Int J Earth Sci (Geol Rundsch) (2008) 97:785–797 DOI 10.1007/s00531-007-0186-y
ORIGINAL PAPER
Coastal sediments from the Algarve: low-latitude climate archive for the Aptian-Albian Ulrich Heimhofer Æ Thierry Adatte Æ Peter A. Hochuli Æ Stefan Burla Æ Helmut Weissert
Received: 27 September 2006 / Accepted: 28 February 2007 / Published online: 3 April 2007 Springer-Verlag 2007
Abstract The Late Aptian to Early Albian transition has previously been identified as a possible example of substantial climate cooling within the mid-Cretaceous greenhouse period. To study the response of continental weathering and terrestrial vegetation to this cooling episode at low- to mid-latitudes, marine nearshore deposits from the Algarve Basin (SW Portugal) have been investigated with a combined approach including palynology, clay mineralogy and bulk-rock geochemistry. In the Lower Aptian part of the succession, quartz-rich sandstone facies is accompanied by high abundances of early diagenetic kaolinite, which is interpreted to reflect episodes of enhanced humidity and high meteoric flow-through. In contrast, the Late Aptian to Early Albian deposits are characterized by high abundances of detrital clay minerals (mica and chlorite) indicating the dominance of physical weathering processes in the source area, most probably related to low precipitation rates in conjunction with tectonically enhanced erosion. Palynological data show a U. Heimhofer (&) Institute for Geology, Mineralogy and Geophysics, Ruhr-Universita¨t Bochum, Universita¨tsstr. 150, Bochum 44801, Germany e-mail:
[email protected] T. Adatte Institute of Geology, University of Neuchaˆtel, Emile-Argand 11, Case Postale 158, Neuchaˆtel CH2009, Switzerland P. A. Hochuli Palaeontological Institute, University of Zu¨rich, Karl Schmid-Strasse 4, Zu¨rich 8006, Switzerland S. Burla H. Weissert Geological Institute, ETH Zu¨rich, Sonneggstr. 5, Zu¨rich 8092, Switzerland
strong dominance of Classopollis pollen associated with low pteridophyte spore abundances, suggesting warm semi-arid to arid palaeoenvironments. Changes in sedimentation patterns from varicoloured lagoonal marls to thick-bedded shallow-water carbonates are neither expressed in the spore-pollen assemblages nor in the distributions of clay minerals which both remain essentially stable throughout the Late Aptian to Early Albian. These relatively stable patterns are in contrast with various lines of evidence, predominantly from high-latitude areas, that suggest a significant cooling during this time interval. Our study demonstrates that terrestrial environments of low- to mid-latitude regions were not significantly affected by the Late Aptian - Early Albian ‘‘cold snap’’.
Keywords Early Cretaceous Algarve Basin Palaeoclimate Palynology Clay mineralogy
Introduction The mid-Cretaceous is generally referred to as a period of extreme global warmth and represents one of the best examples of ‘‘greenhouse’’ conditions in the geological record. However, the stability of the Cretaceous greenhouse is still a matter of debate and the long-held view of an equable and stable Cretaceous climate has been questioned (e.g. Kemper 1987). The Aptian to Early Albian interval (121–106 Ma, Gradstein et al. 1995) has been identified as a possible period of substantial climatic change within the mid-Cretaceous greenhouse period (Fig. 1). Mean temperature estimates derived from d18O measurements of phosphatic fish remains (Puce´at et al. 2003), rudist shells (Steuber et al. 2005), benthic forami-
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Fig. 1 Palaeogeographic reconstruction during the mid-Cretaceous at 112.0 Ma (modified after Geomar map generator; http:// www.odsn.de). Asterisks mark the studied section and successions used for comparison; circles show locations bearing evidence for climate cooling during the Aptian to Early Albian interval. 1 Algarve Basin, Portugal (this study); 2 Agadir and Essaouira Basins, Morocco (Daoudi and Deconinck 1994); 3 Vocontian Basin, France (Bre´he´ret 1997); 4 Vocontian Basin, France (Herrle and Mutterlose 2003); 5
Central North Sea Basin (Ru¨ckheim et al. 2006); 6 Spitsbergen (Pickton 1981); 7 Sverdrup Basin, Canadian Arctic (Kemper 1987); 8 NE Alaska (Dettermann et al. 1975); 9 South Primorye, E Russia (Krassilov 1973); 10 Arroyo Calafate, Patagonia (Pirrie et al. 2004); 11 Exmouth Plateau (Clarke and Jenkyns 1999); 12 Eromanga Basin, Central Australia (De Lurio and Frakes 1999; Frakes and Francis 1988); 13 Gippsland Basin, E Australia (Constantine et al. 1998); 14 James Ross Island, Antarctica (Ditchfield et al. 1994)
