Palaeontological framework from Pirabas Formation (North Brazil) used as potential model for equatorial carbonate platform

Highlights

• Microfossil assemblages suggest a middle Miocene age for the uppermost Pirabas Formation at the Atalaia outcrop.

• The palaeoenvironment are characterized by coastal lagoons with mangrove forests and shallow inner platform.

• Microfossil frameworks are responsible for the heterogeneity in the porosity values

• The ultimate driver of the collapse of Pirabas carbonate production is still uncertaing

• The Pirabas Formation could become a facies model for Neogene tropical carbonate deposits of the tropical Atlantic

Abstract

The Pirabas Formation (early to middle Miocene) from the equatorial margin of North Brazil is characterized by a shallow-marine carbonate platform with high fossil diversity and abundant micro- and macrofossil remains. The Pirabas Formation represents a unique carbonate system along the Atlantic margin of South America that developed before the onset of the Amazon delta. We studied the palaeontology and lithofacies of outcrops of the uppermost Pirabas Formation and found that was deposited in a coastal marine environment with marginal lagoons under the influence of a tidal regime and tropical storms. The remains of calcareous algae, molluscs, crustaceans, echinoiderms, bryozoans, solitary corals, fish and marine mammals, together with foraminifera, ostracods and other marine microfossils, shaped a biogenic framework, that together with the post-depositional processes of dissolution of skeletal grains, is responsible for the mean packstone-floatstone porosity of 14.9%. The palaeontological framework and the petrophysical characterization of the carbonate rocks from the uppermost Pirabas Formation outcrop represent a baseline to interpret the entire Pirabas Formation in the subsurface stratigraphic sections (cores) of this important Neogene unit. Considering that carbonate rocks account for ~50% of oil and gas reservoirs around the world, this research provides a model for Neogene tropical carbonate deposits useful for carbonate petroliferous reservoirs in the Brazilian equatorial basins.

Introduction

The carbonate platform in the equatorial margin of North Brazil (Soares Jr. et al., 2011) includes the Foz do Amazonas Basin (Figueiredo et al., 2007), Pará-Maranhão Basin (Soares et al., 2007) and Barreirinhas Basin (Trosdtorf Jr. et al., 2007) in the coastal plain of the states of Pará and Maranhão. Oligocene-Miocene circumtropical carbonate deposits have similar stratigraphic characteristics across tropical America (Leigh et al., 2013), Asia (Vahrenkamp, 1998; Zampetto et al., 2003; Janjuhah et al., 2017a; Dill et al., 2018), Africa (Buchbinder, 1996; John et al., 2003) and Australia (Ehrenberg et al., 2006), and most of these Cenozoic carbonate sequences are potential reservoirs for gas and oil exploration.

Miocene carbonate deposits from northern Brazil are exposed along the onshore coastal plain of Pará State and are represented by the Pirabas Formation (Maury, 1925). The Pirabas Formation accumulated in a western Atlantic shallow-marine setting until carbonate production was terminated as consequence of the massive input of siliciclastic sediments from both the Amazon delta and the coastal plain drainages during the late Miocene (Damuth and Kumar, 1975; Wolff and Carozzi, 1984; Brandão and Feijó, 1994; Silva et al., 1998; Figueiredo et al., 2009; Watts et al., 2009). Andean alluvial terrigenous sediments (Hoorn et al., 2017) filled the Marajó Basin following the progradation of the Barreiras Formation which overlaps the Pirabas carbonates (Rossetti et al., 2013; Aguilera et al., 2017a)

The Pirabas Formation outcrops, firstly studied by Ferreira-Penna (1876), have a high density and diversity of fossils. White (1887) and Maury (1925) conducted the first studies of the mollusc, bryozoan, and coral assemblages from this formation. Palaeontological contributions by Petri (1957) on foraminifera, Beurlen, 1958a, Beurlen, 1958b on crustaceans, Santos, 1958, Santos, 1967 on echinoids, Barbosa (1961) on bryozoans, Fernandes, 1979, Fernandes, 1981 on corals, Santos and Travassos (1960) on fish, Paula-Couto (1967) on sirenids, and Duarte (2004) on the palaeoflora further improved the knowledge of the Pirabas Formation. Additional contributions included new species descriptions and new records compiled by Távora et al. (2010), Aguilera and Páes (2012), Aguilera et al., 2013a, Aguilera et al., 2013b, Aguilera et al., 2013c, Aguilera et al., 2014, Aguilera et al., 2017a, Aguilera et al., 2017b and palaeontological reviews of bryozoans (Távora et al., 2014; Zágoršek et al., 2014; Ramalho et al., 2015, Ramalho et al., 2017; Muricy et al., 2016), echinoids (Mooi et al., 2018) and ostracods (Nogueira and Nogueira, 2017). Palaeoclimatic interpretations based on taphoflora show mean annual atmospheric temperatures ranging between 24.6 °C and 25.0 °C and mean annual precipitation between 1849 and 2423 mm (Santiago and Ricardi-Branco, 2018). Isotopic analyses of teeth from fossil elasmobranchs (derived δ¹⁸O temperature) indicated a mean seawater palaeotemperature of 26.3 °C, ranging between 21.7 °C and 30.1 °C (Aguilera et al., 2017a). Both terrestrial palaeoclimate and the ocean palaeotemperatures could be related to the final stages of an abrupt episode of global cooling at the Oligocene-Miocene boundary, the Mi-1 glaciation (Stewart et al., 2017; Egger et al., 2018) and the global warming period of the middle Miocene climate maximum (You et al., 2009; Goldner et al., 2014).

In spite of the abundant and excellent preservation of the fossil record in the Pirabas Formation, palaeontological research in the Pirabas Formation over the past two decades has been scarce, isolated, and restricted to outcrop surveys and has produced doubtful taxonomic identifications (e.g., see Muricy et al., 2016 for bryozoans; Luque et al., 2017 for crustaceans). Furthermore, both the stratigraphic framework of the carbonate deposits and the influence of diagenesis on the carbonate porosity remain unknown.

The main aims of this work are to provide an accurate description of the biogenic framework of carbonate rocks, detailed palaeoenvironmental interpretation, palaeontological assemblage descriptions and a stratigraphic facies model for Pirabas Formation. We used a wide arrange of tools in multiple scientific fields to reach accurate palaeoenvironmental results using high technological resources and innovative solutions in the field of micropalaeontology. The new dataset acquired by microcomputer tomography (CT) allow high-resolution recovery of fossil arrangement in the matrix and can provide an important baseline for core research on analogous equatorial Brazilian carbonate platforms.