the Neotethys Ocean, which ended with the continental collision between Arabia and central Iran and the formation of the present Zagros fold belt. This tectonic stacking produced a flexural basin (the Amiran Basin: 400 × 200 km in size) in the northwest Zagros that was filled with a 1225-m-thick shallowing-upward detrital succession made up of the Amiran, Taleh Zang, and Kashkan Formations. This succession sits unconformably above the Late Cretaceous Gurpi Formation and is overlain by the Oligocene-Miocene Shahbazan-Asmari carbonate succession. Dating of the Amiran-Kashkan succession is based on detailed biostratigraphy using large foraminifera and calcareous nannoplankton. The Cretaceous-Tertiary (K-T) boundary is located within the uppermost 25-45 m of the Gurpi Formation. The overlying Amiran and Taleh Zang Formations have been dated as Paleocene in age. However, the base of the Paleocene within the Gurpi Formation lacks NP1 and NP2 zones, implying a hiatus of ∼2 m.y. at ca. 65.5 Ma, which is inferred to correspond to an early folding phase near the Cretaceous-Paleocene boundary. The upper part of the Kashkan Formation is dated to the earliest Eocene by palynostratigraphy. A large hiatus (or very slow deposition) lasting about 15 m.y. occurs between the Kashkan and Shahbazan Formations in the studied region. The base of the prograding Shahbazan platform deposits is dated by 87Sr/86Sr stratigraphy at ca. 33.9 Ma. The upper part of the Asmari Formation is dated as early-middle Miocene using foraminifera associations. Reconstruction of the Amiran-Taleh Zang-Kashkan succession of the Amiran Basin indicates a thickening of the basin fill from the southern pinch-out along the SE flank of the Kabir Kuh anticline to SW of the Khorramabad anticline, where the flexure is at least 900 m. In contrast, the NE part of the basin underwent coeval contraction and uplift of ∼1300 m. Superimposed smaller undulations onto the large-scale flexure are interpreted as Late Cretaceous-Paleocene folds. Regional comparisons (SE Zagros, Oman, and Turkey) indicate that Late Cretaceous-Early Tertiary deformation affected the entire NE margin of Arabia but that compression was not synchronous, being younger in Lurestan than in the NW Persian Gulf where inversion tectonics occurred from Turonian to mid-Campanian times. The long sedimentary hiatus spanning most of the middle and late Eocene must have been related to deep lithospheric processes linked to the initial events of the protracted closure of the Neotethys Ocean between Arabia and central Iran. The tectono-sedimentary history recorded in the Zagros Basin may help to understand early foreland basin growth in other orogens in which subsequent continental collision has obliterated these early events.

Untangling the Palaeocene climatic rhythm: an astronomically calibrated Early Palaeocene magnetostratigraphy and biostratigraphy at Zumaia (Basque basin, northern Spain) ☆

Abstract The magnetostratigraphy of a 54-m-long section above the Cretaceous–Tertiary boundary at the sea-cliff section of Zumaia in the Basque basin (northern Spain) has been established. The section encompasses the entire Danian and the lower part of the Selandian stages as indicated by calcareous plankton biostratigraphy. The studied interval consists of (hemi)pelagic limestone–marl alternations in the form of couplets and bundles, which range from centimetre/decimetre to metre scale respectively and a few thin-bedded calcareous turbidites. The magnetostratigraphy, based on samples from about 200 stratigraphic levels, allows the identification of six reversal boundaries from chron C29r to C26r at a bed level. The spatial (or temporal) evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique. A preliminary age model based on the standard CK95 GPTS indicates that the basic lithologic carbonate–marl couplet corresponds to the 19–23-kyr precession cycle (21–31-cm cycle in the depth domain) and that a bundle cycle (usually groups of four to six basic couplets) with global periodicity centred at 1.22 m corresponds to the ∼110-kyr eccentricity cycle. We have tuned the bundle cycles to the Va03_R7 eccentricity orbital solution [Astrophys. J. 592 (2003) 620–630] following an initial match of a node of the ∼2.4-Ma eccentricity modulatory cycle in the target time series to particularly carbonate-rich bundles from the upper part of the Zumaia section that displays significant power of a 4.4-m-period cycle corresponding to the ∼404-kyr eccentricity cycle. Consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the ∼404-kyr eccentricity cycle is not persistent throughout. The tuning, however, appears robust as it brings the age of the K/T boundary at ∼65.8 Ma. It is argued that a sea-level signal (tectonically driven?) is superimposed on the climatic forcing at the Milankovitch band masking the full expression of the low-frequency astronomical periods. We provide a cycle-tuned duration for all intervening Early Palaeocene polarity chrons and estimate relative ages for bioevents. The cycle-tuned chronology indicates that the CK95 GPTS overestimates the duration of chrons C28 and C27 by 20 and 26% respectively. Our data may prove useful to better constrain Early Palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the Danian and Selandian stages.

