The Age of the Cretaceous Santana Formation Fossil Konservat
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Cretaceous Research 28 (2007) 895e920 www.elsevier.com/locate/CretRes The age of the Cretaceous Santana Formation fossil Konservat Lagersta¨tte of north-east Brazil: a historical review and an appraisal of the biochronostratigraphic utility of its palaeobiota David M. Martill Palaeobiology Research Group, School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK Received 10 April 2006; accepted in revised form 11 January 2007 Available online 10 July 2007 Abstract This paper is concerned with the famous fossil-bearing carbonate concretions of the Romualdo Member of the Santana Formation Konservat Lagersta¨tten of north-east Brazil. This palaeontologically important horizon was first dated as Cretaceous by the French palaeoichthyologist Louis Agassiz on the basis of fish fossils obtained by Bavarian explorers Spix and Martius between 1817 and 1820 and Scottish botanist and explorer George Gardner between 1836 and 1841. Gardner equated the concretion level with the English Albian ‘Upper Greensands’ on the basis of an imagined similarity of stratigraphic sequence with that of the Isle of Wight, southern England. Since then high precision dating of this remarkable deposit has proved elusive and the concretion-bearing part of the Santana Formation has been variously dated as early Late Cretaceous or late Early Cretaceous. Attempts at greater precision over the last 30 years have cited its age variously as Aptian, Albian or possibly Cenomanian, but few reliable data have been presented to support these dates. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Brazil; Araripe Basin; Santana Formation; Cretaceous; Konservat Lagersta¨tten; Geochronology 1. Introduction and understanding of Gondwanan Cretaceous palaeobiotas (Martill, 1988, 1993; Wenz and Brito, 1990; Maisey, 1991, The Santana Formation of north-east Brazil contains one of 1993; and many references herein). Only the age of the the most important Mesozoic fossil Konservat Lagersta¨tten on Romualdo Member of the Santana Formation, a dominantly Gondwana (Maisey, 1991; Martill, 1993, 1997, 2001; Kellner, silty shale sequence that includes the highly fossiliferous 2002; Fara et al., 2005). The formation crops out on the flanks carbonate concretion-bearing unit, is considered here. of the Chapada do Araripe in southern Ceara´, western Pernam- Although the Santana Formation concretions have been fa- buco and a small part of eastern Piaui in the north-eastern mous for their enclosed fossils, especially fishes (Fig. 2), for Brazilian Caatinga (Fig. 1). It forms part of a heterogenous over 150 years, in more recent times they have become known assemblage of spectacularly fossiliferous rocks of Cretaceous for a diversity of dinosaur and pterosaur remains in an excellent age (Gardner, 1841; Brito, 1984; Maisey, 1991; Martill, 1993). state of preservation (Martill, 1998; Martill and Unwin, 1989; In fact, two formations within the basin are well-known as Kellner, 1996a,b; Frey et al., 2003a,b) comparable with, and Konservat Lagerstatten; the Nova Olinda Member of the Crato sometimes exceeding, that of the Jurassic Solnhofen Limestone Formation lies a few tens of metres below the Santana Forma- of Bavaria (Barthel et al., 1990), especially in their three- tion, and both have contributed considerably to our knowledge dimensionality. The so-called ‘‘Lagersta¨tten effect’’ on palaeobiodiversity E-mail address: [email protected] patterns makes it vital that such deposits are precisely dated. 0195-6671/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2007.01.002 896 D.M. Martill / Cretaceous Research 28 (2007) 895e920 AB C Fig. 1. A, location of the Chapada do Araripe in South America. B, the Chapada do Araripe lies on the borders of the Brazilian states of Ceara´, Piauı´ and Pernambuco. C, outline map of the Chapada do Araripe with the Santana Group (of Neumann and Cabrera, 1999) shaded grey. The dashed line indicates the route taken by George Gardner between 1839 and 1841. Unfortunately, the Santana Formation, represented largely Formation sensu lato as Wealden; a term that should not be by non-marine or quasi-marine strata, lacks biostratigraphi- used in a chronostratigraphic context, but is often taken to be cally useful fossils for high precision biostratigraphy and has BerriasianeBarremian by European stratigraphers. The fossil- not been radiometrically dated owing to a lack of suitable iferous concretion-bearing part of the Santana Formation (the igneous rocks. Notwithstanding this, there have been a number Fossil Lagersta¨tte) has variously been cited as Aptian (Brito, of attempts to date the deposit more precisely, most of which 1984; Wellnhofer, 1985, 