Palynofacies analysis of a lacustrine succession: The Maracangalha Formation (Lower Cretaceous), Recôncavo Basin, Brazil

Abstract

A palynofacies analysis was carried out using samples collected from the Manguinhos, Gameleira, Praia da Falha and Penha outcrops of the Lower Cretaceous Maracangalha Formation, Recôncavo Basin, NE Brazil. Qualitative and quantitative analysis of the organic particles, as well as grouping analysis based on the characteristics of the particles found, were conducted to establish the present database. The organic particles were grouped into amorphous organic matter (AOM); opaque phytoclasts; non-opaque phytoclasts and palynomorphs and four distinct palynofacies were recognized. The palynofacies I (P I), present in the Praia da Falha outcrop, is composed of a high abundance of non-opaque phytoclasts (51.22%), followed by palynomorphs (23.61%). The palynofacies II (P II), occurring in the Penha outcrop, shows a high abundance of non-opaque phytoclasts (71.67%), followed by opaque phytoclasts (12%). The palynofacies III (P III), occurring in the Manguinhos outcrop, consists of a high abundance of AOM (48.07%), followed by opaque phytoclasts (32,6%). The palynofacies IV (P IV), occurring in the Gameleira outcrop, also has a high abundance of AOM (47.05%), followed by non-opaque phytoclasts (28.76%). According to the associations of sedimentary organic matter, essentially represented by particles of continental origin, a lacustrine setting is well characterized, corroborating what has been previously suggested in the literature. The palynofacies interpretation also indicates deposition strongly controlled by fluvio-deltaic input. Overall, the Praia da Falha and Penha outcrops suggest proximity to the depositional source in a high-energy environment. In these localities, the predominance of non-opaque phytoclasts indicates deposition under oxic conditions probably near the lake shore. On the other hand, in the Gameleira and Manguinhos outcrops, the predominance of AOM suggests deposition away from the sediment source in a more distal, low-energy environment, with prevalence of suboxic-to-anoxic conditions.