Evidence of an abrupt environmental disruption during the mid-Paleocene biotic event (Zumaia section, western Pyrenees)

An abrupt environmental disruption occurred in the photic zone and at the seafloor during the mid-Paleocene biotic event (MPBE). Calcareous nannoplankton, planktic foraminifer, and benthic foraminifer assemblages at Zumaia section (western Pyrenees) underwent a rapid and remarkable transformation. The major calcareous plankton assemblage changes suggest a shift from relatively cooler mesotrophic to warmer, more oligotrophic conditions, indicating a disturbed environment due to the warming of the ocean. Benthic foraminifer assemblages were also significantly affected by the MPBE; diversity of the assemblages and buliminids show net decline and the low food and opportunistic taxa increase in abundance. The reorganization of the planktic ecosystem possibly involved changes in the food flux (type and quantity) to the seafloor, thus triggering changes in the benthic communities. A 1‰ negative δ 13 C shift and a 30% carbonate content decrease are recorded in connection with the biotic event. This suggests that during the MPBE, as in the Paleocene-Eocene Thermal Maximum (PETM), an input of a large mass of isotopically depleted carbon into the ocean and atmosphere could have lowered the deep-sea pH, triggering a rapid shoaling of the lysocline and contributing to greenhouse warming. The MPBE was short lived: according to the counting of limestonemarl couplets, the stratigraphic expression of precession cycles throughout the Zumaia section, the MPBE lasted for ∼52–53 k.y., with the core of the event representing ∼10–11 k.y. The Zumaia section is the first land-based locality in which the MPBE is recognized and described in detail. Due to its expanded character and excellent paleontological record, this section may prove to be a global reference section for the study of this short-lived event.

THE PALEOCENE–EOCENE THERMAL MAXIMUM: NEW DATA ON MICROFOSSIL TURNOVER AT THE ZUMAIA SECTION, SPAIN

The benthic foraminiferal turnover and extinction event (BEE) associated with the negative carbon isotope excursion (CIE) across the Paleocene–Eocene Thermal Maximum (PETM) is analyzed in the Zumaia section (Spain), one of the most complete and expanded deep-water sequences known worldwide. New biostratigraphic, paleoecologic, and paleoenvironmental data on benthic foraminifera are correlated to information on planktic foraminiferal and calcareous nannofossil turnover in order to evaluate possible causes and consequences of the PETM. Gradual but rapid extinction of 18% of the benthic foraminiferal species starts at the onset of the CIE, after the initial ocean warming (as inferred from calcareous nannofossils) recorded in the last 46 kyr of the Paleocene. This gradual extinction event culminated ∼10.5 kyr after the onset of the CIE and led to the main BEE, affecting 37% of the species. Therefore, extinctions across the PETM affected a total of 55% of the benthic foraminiferal species at Zumaia. The gradual extinction occurred under inferred oxic conditions without evidence for carbonate dissolution, indicating that carbonate corrosivity and oxygenation of the ocean bottom waters were not the main cause of the event. An interval characterized by dissolution occurs above the main BEE, suggesting that bottom waters became corrosive after the main extinction. Carbonate is progressively better preserved through the overlying deposits, and carbon isotope values gradually return to background levels. These data are consistent with a slow deepening of the carbonate compensation depth after its initial rise owing to abrupt acidification of the oceans. Microfossil data support a rapid onset of the PETM, followed by long-term effects on calcareous plankton and benthic foraminifera.