nifera (Abreu et al. 1998) and carbonate fine-fraction (Clarke and Jenkyns 1999; Weissert and Erba 2004) show relatively low temperatures for the Aptian. This rather cool period is followed by an interval of temperature rise beginning with the Early Albian and eventually culminating in the Cenomanian/Turonian ‘‘hothouse’’ (Norris et al. 2002). Evidence for at least seasonally sub-freezing conditions has been inferred from the occurrence of ice-rafted debris and glendonites from high-latitude sites of both hemispheres during the Aptian-Albian transition (De Lurio and Frakes 1999; Frakes and Francis 1988; Kemper 1987; Price 1999). Stratigraphically more constrained evidence for a cooling episode is provided by an increase of boreal nannoplankton taxa in the Central North Sea and Vocontian Basins, indicating migration towards lower palaeolatitudes during the latest Aptian to earliest Albian (Herrle and Mutterlose 2003; Ru¨ckheim et al. 2006). Information on changes in humidity-aridity patterns during the corresponding time interval is mainly based on sedimentological, palynological and clay mineralogical data. A change from semi-arid conditions during the Early Aptian towards a more humid climate in the Late Aptian to Early Albian at mid-latitudes (western Europe) has been reported by Ruffell and Batten (1990). Based on changes in siliciclastic depositional patterns, Weissert (1990) and Wortmann et al. (2004) proposed an episode of accelerated hydrological cycling during the Aptian. In general, long-term changes in climate during the Aptian to Albian interval and subsequent changes in weathering patterns have been
linked to fluctuating palaeoatmospheric CO2 levels (e.g. Weissert and Erba 2004; Weissert and Lini 1991). For a more detailed discussion of the evidence for cool intervals during the mid-Cretaceous we refer to the compilations of Kemper (1987), Frakes and Francis (1988) and Price (1999). The extent and significance of the Aptian to Early Albian climatic fluctuations have not been fully examined, especially with regard to the low- to mid-latitudes. Shallow-water deposits from the Portuguese Algarve Basin show distinct changes in sedimentation patterns during Aptian to Early Albian times. A prominent shift from coastal sandstones (Early Aptian) to varicoloured marls and claystones (Late Aptian) to thick-bedded carbonates (Early Albian) is prominently exposed in that region. These deposits represent an ideal archive to test if the distinct changes in lithology and facies are controlled by sea-level fluctuations, as proposed by Rey (1986), and/or reflect a major palaeoclimatic change during the corresponding time interval as documented from higher-latitude sites. To study the palaeoclimatic evolution of the Portuguese succession, we chose a twofold approach. (1) Palynological analysis of spore-pollen assemblages offers the opportunity to investigate climatically induced variations in past vegetation patterns. The distribution and abundance of certain sporomorph groups (incl. Classopollis pollen, pteridophyte spores) are strongly controlled by the prevailing climatic conditions. (2) Clay mineral assemblages are significantly influenced by the dominant weathering processes and
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provide information on changes in aridity/humidity patterns. These two data sets are complemented by sedimentological, palynofacies and bulk-rock geochemical results to investigate the response of the depositional system to changes in sea-level and detrital input.
Geological setting The Algarve Basin is located at the south-western tip of the Iberian peninsula, accessible in outcrop along the south coast of Portugal (Fig. 2). During the Mesozoic, the opening of the Atlantic and Tethys oceans resulted in the formation of passive margins in western and south-western Iberia (the Lusitanian and Algarve Basins, respectively). Subsidence along the Algarve margin was controlled by extensional tectonics and the resulting Cretaceous palaeotopography was a smoothly inclined sedimentary ramp, slowly deepening towards the open ocean (Rey 1983, 1986). The studied succession covers more than 260 m of Aptian to Early Albian strata, well exposed along the coastal cliffs southwest of the village Lagos in the western Algarve region (Fig. 3) and accessible along a ~2.5 km long strip between the Praia da Luz (east of the village Luz) and the Praia da Porto de Mo´s (2 km southwest of Lagos). Following Rey (1983, 1986), the sedimentary succession can be separated into 4 lithostratigraphic units, including the Choffatella decipiens Marls, Palorbitolina Beds, Luz Marls and Porto de Mo´s Formation (Fig. 4). The upper part of the Choffatella decipiens Marls (19 m thickness) encompasses a succession of alternating beds of gypsiferous marls, bioclastic limestones and dolomicrites which have been
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Fig. 2 a Location of the Algarve Basin in southern Portugal. b Map of the Algarve region with the location of the studied Luz section (arrow)
