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Cainozoic Research November 2002 , 1(1-2) (2001), pp. 35-81, Lake Pebas: a palaeoecological reconstruction of a Miocene, long-lived lake complex in western Amazonia 1,7 4 4, F.P. M.E. Räsänen G. Irion H.B. Vonhof R. W. Renema¹, Wesselingh , ², ³, , Kaandorp 5 6 L.+Romero Pittman & M. Gingras 1 NationaalNatuurhistorischMuseum, P.O. Box 9517, NL-2300 RA Leiden,, the Netherlands: e-mail: [email protected] department ofGeology, University of Turku, SF 20014 Turku, Finland 3 Senckenberg Meeresforschungsinstitut, D 26382 Wilhelmshaven, Germany 4 Department ofEarth Sciences, Vrije Universiteit, de Boelelaan 1085, NL-1081 HV Amsterdam, the Netherlands S INGEMMET, Avenida Canada 1470, San Borja, Lima 12, Peru 6 Departmentof Geology, University ofNew Brunswick, P.O. Box 4400, Fredericton-NB E3B#5A3, Canada 1 Biodiversity Centre, University of Turku, SF20014 Turku, Finland Received 20 February 2001; revised version accepted 19 February 2002 The taxonomic composition and palaeoecological signature of molluscan faunas from the Miocene Pebas Formation of Peruvian Amazonia are assessed. The Pebas fauna is almost entirely made up of extinct, obligate aquatic taxa, and is dominatedin numbers of species and specimens by endemic cochliopine hydrobiid gastropods and pachydontine corbulid bivalves. Molluscan assemblages are defined and linked to depositional environments. Isotope data from the shells indicate freshwater settings during deposition of with the Pebas Formation, the exception of a few incursion levels that were deposited under oligohaline-mesohaline conditions. Faunal and data to a freshwater lake at level with and to isotope geochemical point large, long-lived system sea swamps deltas, open marine settings in the north (Llanos Basin). Sedimentological data and ichnofossils point to (restricted) marine settings. These dif- ferent interpretations are discussed, and it is concluded that faunas (including ichnofabrics) from evolutionary isolated and long- lived be assessed systems cannot in a straightforward actualistic mode, using taxa from non-long-lived environments for compari- son. Aspects of Lake Pebas are compared with modem depositional environments. Lake Pebas is among the largest and longest- lived lake complexes in Phanerozoic history; it was an important stage for the evolution of endemic molluscan and ostracod faunas. It may have played some role in the transition of marine biota to Amazonian freshwater environments during the Miocene, and likely was an important, hitherto unrecognised, dispersal barrier for terrestrial organisms in northwest South America during the Miocene. Key words Palaeoecology, molluscs, Miocene, Amazonia, Pebas Formation, community palaeoecology, long-lived lake, Lake Pebas. Introduction away from modern coastal areas, was questioned. Gabb’s work was the first of some fifteenpapers dealing with the In 1869, the American palaeontologist W.M. Gabb re- remarkable fossil molluscs from the Pebas Formation of remarkable fossil molluscan fauna from Pe- western Amazonia. Different authors with dif- ported on a came up bas in Peruvian Amazonia. All but one of the species ferent interpretations of the environment that sustained described Gabb and the bore by were new, assemblage these unusual faunas (terrestrial, freshwater, brackish but little resemblance to any modem South American and/or marine settings) and the origin of possible marine fauna. Almost all but the species were extinct, exquisite influence (Pacific, southern Atlantic, eastern Atlantic preservation of the fauna led Gabb to believe that the approximately through the course of the present-day fossils were ofrelatively recent age. Gabb described sev- Amazon River, and Caribbean). eral taxa whose relatives at present occur in settings with In 1990, Patrick Nuttall published his seminal mono- fluctuating salinities, others that are marine, and still oth- graph on the Neogene molluscs from western Amazonia with freshwater The in what ers a affinity. question arose and adjacent regions (Nuttall, 1990a). He clarified much kind of environment the Pebas molluscs had lived, and of the systematic uncertainties that complicated age esti- the inferred of marine environmental given presence taxa (the gastropod mates and interpretations. Based on a me- genera Turbonilla and Mesalia and the bivalve Corbula), ticulous region-wide stratigraphic correlation he assigned the origin of marine settings in lowland Amazonia, far a Middle Miocene age to the fauna. Nuttall (1990a) in- -36- terpreted the depositional environment in Miocene west- employed here. Finally, implications for landscape evo- ern Amazonia as a continually shifting pattern of lution and biotic evolution in Miocene western Amazonia streams, swamps and