The diachroneity of alluvial-fan lithostratigraphy? A test case from southeastern Ebro basin magnetostratigraphy

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Abstract

Alluvial-fan strata contain valuable proxy data for kinematic, climatic and sediment-flux phenomena associated with high-relief source regions. Proper application of this data is dependent upon acquisition of accurate high-resolution chronostratigraphic data, best achieved through magnetostratigraphy. Collection of magnetostratigraphic and other proxy data from composite alluvial-fan sections assumes the chronostratigraphic significance of lithostratigraphy. We present the first test of this assumption with new magnetostratigraphy collected from Paleogene rocks of the southeastern Ebro basin. We report five new magnetostratigraphic sections that collectively sample 850 m of alluvial-fan and associated fluvial strata. One-hundred meters of lithostratigraphic overlap between these sections reveals equivalent magnetic polarities over short along-strike distances (∼200–2000 m). However, lithostratigraphic correlation of these sections with the magnetostratigraphy of continuously exposed strata eight km away reveals discrepancies in magnetic polarity trends and age interpretations.

Correlation of our 760 m composite magnetostratigraphic section with the Paleogene geomagnetic polarity time scale suggests that this section spans from ca. 31.3 to 27.7 Ma (C12r–C9n; early to late Oligocene). This correlation implies steady sedimentation rates of ∼12 cm/ka for fluvial facies and ∼32 cm/ka for alluvial-fan facies. Alternative interpretations that force chronostratigraphic correlation with the lithostratigraphically equivalent interval generate unsteady and/or unlikely sedimentation rates in one or both of the stratigraphies and require rejection of multiple well-constrained polarity zones. The absence of significant surfaces of erosion or paleosol development argue against these alternative interpretations and suggest that even in the best exposed study areas, alluvial-fan successions may be diachronous and preclude lateral extrapolation of chronostratigraphic and other proxy data. Interpretations and models based upon data collected from alluvial-fan strata may be tenuous unless supported by sufficient spatial overlap to constrain the relationship between time surfaces and rock surfaces.

Introduction

Much of what we know about Earth history is derived from proxy records of regional and global change preserved in sedimentary rocks. Imperative to accurate collection and application of these proxy data is an understanding of the lateral and vertical relationships between strata and time in the rock record. To address this need, chronostratigraphic models have been developed and applied to marine (Vail et al., 1977, Van Wagoner et al., 1988), lacustrine (Bohacs et al., 2000, Keighley et al., 2003), fluvial (Shanley and McCabe, 1994, Currie, 1997), deltaic (Gani and Bhattacharya, 2005) and alluvial-fan strata (DeCelles et al., 1991a, Bourquin et al., 1998, Weissmann et al., 2002). Despite the elegance and utility of these models, they have only rarely been subjected to the test of independent geochronologic analysis (e.g., Vail et al., 1977, Behrensmeyer and Tauxe, 1982, Zechun et al., 1998, Prothero, 2001). Moreover, many of the models continue to be applied even after the validity of their fundamental assumptions is called into question (cf. Miall and Miall, 2000, Prothero, 2001, Dickinson, 2003). In this paper, we test the length scales of correlation between chronostratigraphy and lithostratigraphy in alluvial-fan strata, which contain proxy data that are widely applied to tectonic, climatic, and kinematic problems.

Alluvial fans are proximal depositional systems that accumulate sediments in the vicinity of high-relief source terrains and occur near the interface between drainage and depositional basins. Ancient alluvial-fan strata have been used to interpret climatic, plate tectonic, structural and autocyclic phenomena (Schumm et al., 1987, DeCelles et al., 1991a, Fraser and DeCelles, 1992, Burbank et al., 1992, Weissmann et al., 2002) and to develop influential theoretical sedimentary models (Heller et al., 1988, Blair and Bilodeau, 1988, DeCelles et al., 1991b, DeCelles and Mitra, 1995, Brozovíc and Burbank, 2000, Jones et al., 2004).

However, the cogency of such interpretations and models is often dependent upon high-resolution geochronology and the validity of assumptions used to establish the temporal relationship between data points. Alluvial fans present a challenge to both of these requirements of accurate application of proxy data. Datable volcanogenic strata are uncommon in alluvial-fan successions. The high-energy deposition of alluvial fans inhibits fossil preservation, which can generate age constraints in more calm terrestrial deposystems (Woodburne, 1987). Whereas fossils from more distal lacustrine and fluvial strata can assist in the dating of proximal successions, correlations across facies boundaries are difficult to achieve with high confidence. Provided sufficient abundance of suitable lithologies, magnetostratigraphy offers an opportunity to generate detailed chronostratigraphy from alluvial-fan successions (Burbank et al., 1992, Meigs et al., 1996, Lopez-Blanco et al., 2000, Jones et al., 2004). However, the local magnetic polarity stratigraphies (LMPS) that allow correlation to the geomagnetic polarity timescale (GPTS) are often collected by combining individual sections into a composite section. This correlation technique, which is also widely applied in the collection of other proxy data from alluvial-fan and other sedimentary rocks, exploits lithostratigraphic markers that are believed to have chronostratigraphic significance. Despite elegant depositional models for alluvial-fan chronostratigraphy (Schumm et al., 1987, DeCelles et al., 1991a, Weissmann et al., 2002), the validity of this assumption remains largely untested.