Did the Late Paleocene thermal maximum affect the evolution of larger foraminifers? Evidence from calcareous plankton of the Campo Section (Pyrenees, Spain)

The larger foraminifer turnover (LFT), which marks the base of the Ilerdian stage, may be related to the Late Paleocene Thermal Maximum (LPTM), or be at least nearly coeval with that climatic event. Thus, the impact of the LPTM may have been greater than hitherto realised, having also affected mid-latitude shallow-marine biota. This conclusion has been reached after a re-study of the calcareous plankton of the uppermost Paleocene and lowermost Eocene interval of the Campo section in the central southern Pyrenees. Campo is an important reference section because it contains larger foraminifers, planktic foraminifers and calcareous nannofossils, and their co-occurrence was used to intercalibrate their respective zonal schemes. Previous studies at Campo placed the onset of planktic foraminiferal Zone P5 near the base of the Ilerdian, and the calcareous nannofossil NP9/NP10 chronal boundary (sensu Bybell, L.M., Self-Trail, J.M., 1995. Evolutionary, biostratigraphic and taxonomic study of calcareous nannofossils from a continuous Paleocene/Eocene boundary section in New Jersey. US Geol. Surv. Prof. Pap. 1554, pp. 1‐36) not less than 150 m above the Ilerdian lower limit. By these estimates, the LPTM (known to have occurred in the middle part of Zone P5 and just before the NP9/NP10 boundary) would be an event much younger than the LFT. However, our reexamination of planktic foraminifers suggests that the base of the Ilerdian is probably situated at the middle of Zone P5 (a possibility proposed by Hillebrandt in 1965, but denied by later authors). For instance, Morozovella occlusa has been found for the first time in the Campo section. Its Last Appearance Datum (LAD), which in the Pyrenees was approximately coeval with that of Morozovella velascoensis (event used to place the top of Zone P5), has been identified in beds situated less than 70 m above the base of the Ilerdian. Such thickness represents a time span of a similar magnitude as the one which separated the LPTM and the LAD of M. occlusa in the deep-water hemipelagic succession of the Basque Basin, in the western Pyrenees. Autochthonous calcareous nannofossils are neither abundant nor well preserved in most of the studied interval, with Rhomboaster bramlettei (the marker of the base of Zone NP10) being extremely rare in lower and middle Ilerdian beds, a fact that makes it very difficult to fix the position of the NP9/NP10 boundary in the Campo section. However, the bases of zones NP9 and NP11 have been located, and they support the zonation with planktic foraminifers. These new data suggest that the LFT and the LPTM may have been coeval or nearly so, a possibility reinforced by correlation with sections of the Basque Basin. Specialists of larger benthic foraminifers can easily delineate the LFT in shallow water carbonate successions of the Tethys domain, and they propose to place the Paleocene/Eocene boundary at the base of the Ilerdian stage. On the other hand, the deep

Basin architecture and growth folding of the NW Zagros early foreland basin during the Late Cretaceous and early Tertiary