lakes of varying salinity, with ma- are discussed. rine influence from the north (Caribbean). Nuttall’s age assignments were confirmed and refined by Hoorn (1993, 1994a, b) on the basis of pollen. Hoorn concluded Material and methods that the Pebas Formation included at least three pollen between late Miocene and sediment zones, spanning an age Early Fossil-bearing samples, collected (in 1996) in early Late Miocene (c. 17-10 Ma). Based on pollen and outcrops of Pebas Formation deposits in Peruvian sediments, Hoorn (1994a) interpreted the depositional Loreto, were weighed and washed (minimum sieve mesh environment ‘fluviolacustrine with marine influ- 0.3 locations in as some mm). Sampling are given Appendix 1. ence’. Samples typically weighed about 1 kg. A total of 285 The discovery of supposed Miocene tidal deposits in samples were assessed for taxonomic composition. The Brazilian led Webb distribution of ‘Pebas’ and Acre (Rasanen et al., 1995) (1995) geographic contemporanous to Miocene western Amazonia of non-marine faunas is shown in interpret as part an Figure 1. The age of the intracontinental seaway that would have occupied all samples is between late Early Miocene and early Late basins of inland South America. Miocene major sedimentary (Figure 2; see Hoom 1993, 1994a). Webb’s views severe criticism Hoom, The Pebas Formation is referred lithostrati- provoked (ie.g., to as a 1996; Paxton et al., 1996; L.G. Marshall & Lundberg, graphic unit comprising predominantly turquoise-blue 1996). Nevertheless, more tidal deposits have been dis- smectitic clays; immature, feldspar-rich, usually grey covered since then in Peruvian Amazonia (Rebata, 1997; sands, and brown-black organic clays and lignites. The Rasanen et al., 1998), and in the Pebas Formation. The formation is characterised by 3-7 m thick, predominantly predominantly endemic molluscan and ostracod faunas coarsening-upwards (CU) cycles, the brilliantly turquoise (Nuttall, 1990a; Wesselingh, 1993; Whatley et al., 1996; smectite-rich clays, as well as the common occurrence of Munoz-Torres et al., 1998; Vonhof et al., 1998) conflict lignitic intervals (Rasanen et al., 1998). The Pebas For- with interior envi- mation is an interpretation as a possible seaway extremely rich in fossils, often beautifully pre- ronment Rasanen et al. and Webb served. wood well as proposed by (1995) e.g. molluscs, ostracods, fragments as (1995), as well as with fluvio-lacustrine conditions as as fish, amphibian and reptilian remains. proposed by Hoorn (1993, 1994a, b). Based on stron- In Peru, this formation is properly known as the Pe- carbon and well bas Formation tium, oxygen isotopes, as as on fauna, (see e.g., Rasanen et al., 1998). The Cura- Vonhof et al. (1998) concluded that the Pebas Formation ray Formation of eastern Ecuador (Tschopp, 1953; Bal- had been deposited in a lacustrine system, occasionally dock, 1982) is the time equivalent of the Pebas Forma- reached by marine incursions. A Caribbean (northerly) tion. In ColombianAmazonia, deposits formerly referred of the incursions Nuttall ‘Terciario inferior Amazonico’ attributed origin as proposed by (1990a) to as were by and Hoom (1994b; see also Hoorn et al., 1995) was sup- Hoom (1994b) to the Pebas Formation. The La Tagua these faunal and data. The ostracod Beds ported by isotope (Nuttall, 1990a; Hoom, 1994b) are included in the fauna from the Pebas Formation was studied by Whatley Pebas Formation as well. In Brazil, Pebas Formation et al. (1996), who proposed a low energetic athalassic deposits have usually been included in the Solimoes based the taxonomic (inland-sea) environment, on com- Group/Formation, an informal unit comprising a plethora position and exceptional preservation of the ostracod of western Brazilian Amazonian Cainozoic strata (eg., fauna. Compilations of Pebas molluscan taxa autecologi- Maia et al., 1977), but some authors (Costa, 1981; Petri cal such Nuttall table signatures, as provided by (1990a, & Fulfaro, 1983) have indeed distinguished the Pebas 2) and Vonhof et al. (1998, table 1) show an admixture Formation as such in Brazil. of (inferred) ecological preferences. Almost all species In the present paper we use the terms ‘lowermost’, from the Pebas Formation several of are extinct, and the ‘lower’, ‘middle’ and ‘upper’ Pebas Formation loosely; genera dominating the fauna are extinct or have survived all in lower case, in order not to be confused with as relics only (Wesselingh, 2000), limiting their actualis- ‘Lower’, ‘Middle’ and ‘Upper’ in lithostratigraphic us- tic This has into the For the Pebas ecological use. complicated insight age. Formation, ‘lower’, ‘middle’ and ‘up- environmental system in which these faunas lived. per’ correspond to the Psiladiporites/Crototricolpites The aim of the is the present paper