Abstract
Abstract Carboniferous–Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (∼1.5 km) succession of exclusively fine-grained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ∼25% of the section. In contrast, data presented here indicate that the lower-middle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and τNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ∼4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation.
Highlights
Global paleogeography in the late Paleozoic was governed by the assembly of Pangea, and formation of the low-latitude Central Pangean Mountains, which spanned from the VariscanHercynian system of Europe (Ziegler et al, 1979; Ziegler, 1996) west to the AppalachianOuachita-Marathon uplifts of North America
Building on previous discussions by Soreghan et al (2008a), Muhs (2013), and others, we reviewed the suitability of these processes and examined them in context of the Salagou Formation
The methods applied here are applicable for recognizing well-lithified and diagenetically altered ancient loessite. This is the first documentation of Upper Paleozoic loessite in eastern equatorial Pangea and represents an important contribution in an increasingly recognized record of pre-Quaternary loess for the late Paleozoic ice age
Summary
Ancient (pre-Quaternary) loess deposits record valuable information about paleoclimate and are well recognized in the Upper Paleozoic record of western equatorial Pangea (e.g., Murphy, 1987; Johnson, 1989; Kessler et al, 2001; Soreghan et al, 2002, 2008a, 2014b; Soreghan and Soreghan, 2007; Mack and Dinterman, 2002; Tramp et al, 2004; Sweet et al, 2013; Giles et al, 2013; Foster et al, 2014). Soreghan and Soreghan, 2007; Sweet and Soreghan, 2008; Soreghan et al, 2008a, 2008b, 2009, 2014a, 2014b), deposits in the Central Pangean Mountains in eastern equatorial Pangea (western Europe) might reflect similar controls on sedimentation across the low-latitude upland regions of the supercontinent. Carboniferous–Permian strata in basins that formed as a result of postorogenic collapse of the Variscan Mountains (eastern Central Pangean Mountains) in France preserve an unusually complete record of highland-proximal sedimentation in the late Paleozoic. This work presents new interpretations for these strata that have implications for our understanding of paleoequatorial climate during the decline of the late Paleozoic icehouse, and it contributes to our repertoire of methods for recognizing well-lithified, diagenetically altered loessite in the deep-time record
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