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

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.

冲积扇地层包含了与高地势源区相关的运动学、气候和沉积物通量现象的有价值的替代数据。这些数据的正确应用取决于获取准确的高分辨率年代地层数据，而通过磁性地层学最能实现这一点。从复合冲积扇剖面收集磁性地层学和其他替代数据是以岩石地层学的年代地层意义为前提的。我们利用从埃布罗盆地东南部古近纪岩石中收集的新磁性地层学数据，对这一假设进行了首次检验。我们报道了五个新的磁性地层剖面，它们总共对850米的冲积扇及相关河流地层进行了采样。这些剖面之间100米的岩石地层重叠显示，在短的走向距离（约200 - 2000米）内磁性极性相同。然而，将这些剖面与8公里外连续出露地层的磁性地层学进行岩石地层对比，发现磁性极性趋势和年龄解释存在差异。 将我们760米的复合磁性地层剖面与古近纪地磁极性年代表进行对比表明，该剖面跨度约为31.3至27.7百万年（C12r - C9n；早渐新世到晚渐新世）。这种对比意味着河流相的沉积速率约为12厘米/千年，冲积扇相的沉积速率约为32厘米/千年。强制与岩石地层等效区间进行年代地层对比的其他解释会导致一种或两种地层中不稳定和/或不太可能的沉积速率，并且需要否定多个约束良好的极性带。缺乏明显的侵蚀面或古土壤发育面与这些其他解释相悖，并表明即使在出露最好的研究区域，冲积扇层序也可能是穿时的，从而妨碍了年代地层和其他替代数据的横向外推。基于从冲积扇地层收集的数据所做的解释和模型可能是不可靠的，除非有足够的空间重叠来约束时间面和岩石面之间的关系。