Carboniferous age for the East Greenland “Devonian” basin: Paleomagnetic and isotopic constraints on age, stratigraphy, and plate reconstructions

Abstract

New paleomagnetic and isotopic data from East Greenland indicate that this classical “Devonian” basin was partly formed in Carboniferous. The basin preserves a stratigraphically linked magnetic reversal pattern of primary character. Paleomagnetic data indicate that the two stratigraphically lowermost intrabasinal angular unconformities, identified on each side of the basin, in fact correlate as one unconformity. This implies a 2 km reduction of the estimated basin thickness, and thus that the unconformity represents a major depositional hiatus. Successions below the unconformity are taken to be Devonian (Givetian) in age, on the basis of correlation with paleomagnetic reference poles. However, we argue that the overlying strata are Carboniferous, rather than Devonian, in age, on the basis of a ca. 336 Ma 40Ar/39Ar extrusive age for a basalt flow and paleomagnetic data. A Carboniferous age for the strata has significant implications for vertebrate evolution; fossils of a terrestial tetrapod, Ichthyostega , are found above the unconformity. Ichthyostega is regarded as the earliest fossil of an animal known to walk on land; however, our data suggest that these dry footsteps are much younger than previously believed. Our results are also significant for plate reconstructions. Paleomagnetic data indicate that the lower part of the basin was deposited at low southerly latitudes. Sediments above our Early Carboniferous unconformity were deposited approximately at lat 4°N, indicating that the continent had drifted northward. A minor pole-longitude misfit between Devonian and Carboniferous poles from East Greenland and North America implies (1) a closer pre–Labrador Sea Greenland–North America fit; (2) counterclockwise block rotations (10°–15°) of the study area; or (3) a combination of both. The East Greenland “Devonian” basin formed along the Caledonian spine of Euramerica, and counterclockwise block rotation may have occurred between sinistral faults resulting from continued relative movement between Baltica and Laurentia during Devonian and Carboniferous time.