The Pirabas Formation ( Early Miocene from Brazil ) and the Tropical Western Central Atlantic Subprovince

Abstract: the neogene tropical Western Central Atlantic-south American faunal assemblage from the Pirabas formation (Early Miocene) has specific and unique features, which distinguishes it from the rest of the Proto-Caribbean subprovince in diversity, paleoceanographic condition and ecosystems. Principal component analysis (PCA) distinguished three groups of localities with similar faunas (Group A: ilha de fortaleza, Colônia Pedro teixeira and Praia de fortalezinha; Group B: Estação Agronômica, ilha de Marajó, turiacu, Baixo Parnaíba, Aricuru and salinópolis; Group C: Capanema B-17 Mine) and five different depositional facies (surf zone, beach, lagoons, tidal channel and tidal delta), in agreement with previous geological studies showing heterogeneous paleoenvironments among the outcrops. these heterogeneous lithostratigraphic sequences suggest different members, and may be related to different ages along the Pirabas sequence. the neogene northwestern Atlantic subprovince is erected to encompass this entire geographical area.


INTRODuCTION
since the 19th century, the fossiliferous limestone studied by ferreira Penna (1876) has yielded a high species diversity from the exclusively Cenozoic deposits (figure 1) along the northern Atlantic coast of south America in Brazil (Pará, Maranhão and Piauí states).
All of these authors suggest close relationships between the faunal assemblages from the Pirabas formation and the Caribbean neogene fauna, based on qualitative analysis, species co-occurrence and personal conclusions.However, based on fossil fish faunas, Aguilera et al. (2011 and in progress) conclude that the Pirabas formation has a distant relationship with the Gatunian Province in the Caribbean region.this is true even when the Pirabas formation is compared with the Early Miocene Husillo and Cojimar formations from Cuba, the Mao formation from the dominican republic, the Cantaure and Castillo (lower section) formations from Venezuela, the tuira formation from Panama, the Brasso formation from trinidad, the Kendeace formation from Grenada, and the Viche formation from Ecuador.
the topics considered here are the faunal composition, the influence of the tropical Atlantic water mass on the north Brazilian Current (Johns et al., 1998(Johns et al., , 2002)), the high tidal range (Beardsley et al., 1995;dHn, s. d.), the eustatic sea level changes (Haq et al., 1988;Carter, 1998;Kominz et al., 2008) and the sea level fluctuations in northern Brazil (rossetti, 2001), the early Miocene age of the Pirabas formation, and the geographic location of the Pirabas Basin previous to the present configuration of the Caribbean sea.
these heterogeneous sequences and the faunal assemblages recorded in different outcrops suggest that different members should be recognized, and accurate geochronological research may reveal different ages within the Pirabas sequence.
Based on this heterogeneous lithostratigraphic profile and the diverse fossil record (which includes 214 invertebrates and vertebrates from 21 outcrop-localities), our objective is a quantitative analysis aimed at producing an appropriate interpretation to support future research on the context of geochronology, paleoenvironments, paleobiogeography and stratigraphic review for the Pirabas formation.
We grouped localities according to the outcrops studied, which are deemed good geographic and paleoenvironmental representatives of the early Miocene Pirabas formation sections.A few samples of taxa in certain localities are biased or absent due to lack of preservation or study.the binary data matrix includes 214 fossil invertebrates and vertebrates from 21 outcrop-localities (table 1).
one of the most important multivariate methods of data analysis is PCA (ferreira et al., 1999;Malinowski, 1991) based on the correlation between variables.it aims to group these correlated variables, generating a new set of variables called principal components (PC) onto which the data is projected.these PC are built as linear combination of original variables and have the important property of being completely uncorrelated.the first new axis is chosen in such a direction that it maximizes the variance along the axis, the second must be chosen orthogonal to the first one and the direction to describe as much variance left as possible so on.
the Principal Components Analysis (PCA) uses the calculated covariance and variance between localities; for this reason this is a Q-PCA according to fasham (1977).the Q-PCA analysis was run using the free software PAst (Hammer et al., 2001), and the taxonomic groups was expressed in the PCA's planes using the concentration ellipse level of 30%.following Jackson et al. (1989), the size effect of the linear relationship between the PCA axis 1 and the relative frequency of species occurrence were established with the largest occurrence.the coefficient from each PCA are present in the table 2.
the confidence ellipses of genera groups were formed by computing the three following quantities using the PAst software (Hammer et al., 2001), as proposed by Abdi et al. (2009): (1) the coordinates of the center of the ellipse of each group in the factorial plane (Axis ii and Axis iii), (2) the angle between the major axis of the ellipse and the first dimension of the plane, and (3) the relative size of the minor axis compared to the major axis of the ellipse.to perform these procedures one must set the center of the ellipse at the center of mass of the points.thereafter, the ratio between the minor axis and the major axis is calculated as the ratio of the second eigenvalue to the first eigenvalue.finally, the angle of rotation is given by the first eigenvector.for a 30% confidence interval, the lengths of the axes of the ellipse are set to ensure that the ellipse comprehends 30% of the points.the value of 30% was chosen for visual clarification so as to avoid overlapping the ellipses and better identify the centroids of taxonomic groups in factorial plane.