1991), Aptian/Albian (Arai et al., have favoured a late Early Cretaceous age (Braun, 1966; Mabe- 2000), Albian (Campos and Wenz, 1982; Berthou, 1990), pos- soone and Tinoco, 1973; Lima, 1978, 1979a,b, 1980; Berthou, sibly Cenomanian (Martill and Wilby, 1993) and Turonian 1990). Many of the dates applied have referred to the Santana (Beurlen, 1962); a range in the order of 30 million years. The Formation as a whole, or without defining the scope of the for- following is a review of the history of dating of the Santana mation, and with little attempt made to indicate from where Formation, the methods used, with statements on their accu- diagnostic fossils came from within the section. Furthermore, racy, and an evaluation of the various palaeontological groups at the time many of these studies were undertaken the name encountered in the formation with comments on their utility Santana Formation was applied to a wide range of strata within for dating. Finally, remarks on the possible utility of stable the Araripe Basin (Fig. 3). In addition, confusion was intro- isotope geochemistry, palaeomagnetism and sequence/event duced by the use of inappropriate terms as chronostratigraphic stratigraphy as potential, but as yet untried methods for dating labels. Krommelbein and Weber (1971) regarded the Santana the Santana Formation are provided. D.M. Martill / Cretaceous Research 28 (2007) 895e920 897 a heterolithic sequence that lies above a series of evaporites (the Ipubi Formation) and beneath a series of prominent red beds (the Arajara and Exu formations) that form an extensive tableland that extends in places beyond the confines of the fault bounded basin. This paper follows Martill and Wilby (1993) and expressly excludes the Ipubi and Crato formations (previously considered as members of the Santana Formation) from the Santana Formation. Thus the definition of the San- tana Formation used here corresponds approximately with the Romualdo Member of Beurlen (1963) (Fig. 3). Martill and Wilby (1993) retained the term Romualdo Member for a series of concretion-bearing fissile shales ca. 4e20 m thick (Fig. 4) that lie approximately within the middle of the Santana Formation sequence at most localities around the Chapada do Araripe. Two major unconformities can be readily identified in the Araripe Basin infill. The oldest of these lies between the metamorphic basement and a series of probably Ordoviciane ?Devonian coarse clastic fluvial sediments of the Vale do Cariri Group (Brito, 1990). Ayounger unconformity between the Vale do Cariri Group and the metamorphic basement and the Meso- Fig. 2. Typical Romualdo Member concretions from the Santana Formation. A, zoic infill which comprises the Brejo Santo and Santana a ‘split’ concretion with a specimen of Notelops sp. in which extensive soft groups (Fig. 5) marks the initial stages of rift filling. This un- tissue (white) is preserved. B, a concretion prior to preparation outlines well conformity is complex, and comprises both onlap and overlap the enclosed fish, in this case a specimen of the pycnodont Neoproscinetes sp. relationships with the underlying strata and includes a buried topography of significant relief where it separates the Meso- zoic sequence from the pre-Mesozoic basement (Martill, 2. Brief geological overview 1993). Minor disconformities and non-sequences occur throughout The Santana Formation owes its name to Small (1913) who the Mesozoic sequence, reflecting eustatic as well as tectonic identified a series of variable strata between two prominent events within the basin (da Silva, 1986a,b; Berthou, 1994; sandstone sequences in the Val do Cariri, Ceara´; he called Ponte and Ponte Filho, 1996). Significantly, one of these dis- this sequence the Calca´reo de Sant’Anna, a name that presum- conformities appears within the Araripe Group, and its recog- ably alludes to the laminated limestone-bearing Crato Forma- nition led da Silva (1986a,b) to divide the Araripe Group tion of recent nomenclature. Small’s informal unit was sequence into two distinct formations. She removed the Crato subdivided by later workers and the Santana Formation is and Ipubi members from the Santana Formation, uniting them now just one of several formations recognised within an Ara- in a single Araripina Formation (see below for details). More ripe Group, a stratigraphic division erected by Gomez et al. recent workers have agreed with da Silva’s removal of these (1981) and reviewed by Ponte and Appi (1990) and Assine lithostratigraphic units from the Santana Formation, but have (1992). The Araripe Group forms part of the infill of a Mesozoic not accepted their unification as a single formation and as extensional rift basin developed between two major east-west such the term Araripina Formation is hardly ever used.