Reconstructing environmental signals across the Permian-Triassic boundary in the SE Germanic Basin: A Quantitative Provenance Analysis (QPA) approach

Highlights

• Drainage modifications are efficiently identified via Quantitative Provenance Analysis.

• Constraining Environmental Signals in deep-time requires paleoclimatic/tectonic models.

• Paleodrainage analysis provides the parameters to calculate sediment flux.

• Sediment flux increases in the Lower Triassic in response to climate change.

Abstract

Global tectonic and climatic models for the Permian-Triassic boundary (PTB) are highly debated. One of the most disputed topics is the temperature increase associated with CO₂ emissions generated by the Siberian Trap volcanism and its potential influence on chemical weathering and associated variations in sediment fluxes. By integrating crustal architecture, plate modelling, structural kinematics, and two climatic models, we reconstruct the drainage evolution of Variscan tectonostratigraphic units from the SE portion of the Germanic Basin, from which we also extract the parameters necessary to calculate sediment flux across a time-scale of 28 Ma (Guadalupian-Lower Triassic). We reconstruct the sedimentary response to climatic and tectonic perturbations using Quantitative Provenance Analysis (QPA) and integrate compositional data into a sedimentological framework, paleodrainage and paleoclimatic models. Raman heavy mineral analysis, as well as geochronology and geochemistry of detrital apatite, zircon, and rutile, document variation in drainage lithologies and sediment flux which are controlled by regional extensional tectonics and increasingly humid conditions at the PTB. The sedimentary successions of the SE Germanic Basin record climatic perturbations on a 10⁴ years timescale, while the effects of tectonics are visible on a 10⁶ years timescale. The interplay of climate, tectonics and lithology, and their effects on sediment production and drainage evolution resulted in changes in sediment flux from 2.3 Mt./yr during the Guadalupian (Capitanian), to 3.80 Mt./yr in the Lopingian (Changhsingian) to 7.44 Mt./yr at the end of the Lower Triassic (Olenekian). The multifaceted workflow provided in this study represents the first step towards more precise reconstructions of sediment routing systems in deep-time and provides the first ground-truthed quantification of sediment flux across the Permian-Triassic Boundary.

Introduction

The identification of tectonic and climatic perturbations in deep-time and their effect on the dynamics regulating the development of sediment routing systems (SRS) and sediment fluxes is one of the most challenging topics in sedimentary geology (Weltje et al., 1998; Castelltort and Van Den Driessche, 2003; Romans et al., 2016; Allen, 2017; Caracciolo, 2020). Limiting factors complicating the reliable reconstruction of the evolution of paleo-SRS and the quantification of sediment fluxes are processes that shred environmental signals in the SRS (e.g. Jerolmack and Paola, 2010), the incompleteness of the stratigraphic record (e.g. Miall, 2015), and/or compositional signatures which are not diagnostic of drainage lithologies, as well as uncertainties associated with reconstructions of climatic and tectonic settings (Brewer et al., 2020; Caracciolo, 2020; Ravidà et al., 2021a). Besides tectonics and climate, the compositional signatures of the erosional products are dramatically influenced by the type of lithology, with different rock types producing different amounts of gravel, sand, silt, clay, and solutes (Palomares and Arribas, 1993, Arribas and Tortosa, 2003, Caracciolo et al., 2012, Allen and Allen, 2013, Caracciolo et al., 2015), which in turn has significant implications for detrital single-grain provenance studies (e.g. Chew et al., 2020). Furthermore, geological processes operating on a 10⁶–10⁸ time scale might considerably alter the preservation of both the architectural elements and the sandstone components diagnostic of sedimentary provenance, or the resolution to which analytical tools decrease in deep-time (Fig. 1). Quantifying sediment fluxes in ancient systems can be done using a mass-balance approach (Hinderer, 2012, Mansouri and Hinderer, 2020), although precise estimations require both closed sedimentary systems and the full preservation of the sediment volume. Methods which are independent from the amount of preserved sedimentary basin fill are generally based on three different approaches, namely (i) the BQART model (Syvitski and Milliman, 2007), (ii) the Geomorphological Scaling (Nyberg et al., 2018, Sømme et al., 2009), and (iii) the Fulcrum model (Holbrook and Wanas, 2014) the use of which is conditioned by the availability of palaeohydrological parameters (Brewer et al., 2020). In mass balance calculations, the generation of sediments in drainage lithologies is traditionally seen as a boundary condition for sediment supply rather than a dynamic system determining specific textural and compositional properties and, ultimately, the sediment cascade (Weltje, 2012). Quantitative Provenance Analysis (QPA) - the quantitative assessment of the controlling factors on sediment character and composition, and the quantification of the amount and rate of supply of detrital material from identifiable parent-rock assemblages to a basin fill (Basu, 2003, Weltje and von Eynatten, 2004; Caracciolo et al., 2020) – is arguably the most suitable approach as it allows extracting data from the sedimentary archive and produce inverse models which use the physical and chemical properties of detritus of the basin fill to infer the volumes and nature of the parent rocks in the source region (Weltje, 2012; Caracciolo et al., 2020).