RESuLTS AND DISCuSSION
the Early Miocene faunal assemblage identified in northern Brazil is called here the Proto-Caribbean Pirabas fauna (not Caribbean), based on the nature of this fauna and because the Caribbean sea proper was only formed after the Pacific and Atlantic were completely isolated in the Pliocene (Coates et al., 1992(Coates et al., , 2004;;Coates & obando, 1996).the paleobiogeographic model for the southern Caribbean comprises the lower Miocene-lower Pleistocene Gatunian Province and the middle Pleistocene-upper Pleistocene Caribbean Province (Landau et al., 2008).According to this system the neogene tropical Western Central Atlantic subprovince can be erected to encompass this entire geographical area.

Rhyncholampas
is closely related to the co-occurrence of echinoids, crustaceans and vertebrates.

CONCLuSION
the neogene tropical Western Central Atlantic-south American faunal assemblage from the Early Miocene Pirabas formation has specific and unique features, which distinguish it from the rest of the Proto-Caribbean subprovince in diversity, paleoceanographic condition and ecosystems.
the proposed tropical Western Central Atlantic subprovince is supported by the faunal assemblage analysis for the Proto-Caribbean Pirabas formation, which is an exclusively Cenozoic unit cropping out along the northern Atlantic coast of Brazil in south America.
As a result of the PCA analysis, three faunal groups and five different facies were observed, in agreement with previous geological studies that indicated heterogeneous paleoenvironments along the section.these heterogeneous sequences in this lithostratigraphic unit could support a division into different formal or informal members, and/ or indicate different ages of parts of the Pirabas formation.
this study showed the diagnostic faunal assemblages and priority areas for future research in taxonomy, taphonomy, geochemistry, stratigraphy needed to establish detailed relationships between the different outcrops assigned to the Pirabas formation.the paleoenvironments and paleobiogeography must be combined with complementary geochronologic research to provide absolute age determinations.

ACKNOWLEGEDMENTS
the Museu Paraense Emílio Goeldi, the Universidade federal rural do Pará and the Conselho nacional de desenvolvimento Científico e tecnológico (CnPq), Brazil, support this research.We thank to daryl domning for the early reviewing and English corrections, Maria inês ramos, Heloísa Moraes-santos and Maria de Lourdes ruivo for support under the CnPq project, francileila Jatene Cavalcante for the library assistance, the Editors Board from the Boletim do Museu Paraense Emílio Goeldi for allowing the reproduction (modified from rossetti &Góes, 2004 andtávora et al., 2010) of the lithostratigraphic section and the outcrops map. the authors would like to thank two anonymous reviewers for comments and suggestions that improved the manuscript.