Application of lithostratigraphic correlation in alluvial-fan strata requires careful stratigraphic analysis because numerous characteristics of alluvial fans may inhibit local stratigraphic completeness and prevent accurate lateral correlations required to produce complete composite sections. Point-sourced sediments, autocyclic fluctuations and extrinsic forcing common to alluvial fans instigate avulsive processes that generate spatio-temporal variations in sediment accumulation (Schumm et al., 1987, Fraser and DeCelles, 1992, DeCelles et al., 1991a, Weissmann et al., 2002). The entrenchment and backfilling processes associated with alluvial-fan deposition (Schumm et al., 1987) can also result in erosion of previously deposited strata leading to further stratigraphic incompleteness. Variations in thrust-belt structural style and kinematic history can introduce along-strike age gradients in synorogenic strata (Butler et al., 2001, Nichols, 2002). Growth structures, which are common in alluvial-fan strata, often contain local intraformational unconformities (Anadón et al., 1986, DeCelles et al., 1991b, Lawton et al., 1999, Barbeau, 2003). Despite these hazards, numerous researchers have utilized alluvial-fan magnetostratigraphy to develop sedimentary histories and theoretical models by assuming lateral continuity and chronostratigraphic significance of proximal strata.

In this paper, we integrate new and existing magnetostratigraphic data with sedimentology and field mapping of a well-exposed Paleogene alluvial-fan succession adjacent to the Catalan Coastal Ranges (CCR) in the southeastern Ebro basin (Spain) in order to examine the synchronicity of alluvial-fan lithostratigraphy and magnetostratigraphy. We achieve these goals through the evaluation of candidate correlations of two composite LMPS to the GPTS in light of resultant sedimentation rates and stratigraphic architecture.

In addition to providing important tests of the efficacy of lithostratigraphic correlations and interpretations within proximal strata, this study attempts to use the new magnetostratigraphic dataset to constrain ages of CCR wedge-top deposition in the southeastern Ebro basin. Since the succession records tectonic activity along the eastern margin of Iberia, this new correlation provides valuable insight into the kinematics of the pre-rollback history of the western Mediterranean and the relative synchronicity of deformation in the CCR and the Pyrenees (Fig. 1).

Section snippets

Plate tectonics, thrust belts and basin formation

The Ebro basin formed concurrently with uplift and crustal shortening attributed to the Late Cretaceous–Paleogene collision of Iberia with western Europe and deformation in the Pyrenean, Iberian and CCR of northeastern Spain (Anadón et al., 1986, Lopez-Blanco, 2002). This deformation was partly coincident with 35° of anticlockwise rotation that translated the Iberian Peninsula from its Cretaceous position west of France to its current location via rotational opening of the Bay of Biscay (

Previous magnetostratigraphic work in the southeastern Ebro Basin

Barberà et al. (2001) constructed a magnetobiostratigraphy from diverse stratigraphic sections in the medial part of the eastern Ebro Basin to the northeast of the study area (Fig. 1). Using the magnetostratigraphy of lacustrine and distal fluvial strata and 53 fossil mammal sites ascribed to local biozones and/or European reference levels, they interpreted the intermediate to marginal facies of the partially exposed Montsant alluvial system to have been deposited from 34.8 to 23.7 Ma (late

Results

We report new paleomagnetic data (Fig. 6 and Fig. 7) from five sections that transect Rodamunts and Muntanya de Santa Bàrbara (Fig. 2, Fig. 3 and Fig. 7), which we compile into a single composite section (RMSB LMPS) covering a net ∼770 m of strata (Fig. 7). The analyzed samples were collected from red siltstone and fine sandstone intervals of the conglomeratic study interval. The methods of sample collection and analysis are included as supplementary material in the online version.

Correlation with the global polarity time scale

As in other proximal depozones (Colombo, 1994, Lawton et al., 1999, Lopez-Blanco et al., 2000, Lopez-Blanco, 2002), neither fossils nor volcanogenic strata have been identified in the immediate study area. Despite the absence of independent age constraints, robust evidence brackets the depositional age of the studied interval to the Paleogene period: (1) Paleocene and Eocene gastropod, ostracode and charophyte (López et al., 1985, Anadón et al., 1992) and Oligo-Miocene mammal faunal assemblages

Conclusions

  • 1.

    Paleomagnetic analysis of ∼865 m of alluvial-fan strata in the southeastern Ebro Basin reveals 15 reversals within a ∼760 m composite succession. Correlation of the resulting local magnetic polarity stratigraphy to the Paleogene global polarity time scale of Cande and Kent (1995) dates deposition from 31.4 to 27.7 Ma (C12r–C9n; early to late Oligocene). This correlation requires an increase in sedimentation rates from ∼12 cm/ka to ∼32 cm/ka at ca. 29.6 Ma (base of C11n.1), which is coincident

Acknowledgments

This work was supported by the Duncan Stewart Fellowship (Carleton College) awarded to NSH and support provided to DLB through ExxonMobil Upstream Research Company and the University of South Carolina. Joe Kirschvink generously provided access to his paleomagnetism laboratory at Caltech. Dave Bice was instrumental in the development of this project. Amy Moragues, Kate Stalker and Ellen Schaal provided assistance in the field. Bob Butler, Tim Lawton, Miguel Garcés, Adam Maloof, Cam Davidson and

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