Fig. 3 Outcrop photograph showing the transition from the Late Aptian to the Early Albian in the Luz section, Algarve Basin, Portugal. Note the distinct shift in lithology from the marls and claystones predominating in the Late Aptian towards the thick-bedded limestones in the Early Albian
deposited in a shallow marine to lagoonal and partly evaporitic setting. The overlying Palorbitolina Beds (25 m thickness) are represented by massive, oblique-bedded coastal sandstones containing abundant nerinean gastropod coquinas. Above a distinct hardground, the Luz Marls (146 m thickness) consist of a monotonous succession of variegated marls and claystones with few intercalated siltand limestone beds that were deposited in a restricted lagoonal-brackish marsh environment with few open-marine episodes. The Porto de Mo´s Formation (70 m thickness) is composed of thick-bedded, bioturbated limestones alternating with calcareous marls. Typical sedimentary structures include laminations, bored hardgrounds and fenestrae, indicating a carbonate-dominated shallow-water depositional environment. Similar lithostratigraphic successions covering the Palorbitolina Beds, the Luz Marls and the Porto de Mo´s Formation have been described by (Rey 1983) from the central and eastern parts of the Algarve Basin (Porches and Alfandanga-Pa˜o Branco sections, respectively). The stratigraphy of the different formations has been mainly based on orbitolinids, calcareous algae, ostracods and dinoflagellate cysts (Berthou and Leereveld 1990; Ramalho and Rey 1981; Rey 1983, 1986). Based on the recent integration of chemo- and palynostratigraphic results, a refined stratigraphic framework of the Luz section is provided (Heimhofer 2004) (Fig. 4). According to the revised stratigraphy, the Choffatella decipiens Marls and the Palorbitolina Beds correspond to the Early Aptian. The Early to Late Aptian boundary is located within a significant hiatus marked by a hardground (D3 of Rey 1986) on
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Int J Earth Sci (Geol Rundsch) (2008) 97:785–797
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Biostratigraphy based on Heimhofer (2004) and Burla et al. (submitted). C. d. Marls, Choffatella decipiens Marls; P. Beds, Palorbitolina Beds
top of the Palorbitolina Beds. An Early Albian age is assigned to the Porto de Mo´s Formation. The position of the Aptian-Albian transition is confirmed by carbon-isotope chemostratigraphy and marked by a distinct decline in d13C values (Heimhofer et al. 2003; Herrle et al. 2004). During the Late Aptian, the Algarve Basin was situated at a palaeolatitude of ~20N (Hay et al. 1999). According to Brenner (1976) and Batten (1984), the corresponding area was located in the southernmost part of the Southern Laurasian floral province, which encompassed the midlatitudes of the northern hemisphere during Aptian to Albian times. A generally warm-temperate to subtropicalhumid climate has been inferred for this floral province, whereas the Northern Gondwana province, adjacent to the south, was characterized by savanna-type vegetation thriving under arid to semi-arid conditions (Brenner 1976; Chumakov et al. 1995; Dino et al. 1999). Evidence for the existence of a tropical everwet belt during the late Early
Cretaceous is scarce, pointing towards a interval of increased aridity (Ziegler et al. 2003).
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Methods Bulk geochemical parameters Inorganic and total carbon contents of 137 samples were determined using a UIC CM 5012 Coulomat. Total organic carbon (TOC) contents were calculated from the difference between total and inorganic carbon contents. Determination of kerogen type and organic matter (OM) maturity was performed on ~100 mg of dried and ground rock sample using whole-rock pyrolysis (Rock-Eval 6 pyrolysis instrument) applying standard cycles (Behar et al. 2001). The hydrogen index (HI in mg HC/g TOC) was calculated from the amount of free hydrocarbons thermo-vaporizable be-
Int J Earth Sci (Geol Rundsch) (2008) 97:785–797
tween 300 and 550C (S2 value) and normalized to the TOC content of the sample. The oxygen index (OI in mg CO2/g TOC) was calculated from the amount of CO2 generated from the organic matter between 300 and 390C (S3 value) based on the TOC content. The Tmax parameter (in C) represents the temperature of maximum rate of hydrocarbon generation during S2. Clay mineralogy Fourty-five samples from the Luz section were analysed for their clay mineralogy. Clay mineral analyses were based on methods described by Adatte et al. (1996). Ground chips were mixed with deionized water (pH 7–8) and agitated. The carbonate fraction was removed with the addition of HCl 10% at room temperature for 20 min or longer until all the carbonate was dissolved. Ultrasonic disaggregation was accomplished during 3 min intervals. The insoluble residue was washed and centrifuged (5–6 times) until a neutral suspension was obtained (pH 7–8). Separation of different grain size fractions (
