Carina E. Colombi

Late Triassic Environmental Evolution in Southwestern Pangea: Plant Taphonomy of the Ischigualasto Formation

Abstract The Ischigualasto Formation was deposited in a fluvial system and contains a very well preserved Triassic flora. This flora comprises seven taphofacies: (1) C(St), autochthonous silicified roots of low-statured, woody plants associated with low-sinuosity channels and crevasse-splay deposits; (2) A(Sm/Fm), autochthonous carbonized roots or root impressions of herbaceous plants in crevasse-splay and levee deposits; (3) B(Fsm/Fm), autochthonous root halos of herbaceous plants associated with levee deposits; (4) EI(St), allochthonous silicified tree trunks and charcoal associated with high- and low-sinuosity channel bars; (5) GI(St), leaf cuticles and charcoal associated with trough cross-bedded sandstone; (6) FH(Fl/C), leaf cuticles and impressions associated with palynomorphs in abandoned-channel deposits; and (7) D(Fl), autochthonous silicified stumps in abandoned-channel deposits. Taphonomically, the Ischigualasto Formation can be divided into four parts, and these partially correspond to changes in the environment. The basal part (∼0–45 m) includes the transition from the underlying Los Rastros Formation and is characterized by taphofacies 1 and 2. The fluvial sediments were deposited during tectonic subsidence of the basin, resulting in development of a fluvioaxial system. The next part (∼45–300 m) is characterized by taphofacies 1, 2, and 3 and associated with calcic paleosols that formed under a dry seasonal climate. The middle-upper portion (∼300–600 m) contains all the taphofacies associated with argillic paleosols, which were formed during a time of increasing humidity. The upper portion (∼600–700 m) is characterized by taphofacies 1, 2, and 3, associated with immature paleosols that formed under a dry seasonal climate. The changes in humidity during deposition of the Ischigualasto Formation may have resulted from an increase in rainfall generated on the western side of Pangea by maximal development of the megamonsoon during the middle Carnian Age. The climatic signal in the Ischigualasto Formation was probably modified by the tectonosedimentary development of the basin.

A New Late Triassic Vertebrate Assemblage from Northwestern Argentina

Abstract. The Quebrada del Barro Formation (QBF) is part of the continental Marayes-El Carrizal Basin, in NW Argentina. Here we report a diverse faunal assemblage recently discovered in the Quebrada del Barro Formation, along with a preliminary discussion of the taxonomic status and affinities of numerous vertebrate specimens found at two localities where this unit is exposed. The new vertebrate association includes remains of at least 12 different new species related to six major vertebrate groups: Cynodontia, Testudinata, Sphenodontia, Pseudosuchia, Pterosauria, and Dinosauromorpha. The most abundant specimens in this faunal assemblage are opisthodontian sphenodonts, tritheledontid cynodonts and basal sauropodomorph dinosaurs, but the assemblage also includes diagnostic remains of lagerpetid dinosauromorphs, theropods, pterosaurs, basal crocodylomorphs, and stem testudinatans. Several of these groups have also been reported for the Los Colorados Formation (LCF), although the two units differ in their taxonomic content at the species level and in the relative abundance of different taxonomic groups. A comparison of these two faunal assemblage suggest the fauna of QBF is younger than that of LCF and we tentatively assess a late Norian-Rhaetian age for the QBF. Some of the specimens reported here are known from well-preserved specimens and yield important new information for understanding the evolution of these groups, which underscores the relevance of the QBF fauna for assessing the dynamics of the major groups of vertebrates that dominated the terrestrial ecosystems during the early Mesozoic in Pangea.

Caracterización sedimentológica de un Sistema Fluvial Distributivo de clima árido: arroyo Papagayos, en el piedemonte oriental de las Sierras La Huerta-Imanas, San Juan, Argentina

Recent studies proposed that distributive fluvial systems (DFS) occupy a great proportion in modern continental sedimentary basins, challenging classical sedimentary models proposed for similar ancient basin. Because of this, arises the importance of studying modern DFS with the aim of obtaining facial models to apply them in ancient deposits. In this paper, we analyze and discuss the sedimentary environments included in the modern deposits of the Arroyo Papagayos, Western Pampean Ranges, San Juan province, interpreted as a Distributive Fluvial System (DFS). We perform the study of the Papagayos creek by describing different architectural elements, which characterize the sub-environments organized in four zones from the mountain front to the distal playa lake. A low sinuosity gravelly-sandy bed braided fluvial system, characterizes the proximal zone. A medial to high sinuosity sandy-gravelly anastomosed-anabranching fluvial system describes the medial zone, where floodplain aggradation is domain by crevasse splays accretion. At this point, 10 km far from the mountain front, the deposits starts to develop a cone shaped morphology. A higher sinuosity flashy ephemeral meandering sandy-muddy fluvial system with eolian interaction defines the distal zone, where the main channel of the system disappears by low gradient of the topography and infiltration depositing muddy terminal splays. Finally, eolian dune and wet interdune deposits characterizes the fourth zone, interpreted as a transitional area between the DFS and the saline playa lake. A particular feature observed along the DFS system is that mudflows are the main sedimentary process in the floodplain, probably as the result of the combination of the seasonal arid climate and the Arroyo Papagayos drainage basin morphometry and the low gradient of the topography. The sedimentary process and environments recognized along the Arroyo Papagayos will allow a better understanding of arid DFS depositional dynamics as well as, to identify them in the geological record. Lastly, high aggrading conditions for DFS development will response to climate interaction between upland catchment area (semiarid) and valley (arid), drainage basin morphometry, smooth piedmont gradient and high tectonic subsidence of the basin, located in the Andean broken foreland.