Massive Corals in Paleocene Siliciclastic Sediments of Chubut (Argentina)
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Facies (2005) 51: 233–241 DOI 10.1007/s10347-005-0023-3 ORIGINAL PAPER Wolfgang Kiessling · Eugenio Aragon´ · Roberto Scasso · Martin Aberhan · Jurgen¨ Kriwet · Francisco Medina · Diego Fracchia Massive corals in Paleocene siliciclastic sediments of Chubut (Argentina) Received: 23 December 2004 / Accepted: 17 February 2005 / Published online: 19 October 2005 C Springer-Verlag 2005 Abstract A horizon with large, massive corals in growth Introduction position was discovered in the Paleocene, probably upper Danian, part of the Maastrichtian–Paleocene Lefipan´ For- The mass extinction at the end of the Cretaceous has mation of Chubut (Patagonia, Argentina). All corals belong affected most groups of marine animals. However, to one species, the cosmopolitan Haimesiastraea conferta significant variation exists between the extinction rates Vaughan, which survived the end-Cretaceous mass of different taxonomic groups. Scleractinian corals were extinction. The occurrence of massive corals at this site surprisingly little affected by the mass extinction and is exceptional both because of the siliciclastic depositional even many corals inferred to have hosted zooxanthellae regime and because of the high palaeolatitude setting. (photosymbiotic algae) in their tissues survived this An unusual autecology of this coral and strongly reduced event Kiessling and Baron-Szabo (2004). Here we report sedimentation rates, were probably the prerequisites for an unexpected occurrence of large and massive corals, coral growth, but a link to palaeoclimate is less likely. identified as Haimesiastraea conferta Vaughan, (1900) in coarse siliciclastic sediments of Paleocene age. This Keywords Bioerosion . Corals . Ecology . Mass discovery sheds light on the autecology of an important extinction . Paleocene . Siliciclastics survivor of the end-Cretaceous mass extinction. W. Kiessling () · M. Aberhan Institut fur¨ Palaontologie,¨ Museum fur¨ Naturkunde, Geological setting and stratigraphy Humboldt-Universitat¨ Berlin, Invalidenstr. 43, The corals occur in the uppermost part of the Lefipan´ D-10115 Berlin, Germany e-mail: [email protected] Formation, the upper unit of a thick, siliciclastic sequence Tel.: +49-30-2093-8576 composed of the Paso del Sapo and Lefipan´ Formations, Fax: +49-30-2093-8868 exposed along the Middle Chubut River Valley near the town of Paso del Sapo (Fig. 1). The Lefipan´ Formation E. Aragon´ Centro de Investigaciones Geologicas,´ Universidad Nacional de is capped by continental beds and rests conformably on La Plata 1 No 644, a thick sequence of cross-bedded sandstones with some 1900 La Plata, Argentina coal beds and mudstones (Paso del Sapo Formation) of Campanian–Maastrichtian age (Fig. 2). The whole · R. Scasso F. Medina sequence is unconformably overlain by a volcanic– Departamento de Ciencias Geologicas,´ FCEN, Universidad de Buenos Aires, Ciudad Universitaria, pyroclastic complex, the Ignimbrita Barda Colorada Pab. 2, 1◦ Piso, (IBC) of the Middle Chubut River volcanic–pyroclastic 1428 Buenos Aires, Argentina complex (Aragon´ and Mazzoni 1997). K/Ar analyses of tuffs from the IBC have indicated a Late Paleocene age J. Kriwet 40 39 Department fur¨ Geo- und Umweltwissenschaften, (Archangelsky 1974). More recent K/Ar and Ar/ Ar Ludwig-Maximilians-Universitat¨ Munchen,¨ analyses of associated tuffs and volcanic rocks have Richard-Wagner-Str. 10, indicated a Late Paleocene to Middle Eocene age for the D-80333 Munchen,¨ Germany complex (Mazzoni et al. 1991; Wilf et al. 2003). D. Fracchia The Lefipan´ Formation is composed of marine, fossil- Instituto de Geolog´ıa y Mineria, Universidad Nacional de Jujuy, iferous sandstones and mudstones with some intercalated C. Correo 258, coquinas and conglomerates, and ranges from the 4600 S.S. de Jujuy, Argentina Maastrichtian to the Paleocene. The biostratigraphic 234 Fig. 1 Geographic and geological setting of the Lefipan´ Formation and location of coral-bearing outcrops Formation, evolving to deltaic systems in the middle and upper part of the unit (Olivero and Medina 1994). Field studies of our working group on a reference sec- tion (San Ramon) and several partial sections in the area suggest that the lower part of the Lefipan´ Formation (Maastrichtian) was mainly deposited in a shallow marine, shoreface environment with strong tidal influence and a conspicuous association of trace fossils (Poire´ and Spalletti 1998) and beds rich in phosphatic concretions (Pereira and Scasso 2002). The palaeoenvironment evolved to a tide- to wave-dominated deltaic system in the middle part of the sequence, which comprises the Cretaceous–Paleogene boundary (Ruiz and Scasso 2004). In the Paleocene, the en- Fig. 2 Schematic geological column