Search

The identification and distinction of fluvial from lacustrine deposits and the recognition of catchment changes are crucial for the reconstruction of climate changes in terrestrial environments. The investigated drill core succession shows a general evolution from red–brown claystones to white–grey marlstones and microcrystalline limestones, which all have previously been considered as relict deposits of an impact ejecta‐dammed lake, falling within the mid‐Miocene Climate Transition. However, recent mammal biostratigraphic dating suggests a likely pre‐impact age. Indeed, no pebbles from impact ejecta have been detected; only local clasts of Mesozoic formations, in addition to rare Palaeozoic lydites from outside of the study area. Lithofacies analysis demonstrates only the absence of lacustrine criteria, except for one charophyte‐bearing mudstone. Instead, the succession is characterized by less diagnostic floodplain fines with palaeosols, palustrine limestones with root voids and intercalated thin sandstone beds. Carbonate isotope signatures of the mottled marlstones, palustrine limestones and mud‐supported conglomerates substantiate the interpretation of a fluvial setting. Low, invariant δ18Ocarb reflects a short water residence time and highly variable δ13Ccarb indicates a variable degree of pedogenesis. Carbonate 87Sr/86Sr ratios of the entire succession show a unidirectional trend from 0.7103 to 0.7112, indicating a change of the solute provenance from Triassic to Jurassic rocks, identical to the provenance trend from extraclasts. The increase in carbonate along the succession is therefore independent from climate changes but reflects a base‐level rise from the level of the siliciclastic Upper Triassic to the carbonate‐bearing Lower to Middle Jurassic bedrocks. This study demonstrates that, when information on sedimentary architecture is limited, a combination of facies criteria (i.e. presence or absence of specific sedimentary structures and diagnostic organisms), component provenance, and stable and radiogenic isotopes is required to unequivocally distinguish between lacustrine and fluvial sediments, and to disentangle regional geological effects in the catchment and climate influences., China Scholarship Council http://dx.doi.org/10.13039/501100003398, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Since its recognition as an impact structure 60 years ago, no volcanics were anticipated in the circular depression of the 14.8 Ma old Nördlinger Ries. Here, we describe for the first time a volcanic ash‐derived clinoptilolite‐heulandite‐buddingtonite bed within the 330 m thick Miocene lacustrine crater fill. Zircon U‐Pb ages of 14.20 ± 0.08 Ma point to the source of the volcanic ash in the Pannonian Basin, 760 km east of the Ries. The diagenetically derived zeolite‐feldspar bed occurs in laminated claystones of the Ries soda‐lake stage and represents the first unequivocal stratigraphic marker bed in this basin, traceable from marginal surface outcrops to 218 m below surface in the crater center. These relationships demonstrate a deeply bowl‐shaped geometry of crater fill sediments, not explainable by sediment compaction and corresponding stratigraphic backstripping alone. Since most of the claystones formed at shallow water depths, the bowl‐shaped geometry must reflect 134 +23/−49 m of sagging of the crater floor. We attribute the sagging to compaction and closure of the dilatant macro‐porosity of the deeply fractured and brecciated crater floor during basin sedimentation and loading, a process that lasted for more than 0.6 Myr. As a result, the outcrop pattern of the lithostratigraphic crater‐fill units in its present erosional plane forms a concentric pattern. Recognition of this volcanic ash stratigraphic marker in the Ries crater provides insights into the temporal and stratigraphic relationships of crater formation and subsidence that have implications for impact‐hosted lakes on Earth and Mars., Plain Language Summary: We describe for the first time a volcanic ash layer from the lake sediment fill of the 14.8 million years old asteroid impact crater Nördlinger Ries. Radiometric age and trace element characteristics of this ash layer are identical to that of a volcanic field in Hungary, so that the ash reflects a volcanic eruption 760 km east of the Ries basin. Recognition of this ash layer enables its use as a marker bed. The ash layer can be traced from surface outcrops to 218 m depth in drillings. This indicates that the strata are significantly inclined toward the crater center. Calculations of sediment compaction by further sediment load and burial only partially explain the observed deeply bowl‐shaped geometry. We attribute the additional sagging to the subsidence of the crater floor substrate, formed of rocks highly shattered by the impact event. Both effects cause a concentric pattern of outcropping strata in the partially eroded crater fill. The presence of the ash layer and its use to help disentangle the source and timing of subsidence (due to compaction of lake sediments, and closure of deeper, impact‐induced fractures), has important implications for lakes formed in impact craters on Earth and Mars., Key Points: A critical question in the evolution of impact‐crater‐hosted lakes is the origin and timing of post‐impact floor subsidence We describe a volcanic ash layer from the Ries impact crater that demonstrates a deeply bowl‐shaped geometry of its lacustrine crater fill This geometry, leading to a concentric outcrop pattern, requires significant crater floor sagging, in addition to sediment compaction, German Research Foundation, Chinese Scholarship Council, Bolyai J. Research Fellowship