Abstract: We present and use the chronostratigraphy of 13 field logs and detailed mapping to constrain the evolution of the early Zagros foreland basin, in NW Iran. Large foraminifera, calcareous nannofossil, palynological and 87Sr/86Sr analysis supplied ages indicating a Campanian–early Eocene age of the basin infill, which is characterizd by a diachronous, southwestward migrating, shallowing upwards, mixed clastic–carbonate succession. Growth synclines and local palaeoslope variations indicate syndepositional folding from Maastrichtian to Eocene time and suggest forelandward migration of the deformation front. We also illustrate the basin architecture with a synthetic stratigraphic transect. From internal to external areas, time lines cross the formation boundaries from continental Kashkan red beds to Taleh Zang mixed clastic–carbonate platforms, Amiran slope deposits and basinal Gurpi–Pabdeh shales and marls. The foreland basin depocentres show a progressive migration from the Campanian to Eocene (c. 83–52.7 Ma), with rates of c. 2.4 mm a−1 during the early–middle Palaeocene (c. 65.5–58.7 Ma) increasing to c. 6 mm a−1 during the late Palaeocene–earliest Eocene (c. 58.7–52.8 Ma). Coeval subsidence remained at c. 0.27 mm a−1 during the first 12.7 Ma and decreased to c. 0.16 mm a−1 during the last 4.2 Ma of basin filling. Finally, we integrate our results with published large-scale maps and discuss their implications in the context of the Zagros orogeny. Supplementary material: Tables with dating results are available at http://www.geolsoc.org.uk/SUP18439.

Correlation of the Thanetian-Ilerdian turnover of larger foraminifera and the Paleocene-Eocene thermal maximum: confirming evidence from the Campo area (Pyrenees, Spain)

It has long been known that a major larger foraminifera turnover (LFT) occurred at the boundary between the Thanetian and Ilerdian stages, but its possible correlation with the Paleocene-Eocene thermal maximum (PETM) was unsuspected until the work of Baceta (1996), and has been controversial ever since. After summarizing the history of this controversy, we present information from three new sections that conclusively resolve the issue, all of them placed less than 2 km to the east of the classical Campo section in the southern Pyrenees. In these three sections, an up to 7 meter-thick intercalation of continental deposits rich in pedogenic carbonate nodules is sandwiched between uppermost Thanetian and lowermost Ilerdian shallow marine carbonates. The d13C composition of 42 pedogenic nodules collected from two of these sections (San Martin and La Cinglera) ranges between –11.4 and -14.3‰ and averages –12.9‰, values that conclusively represent the PETM and for the first time are recorded in sections where the LFT is clearly represented. Further, a high-resolution lithological correlation between Campo and the three new sections across the P-E interval unquestionably demonstrates that the lowermost marine beds with autochthonous specimens of Alveolina vredenburgi (a tell-tale of the LFT) are laterally interfingered –and are therefore coeval- with the nodule-bearing PETM continental deposits. On the basis of the new evidence, the temporal coincidence of the PETM and the LFT can no longer be doubted.

Redefinition of the Ilerdian Stage (early Eocene)

The Ilerdian Stage was created by Hottinger and Schaub in 1960 to accommodate a significant phase in the evolution of larger foraminifera not recorded in the northern European basins, and has since been adopted by most researchers working on shallow marine early Paleogene deposits of the Tethys domain. One of the defining criteria of the stage is a major turnover of larger foraminifera, marked by the FO’s of Alveolina vredenburgi (formerly A. cucumiformis) and Nummulites fraasi. There is now conclusive evidence that this turnover was coeval with the onset of the Carbon Isotope Excursion (CIE) and, consequently, with the Paleocene-Eocene (P-E) boundary, a temporal correspondence that reinforces the usefulness of the Ilerdian as a chronostratigraphic subdivision of the early Eocene in a regional context. However, in addition to the paleontological criteria, the definition of the Ilerdian was also based on the designation of two reference sections in the southern Pyrenees: Tremp (stratotype) and Campo (parastratotype). In both sections, the base of the stage was placed at the lowest marine bed containing A. vredenburgi specimens. Using the CIE as a correlation tool we demonstrate that these two marine beds occur at different chronological levels, being older in Campo than in Tremp. Further, we show that both beds are in turn younger than the lowest strata with Ilerdian larger foraminifera at the deep-water Ermua section in the Basque Basin (western Pyrenees). Since the age of stage boundaries must be the same everywhere, the choice of these stratotype sections was misleading, since in practice it resulted in the Ilerdian being used as a facies term rather than as a chronostratigraphic unit. To eliminate that conflict, and yet be respectful with established tradition, we propose to redefine the Ilerdian Stage following a procedure similar to the one used by the International Commission on Stratigraphy to establish global chronostratigraphic standards, namely: by using a “silver spike” to be placed in the Tremp section at the base of the Claret Conglomerate, a widespread lithological unit that in the Tremp Graus Basin coincides with the onset of the CIE. The redefined regional Ilerdian Stage becomes thus directly correlatable to the lower part of the global Ypresian Stage, as currently defined by the International Commission on Stratigraphy.