The identification of environmental signals across the Permian-Triassic Boundary (PTB) at global scale is particularly challenging due to the uncertainty of available local/regional climatic models, tectonic constraints, and especially the difficulty in determining the precise stratigraphic age of terrestrial sediments. While there is a general agreement that the PTB is amongst the driest periods in Earth's history, the role of exhumation/subduction processes at global scale is considered to be negligible by part of the community working on the PTB (e.g. Algeo and Twitchett, 2010). However, some of the recent literature challenges the commonly held views on the PTB climate and tectonics. Several studies have suggested that global warming across the Permian-Triassic transition greatly enhanced continental weathering thus providing more nutrients to the ocean (e.g. Joachimski et al., 2012; Joachimiski et al., 2019). Increasingly humid conditions would correspond a change in fluvial patterns, with a general switch from ephemeral towards perennial rivers (Smith, 1995; Ward et al., 2000; Wilson et al., 2019; Zhu et al., 2019; Ravidà et al., 2021a) and an increase of sediment flux of up to 700% from the Changhsingian onwards (Algeo and Twitchett, 2010), while in the Central European Basin System a mass-balance model indicate an increase of ca. 470% (Mansouri and Hinderer, 2020). Furthermore, rapid tectonic exhumation across the PTB is documented in several places around the world including the Moroccan-Mauritanian Atlas (Charton et al., 2020), the Appalachians (Hatcher Jr. et al., 2002), and the Urals (Glasmacher et al., 2002). In central-western Europe, and particularly in the Germanic basin, the PTB is considered to tectonically quiescent, but some studies indicate a rather different scenario in which transtensional tectonics (e.g. Mattern, 1995a, Mattern, 1995b, Peterek et al., 1997) and climatic settings (e.g. Jewula et al., 2021) might have changed considerably across the PTB. However, the increase in sediment production and water discharge and the consequent modifications of sediment routing systems in central Europe is still a matter of debate.

In this paper, we investigate the evolution of continental successions deposited between the Middle Permian (Guadalupian) and the Early Triassic (Induan-Anisian) in the southeasternmost portion of the Germanic basin. Identifying changes in tectonic and climatic settings and quantifying sediment flux in this region is difficult due to substantial destruction of the basin following late Mesozoic basin inversion, along with the limited availability of surface exposures. We use a state of the art QPA approach including sandstone petrography (from Ravidà et al., 2021a), Raman heavy mineral analysis (e.g. Andò and Garzanti, 2013; Lünsdorf et al., 2019), multi-varietal UPb geochronology (on detrital apatite, zircon, and rutile grains), and apatite and rutile geochemistry, and integrate the results into a detailed sedimentological, environmental and paleodrainage framework obtained from crustal architecture, plate modelling, structural kinematics and two climatic models. The output data allow to calculate sediment flux using the BQART equation (Ravidà et al., 2021b) and integrate it to detailed QPA. The results provide key constraints on the timing of climatic and tectonic perturbations across the Permian-Triassic boundary in the southern Germanic basin and the associated drainage response. They reveal increasing sediment flux in the early Triassic, with important implications for central European paleogeography and similar successions around the world. Furthermore, the results present a comprehensive and novel approach to increase the resolution by which paleo-SRS can be reconstructed.