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The Jurassic-Cretaceous Boundary (Tithonian - Hauterivian) in the Andean Basin of Central Chile: Ammonites, Bio- and Sequence Stratigraphy and Palaeobiogeography

Salazar Soto, Christian Andrés

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Five sections of the Baños del Flaco and Lo Valdés formations in the High Andean Cordillera of Central Chile have been analyzed with regard to sediment, bio- and sequence stratigraphy, ammonite assemblages and palaeobiogeography, in order to reveal the age and depositional facies of the sediment sequence, and to evaluate the age and important bioevents across the Jurassic-Cretaceous boundary. The Baños del Flaco Formation was investigated at Rio Tinguiririca and in the Rio Maitenes valley, while sections of the Lo Valdés Formation were studied at Rio Volcán and in the Rio Maipo valley. The Baños del Flaco Formation is 369 m thick at Baños del Flaco section and 536 m thick at Rio Maitenes; lithologies consist of conglomerate, limestone, sandy limestone, calcareous sandstone, calcareous siltstone and siltstone. A lower and an upper member are distinguished by their lithological composition. The Lo Valdés Formation is here divided into two formations, based on the lithological composition. The lower part is assigned here as the Cathedral Formation, composed mainly of andesitic units and only restricted levels of sediment. This formation is 760 m thick and divided into two members (lower and upper); it is assigned to the Lower-Middle Tithonian. The Lo Valdés Formation as redefined here overlies the Catedral Formation; it is 539 m thick in the type locality, 582 m thick at Cajón del Morado and 150 m thick at Cruz de Piedra. Three members are differentiated by their predominant lithological composition, from base to top a sandstone, siltstone and limestone member. The ammonite assemblage of the Baños del Flaco Formation is referred to 12 species and 10 genera. Aulacosphinctes proximus, Micracanthoceras spinulosum and Corongoceras evolutum are new records for this formation. Pseudolissoceras cf. zitteli, Euvirgalithacoceras malarguense, Choicensisphinctes windhauseni, Catutosphinctes cf. americanensis, Virgatosphinctes scythicus and Micracanthoceras microcanthum are registered for the first time in Chile. Micracanthoceras spinulosum shows strong ontogenetic changes not recorded previously. Virgatosphinctes scythicus is here considered a morphologically variable species and considered synonymous with classical South American taxa such as Virgatosphinctes andesensis, V. mendozanus, V. mexicanus and V. leñaensis. Virgatosphinctes aff. pseudolictor, V. cf. raja and V. guadalupensis are considered synonymous with E. malarguense, while V. tenuilineatus is synonymous with C. windhauseni and Aulacosphinctes chilensis with A. proximus. Other synonymies include Micracanthoceras lamberti and M. tapiai, with M. microcanthum, and Corongoceras rigali with C. alternans. A detailed biozonation of the Baños del Flaco Formation is here presented for the first time. The following biozones were assigned, from bottom to top: Virgatosphinctes scyticus / Pseudolissoceras zitteli zone, Windhauseniceras internispinosum zone, Micracanthoceras microcanthum / Corongoceras alternans zone and Substeueroceras koeneni zone. Here we suggest that the Baños del Flaco Formation reaches from the lower part of the Middle Tithonian to the lowermost part of the Lower Berriasian.

39 species of ammonoids were differentiated in the Lo Valdés Formation and were referred to 22 genera. Frenguelliceras magister is a new record for the Lo Valdés Formation and central Chile. Pterolytoceras exoticum, Aspidoceras rogoznicense, Micracanthoceras microcanthum, Micracanthoceras vetustum, Corongoceras lotenoense, Corongoceras mendozanum, Spiticeras acutum, Spiticeras pricei, Spiticeras spitiense, Groebericeras rocardi, Berriasella (Berriasella) jacobi, Malbosiceras malbosi, Chigaroceras bardensis, Tirnovella kayseri, Thurmaniceras thurmanni, Crioceratites perditum and Bochianites sp. are first registers for Chile. Lytohoplites paredesi n.sp., Lytohoplites zambranoi n.sp., Lytohoplites varelae n.sp. and Lytohoplites rauloi n.sp. are new taxa. The neotype of Substeueroceras is designated here and corresponds to the lectotype of Substeueroceras koeneni. Parodontoceras is here considered to be a junior synonym of Substeueroceras. We also designate lectotypes of Micracanthoceras spinulosum, Micracanthoceras vetustum, Spiticeras acutum, Substeueroceras calistoide, Argentiniceras fasciculatum, Tirnovella kayseri, Crioceratites andinum, Crioceratites diamantense and Crioceratites perditum. Spiticeras acutum is considered here a morphologically variable species. Berriasella fraudans is considered to be a junior synonym of B. jacobi. Six biozones are here differentiated for the Lo Valdés Formation, from bottom to top: Micracanthoceras microcanthum / Corongoceras alternans, Substeueroceras koeneni (Berriasella jacobi and Groebericeras rocardi sub-zones), Thurmaniceras thurmanni / Argentiniceras fasciculatum, and Crioceratites diamantense. In consequence, the Lo Valdés Formation reaches from the upper part of the Upper Tithonian to the Upper Hauterivian. Biozonations of the Baños del Flaco and the Lo Valdés formations are integrated and correlated, leading to the identification of 9 biozones reaching from Middle Tithonian to Upper Hauterivian times. According to the ecological index, major bioevents are recognized for the Upper Tithonian, with peak relative abundances and diversity, and for the Lower Berriasian, which is characterized by highest species richness. The turn-over analysis indicates that similarity (Jaccard index) is low between the Upper Tithonian and the Lower Berriasian. Species abundances and richness decrease in the Valanginian and even further, during the Hauterivian. Taxa endemic to Central Chile and Tethyan species are the dominant ammonites during the Tithonian. During the Berriasian, Cosmopolitan and Indo-Pacific influences increase and these taxa form the dominant group, while Tethyan and Endemic ammonites dominate during the Valanginian. During the Tithonian, the dominance of endemic taxa suggests that Central Chile may not have been well connected with the Tethyan region via the Hispanic Corridor, but that Indo-Pacific affinities result from an open Indo-Austral seaway. During the Berriasian, the dominance of Cosmopolitans and abundance of Indo-Pacific taxa suggests that new pathways opened between Antarctica and South America–Africa. These connections were likely established via the Indo-Austral seaway and a new connection through the Hispanic Corridor. Subsequently, during the Lower Valanginian, Tethyan, Indo-Pacific (Indo-Austral) and Boreal taxa are present in the Central Chilean fauna and Endemic ammonites are equally abundant. This may correspond to a similar grade of influence from the Hispanic Corridor and the Indo-Austral seaway. The base of the Baños del Flaco Formation represents transgressive facies (sequence 1) during the lowermost Middle Tithonian, with a maximum depth during the uppermost Middle Tithonian. Upsection, sequence 2 represents a second cycle with transgressive facies during the Upper Tithonian and a regression in the Lower Berriasian. In the Lo Valdés Formation, sequence 3 represents a retrogradational and progradational pattern, which results in transgression during the Upper Tithonian to Lower Valanginian and a regression during the Upper Valanginian to Upper Hauterivian.

Item Type: Dissertation
Supervisor: Stinnesbeck, Prof. Dr. Wolfgang
Date of thesis defense: 14 December 2012
Date Deposited: 06 Feb 2013 10:25
Date: October 2012
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 550 Earth sciences
560 Paleontology Paleozoology
Controlled Keywords: Geologie
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