The Bartonian-Priabonian marine record of the eastern South Pyrenean Foreland Basin (NE Spain) : a new calibration of the larger foraminifers and calcareous nannofossil biozonation

This study presents a combined biostratigraphic (calcareous nannofossils, larger foraminifers) and magnetostratigraphic study of the Middle and Late Eocene marine units of the Igualada area, eastern Ebro Basin. The studied sections of Santa Maria de Miralles and La Tossa encompass the complete marine succession of the Santa Maria Group, where assemblages rich in larger foraminifers have been studied since the early 1950s. A total of 224 paleomagnetic sites and 62 biostratigraphic samples were collected along a 1350m-thick section that ranges from chron C20n to chron C16n (~43Ma to ~36Ma). The resulting magnetostratigraphy-based chronology challenges existing chronostratigraphic interpretations of these units and results in a new calibration of the biostratigraphic zonations. The base of calcareous nannofossil Zone NP19-20 is pinned down to an older age than its presently accepted attribution, whereas the time span assigned to Zone NP18 is significantly reduced. A revised calibration of larger foraminifers indicates that Zone SBZ18, formerly assigned exclusively to the late Bartonian, extends its range to the earlymost Priabonian, the Bartonian stage being almost entirely represented by Zone SBZ17. A division of Zone SBZ18 into two subzones is proposed.

On the age of the Early/Middle Eocene boundary and other related events: cyclostratigraphic refinements from the Pyrenean Otsakar section and the Lutetian GSSP

An integrated bio-, magneto- and cyclostratigraphic study of the Ypresian/Lutetian (Early/Middle Eocene) transition along the Otsakar section resulted in the identification of the C22n/C21r chron boundary and of the calcareous nannofossil CP12a/b zonal boundary; the latter is the main correlation criterion of the Lutetian Global Stratotype Section and Point (GSSP) recently defined at Gorrondatxe (Basque Country). By counting precession-related mudstone–marl couplets of 21 ka, the time lapse between both events was calculated to be 819 ka. This suggests that the age of the CP12a/b boundary, and hence that of the Early/Middle Eocene boundary, is 47.76 Ma, 250 ka younger than previously thought. This age agrees with, and is supported by, estimates from Gorrondatxe based on the time lapse between the Lutetian GSSP and the C21r/C21n boundary. The duration of Chron C21r is estimated at 1.326 Ma. Given that the base of the Eocene is dated at 55.8 Ma, the duration of the Early Eocene is 8 Ma, 0.8 Ma longer than in current time scales. The Otsakar results further show that the bases of planktonic foraminiferal zones E8 and P10 are younger than the CP12a/b boundary. The first occurrence of Turborotalia frontosa , being approximately 550 ka older that the CP12a/b boundary, is the planktonic foraminiferal event that lies closest to the Early/Middle Eocene boundary. The larger foraminiferal SBZ12/13 boundary is located close to the CP12a/b boundary and correlates with Chron C21r, not with the C22n/C21r boundary.