Age of the Serra do Martins Formation, Borborema Plateau, northeastern Brazil : constraints from apatite and zircon fission track analysis.

Abstract

Results of apatite and zircon fission track analysis of samples from the Serra de Santana mesa provide quantitative constraints on the depositional age of the nonfossiliferous Serra do Martins Formation. This unit consists of sedimentary remnants preserved at high elevations on the Borborema Plateau and its distribution and age are considered important in understanding the geomorphological evolution and denudation history of northeastern Brazil following the early Cretaceous breakup. We also report apatite fission track results from post-rift units of the Potiguar Basin (Açu and Tibau formations). Apatite fission track analysis (AFTA) of samples from the Serra do Martins Formation suggests that they reached maximum paleotemperatures around 60°C, from which they began to cool some time between 30 and 0 Ma. Due to the high thermal gradients related to the prolonged Cenozoic volcanism in the study area, we hypothesize that the paleotemperatures modeled for those samples may be related dominantly to an anomalous heat flow, rather than to significant burial. Zircon fission track analysis (ZFTA) in two samples of the Serra do Martins Formation yields ages of 135±18 Ma and 165±40 Ma, but both samples show a significant spread in the data, and the youngest population of grains in these samples are characterized by ages of 83±5 Ma and 64±5 Ma, respectively. As the AFTA data show that these ages have not been reset after deposition, the zircon fission track ages must represent inherited provenance ages, demonstrating that those sediments can be no older than Paleocene. Independent lithological observations provide additional support to reject a stratigraphic correlation between the Serra do Martins and Açu formations. Combining the AFTA and ZFTA results, and integrating available geological evidence, we suggest that the Serra do Martins Formation was deposited some time between 64 and 25 Ma (Paleocene-Oligocene). Modeled thermal history solutions from AFTA in the Serra do Martins Formation samples suggest a cooling event during late Cretaceous/early Paleogene, interpreted as reflecting exhumation of sediment provenance terrain. The resulting erosional products form the clastics for the Serra do Martins Formation, which were deposited on an extensive regional planation surface – the Borborema Surface.