Facies architecture and palaeoenvironmental implications of the upper Cretaceous (Santonian) Csehbánya formation at the Iharkút vertebrate locality (Bakony Mountains, Northwestern Hungary)

Abstract

The Csehbánya Formation (Santonian), exposed in the Iharkút open-pit, BakonyMountains, Hungary, is made up of a cyclic alternation of conglomerate, sandstone, and variegated siltstone and clay deposited in a fluviolacustrine environment. As a result of continuous excavation since 2002 it has yielded rich and diverse continental vertebrate and plant assemblages. A facies and architectural analysis of the Csehbánya Formation at this location identified four main lithofacies associations with eight subtypes consisting of (1) lenticular sandstones representing river channels, (2) conglomerates with sandstone (coarse grained likewise representing channel deposits), (3) heterolithic-channel fill (high density flash flow deposits) (4) splay sandstones produced by crevasse splays, (5) dark sandy siltstone (small-scale stagnant pool deposits with high organic content), (6) greenish-grey claystone (deposits of shallow lakes and ponds), (7) reddish (moderately drained) paleosols, (7) yellowish, mottled (hydromorphic) paleosols.The sedimentological investigations revealed that the terrestrial deposits exposed by the Iharkút open-pit were formed in an anastomosing fluvial system because: (i) the alluvial architecture is characterized by large proportion of overbank deposits encasing the channel sandstone bodies, (ii) the ribbon shaped sandstone bodies are dominant, (iii) cross-bedding and lateral accretion are almost completely absent in the channel fill deposits and (iv) the sandstone bodies are clearly isolated from each other, embedded in floodplain sediments, suggesting multiple co-existing channels.The most important vertebrate fossil site (SZ-6) was examined in special detail because it shows peculiar lithological features. The layers richest in fossils (Unit 1) of site is interpreted as a lag deposit formed during an episodic high density flash flood event representing a relatively short time interval, i.e., probably within a single rainy season. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 1 1. INTRODUCTIONA significant part of continental vertebrate fossils is found in alluvial deposits thus the reconstruction of the depositional history of alluvial sequences is the main goal of any sedimentological, paleontological and taphonomical examination of vertebrate sites (Behrensmeyer, 1982(Behrensmeyer, , 1988Retallack, 1984; Badgley, 1986a,b;Eberth and Miall, 1991;Nadon, 1993;Therrien, 2005;Roberts, 2007). One of the reasons for the accumulation of rich vertebrate fossil-bearing strata is that a relatively high number of animals are attracted to the floodplain due to abundant water and food supply (different plants or prey), which can result in a rich accumulation of bones (Behrensmeyer, 1988;Nadon, 1993;Andersson and Kaakinen, 2004). Furthermore, certain floodplain environments are favourable for bone preservation because of the relatively high depositional rate, post-mortem transportation, and reworking of animal remains by various fluvial processes (Behrensmeyer, 1988;Aslan and Behrensmeyer, 1996)....

Collection of the thinnest: A unique eggshell assemblage from the Late Cretaceous vertebrate locality of Iharkút (Hungary)

“…20D) that would indicate the presence of pyrite. Thus, in sharp contrast with most of the fossils known from the Iharkút locality (Botfalvai et al 2015(Botfalvai et al , 2016, pyrite seems to have played no role in the preservation of these fragile eggshell fragments.…”

“…The excavation methods and circumstances and the sedimentological characteristics of the Iharkút locality (Botfalvai et al 2016) usually prevent the in situ recognition, separation, and collection of very small and fragile fossils. Therefore, despite its destructive aspects (e.g., intensive fragmentation resulting in significant size reduction), only screen-washing could reveal the presence of the thin (50-350 μm) eggshell fragments from two different sites, SZ-6 (Unit 1) and SZ-7-8.…”

“…1A), is made up of a cyclic alternation of sandstone and variegated siltstone and clay layers with sporadic intercalation of thin coal seams. This sedimentological setting implies that the exposed layers were deposited by an anastomosing fluvial system in a topographically low-level, wet, alluvial plain environment (Botfalvai et al 2016). The floral assemblage (subtropical floodplain forest vegetation), the absence of desiccation cracks, the common presence of hydromorphic paleosols, and the subordinate amount of secondary (pedogenic) carbonate accumulation indicate that the climate was predominantly humid but seasonal, with flash-flood-like episodes (Botfalvai et al 2015(Botfalvai et al , 2016.…”

“…Along with its taphonomic characteristics, this sedimentology suggests that Unit 1 was deposited by ephemeral high-density flashflood events probably triggered by episodic heavy rainfalls. It acted as a trap where current velocity suddenly decreased and the poorly sorted sand, ripped-up clay clasts, pebbles, bones, and eggshell fragments accumulated (Botfalvai et al 2015(Botfalvai et al , 2016.…”

“…1C), which to date yielded only microfossils (including eggshell fragments), are composed of 10-to 20-cm-thick, dark gray silt, and clay beds enriched in coalified plant material and amber. The depositional area of this site was part of the low-level (poorly drained) floodplain with elevated water table characterized by overall hydromorphy; the dark sandy siltstone was deposited in small-scale stagnant (poorly oxygenated) pools of the floodplain (Botfalvai et al 2015(Botfalvai et al , 2016.…”