of exposed rocks near Paso vironment was deepening, developing into a fully marine del Sapo. Stratigraphic position of the coral locality at Estancia system influenced by wave and storm activity. Don Manuel is indicated. Not to scale. Camp - Campanian; Maa - Maastrichtian; Pal - Paleocene; Eoc - Eocene resolution is currently exclusively based on assemblages of Coral localities bivalves and gastropods and is therefore quite coarse. Three macrofossil assemblages have been separated in previous We have measured and studied several sections in the papers, one in the Maastrichtian and two in the Paleocene Lefipan´ Formation in the area, but corals have been found (Medina et al. 1990; Medina and Olivero 1994). The depo- at only two localities. Previous workers, with the exception sitional environment in the latest Cretaceous changes from of one (Camacho 1967), have not reported on the presence a fluvial to tidally influenced estuarine environment (Paso of corals in the area. At both our localities (Estancia Don del Sapo Formation) to an open marine environment in the Manuel in the Barda Colorado region and an unnamed lo- Lower Lefipan´ Formation (Spalletti 1996). Macrofossils cality in the upper part of the San Ramon section), corals and sedimentary features indicate a shallow marine or occur in a single horizon belonging to the Paleocene part estuarine environment for the lower part of the Lefipan´ of the Lefipan´ Formation. 235 Estancia Don Manuel (Barda Colorado) gressive Systems Tract) capped by the hardground repre- senting the maximum flooding surface. Horizons 4 and 5 Abundant corals were discovered in the Barda Colorado are preliminarily interpreted as early highstand deposits. area at a locality called Estancia Don Manuel (DM; A total of 52 mature coral colonies were found at this 42◦4350S, 69◦5838W). The Lefipan´ Formation in the locality in our 2004 expedition, with up to 60 cm maxi- Barda Colorado area is strongly folded and thrusted. Only mum diameter. The majority of corals were found in scree. parts of the unit are exposed, often in blocks separated by However, several massive corals occur in outcrop as well, thrust planes. As a consequence, the stratigraphic relation- where they are all in growth position forming a biostromal ships between the coral locality and the rest of the Lefipan´ layer. This layer is up to 50 cm thick and can be laterally Formation and to the overlying pyroclastics belonging to traced for 20 m (Fig. 4A and B). Corals found in outcrop the IBC complex are not completely clear. The DM locality (horizon 3) are irregular massive or hemispherical and are is apparently situated near the top of the Lefipan´ Formation. up to 50 cm in diameter and up to 30 cm high (Fig. 4B). From base to top, the sedimentary sequence at this locality One of the authors (EA) has found a dome-shaped coral- is (Fig. 3): lum of 80 cm height in a previous visit to the outcrop. All corals are identified as H. conferta Vaughan (Fig. 5A–H), 1. Brown, friable, mottled (intensely bioturbated) fine a species ranging from the Maastrichtian to the Middle sandstone typical of Lefipan´ Formation. Eocene. The corals are exclusively massive exhibiting do- 2. Conglomerates with interbedded sandstones. Conglom- mal, hemispherical or irregular growth forms (Table 1). The erate with 50% of clasts made of well-rounded gravel smallest, complete adult corallum is hemispherical with a up to 1 cm in diameter, fragmented bioclasts (bivalves diameter of 7 cm. and gastropods). The sandstone matrix has carbonate ce- The corals are moderately well preserved. Most of the ment. Intercalated are two sandstone layers the lower of corals are superficially little affected by bioerosion and which shows hummocky cross-bedding. This group of encrustation. Encrusting bivalves are small (Fig. 4G) and beds is capped by a hardground marked by reddish mud- cover less than 5% of the coral surfaces. Serpulids were also stones infilling borings and encrustations of serpulids found as encrusters, but there are no traces of encrusting and oysters. Gastropods (Turritella malaspina), bivalves algae. We have identified the bioerosion ichnogenera Gas- (Venericardia feruglioi, Pseudolimea sp. and Pycn- trochaenolites, Trypanites and Entobia in the corals. The in- odonte miradonensis), crustacean remains and shark tensity of bioerosion is apparently stronger than visible ex- teeth have been identified in the conglomerates immedi- ternally because most of the cavities are internal (Fig. 4F). ately below the hardground, suggestive of a Paleocene, Internal cavities tend to be aligned with coral growth struc- perhaps Danian age. The hardground is developed on an tures and are thus interpreted as passive “borings”, formed erosive surface. by the attachment of organisms to the live coral surface 3. Fossiliferous reddish conglomerate