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

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.

Autogenic dynamics of alluvial fans in endorheic basins: Outcrop examples and stratigraphic significance

“…Because of the autogenic switching of active fan lobes, stratigraphic successions of fan systems accumulate under spatially inhomogeneous patterns of aggradation and non‐deposition that can lead to local stratigraphic incompleteness and lack of chronological correlation between adjacent deposits. The problem has been frequently underestimated in analyses of thick fan successions, due to the common lack of high‐resolution chronostratigraphic data to constrain the age and correlatability of different depositional units (Swanson‐Hysell & Barbeau, ). Recent evidence from Quaternary piedmont fans in California demonstrates that the absence of an incised fan‐head channel favours spatially uniform aggradation and complete resurfacing over time spans in the order of a few thousand years (Dühnforth et al ., ).…”

“…Went, ), where the shingled stacking of depositional units does not produce notable textural and facies variations (Beaty, ; Blair, ). As observed by several authors, the main uncertainty in this approach to architectural interpretation is determined by the spatial and temporal overlap between stratigraphic products of high‐frequency climate cycles and autogenic mechanisms, and by the difficulty in obtaining chronologies of sufficient resolution to discriminate between architectural elements (Gloppen & Steel, ; Mustard, ; Swanson‐Hysell & Barbeau, ). Studies of Quaternary systems apply a variety of dating methods to estimate patterns of fan‐surface activity and abandonment (e.g.…”

Tectonic and climatic controls on the sequential arrangement of an alluvial fan/fan‐delta complex (Montserrat, Eocene, Ebro Basin, NE Spain)

“…It is worth noting that the compacted accumulation rates obtained for Montserrat (up to 42 cm kyr −1 ) are strikingly higher than average accumulation rates estimated (15–20 cm kyr −1 ) in Priabonian to Oligocene synorogenic alluvial successions in other marginal areas of the SE Ebro Basin (Jones et al ., ; López‐Blanco et al ., ; Swanson‐Hysell & Barbeau, ; as re‐interpreted in Garcés et al ., ; Costa et al ., ). These contrasting patterns of accumulation along the CCR‐Ebro Basin foreland system may be related to differences in subsidence and, therefore, in the structural style.…”

“…Subsidence was distributed along a wider region ahead of the mountain front, resulting in lower average accumulation rates of about 15 cm kyr −1 . Another study in the Gandesa area reported sedimentation rates as high as 32 cm kyr −1 (Swanson‐Hysell & Barbeau (), but we note that these are based on a correlation with the GPTS which is not final as admitted by the same authors (Swanson‐Hysell & Barbeau, ). Despite the criticism of Swanson‐Hysell & Barbeau (, ), we rely on the magnetostratigraphic study of Jones et al .…”

“…Another study in the Gandesa area reported sedimentation rates as high as 32 cm kyr −1 (Swanson‐Hysell & Barbeau (), but we note that these are based on a correlation with the GPTS which is not final as admitted by the same authors (Swanson‐Hysell & Barbeau, ). Despite the criticism of Swanson‐Hysell & Barbeau (, ), we rely on the magnetostratigraphic study of Jones et al . (), because it presents a robust local magnetostratigraphy, with a unique correlation with the GPTS as discussed in Garcés et al .…”