Palaeontologia Electronica PALAEOENVIRONMENTAL
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Palaeontologia Electronica http://palaeo-electronica.org PALAEOENVIRONMENTAL IMPLICATIONS OF ASTERIACITES LUMBRICALIS IN THE COSTE DELL'ANGLONE SINEMURIAN DINOSAUR ICHNOSITE (NE ITALY) Massimo Bernardi, Fabio Massimo Petti, and Marco Avanzini Massimo Bernardi. Museo Tridentino di Scienze Naturali, Via Calepina, 14 - 38122 Trento, Italy. [email protected] Fabio Massimo Petti. Museo Tridentino di Scienze Naturali, Via Calepina, 14 - 38122 Trento, Italy. [email protected] Marco Avanzini. Museo Tridentino di Scienze Naturali, Via Calepina, 14 - 38122 Trento, Italy. [email protected] ABSTRACT The finding of Asteriacites lumbricalis in the Lower Jurassic of the Trento carbon- ate Platform is here integrated with the sedimentological, stratigraphical, and palaeon- tological data for the Coste dell'Anglone ichnosite in order to better reconstruct its palaeoenvironmental setting. The presence of these traces produced by small ophi- uroids support the interpretation, for this geographical sector during the middle-late Sinemurian, of a marginal-marine tidal flat environment. A marginal marine environ- ment is consistent also with the documented existence of sedentary populations of dinosaurs recorded as footprints in the deposits that overlie those preserving Asteria- cites. KEY WORDS: Asteriacites lumbricalis; Sinemurian; Trento Platform; dinosaur ichnosite INTRODUCTION anticline associated with the main E-oriented thrust (Petti et al. 2008, Petti et al. in press). The Coste dell’Anglone ichnosite is located Immediately below the dinosaur trampled bed, along the eastern slope of Monte Brento (Dro, several Asteriacites lumbricalis traces were found Trentino Alto-Adige, NE Italy; Figure 1), in the cen- (see also Bernardi et al. 2010). Asteriacites von tral-eastern sector of the Southern Alps, slightly Schlotheim, 1820 is a well known star-shaped rest- north of the Lake Garda. In this site, hundreds of ing trace (cubichnia) that belongs to the Cruziana dinosaur tracks, mostly arranged in long track- ichnofacies (Seilacher 1964, 1967). In wave-domi- ways, were recognized on a wide surface dipping nated systems this ichnofacies is often present in about 30° SE, representing the flank of a ramp the region between daily wave base and storm PE Article Number: 13.3.17A Copyright: Palaeontological Association November 2010 Submission: 15 January 2009. Acceptance: 26 July 2010 Bernardi, Massimo, Petti, Fabio Massimo, and Avanzini, Marco, 2010. Palaeoenvironmental Implications of Asteriacites lumbricalis in the Coste dell'Anglone Sinemurian Dinosaur Ichnosite (NE Italy). Palaeontologia Electronica Vol. 13, Issue 3; 17A:8p; http://palaeo-electronica.org/2010_3/230/index.html BERNARDI, PETTI, & AVANZINI: JURASSIC ASTERIACITES FIGURE 1. Location map of the Coste dell'Anglone ich- nosite, Trentino Alto-Adige, NE Italy. wave base (Frey and Pemberton 1985, but see Frey et al. 1990) while in tide-dominated environ- ments it may also occur in intertidal positions, grad- ing seaward into the Skolithos ichnofacies (see discussion in Mangano and Buatois 2004). Asteria- cites has a long temporal range extending from Cambrian to recent shallow- to deep-marine facies FIGURE 2. Stratigraphical section of the Lower Jurassic (see Mikuláš 1992 and references therein). In the deposits at Coste dell'Anglone ichnosite. Trace fossils Eastern sector of the Southern Alps, Asteriacites bearing levels belong to the lower portion of the Rotzo has been documented several times (e.g., Broglio Formation (Sinemurian-Pliensbachian, Calcari Grigi Loriga et al. 1983, 1990), and its occurrence has in Group). particular been investigated in the study of the Permo-Triassic aftermath (e.g., Twitchett and Wig- GEOLOGICAL SETTING nall 1996; Wilson and Rigby 2000). Seilacher (1953) suggested that Asteriacites Trace-bearing levels belong to the lower por- lumbricalis was probably made by ophiuroids but tion of the Rotzo Formation (Sinemurian- most subsequent papers did not provide any dis- Pliensbachian; Calcari Grigi Group, see Avanzini et cussion about the possible producer of the trace al. (2007) and references therein; Figure 2) repre- (but see Spencer and Wright 1966; Lawrence sented in this geographic sector by a shallowing 1987; Wilson 1997). The tracemaker was generally upward sequence composed by cycles, each char- intuitively indicated as a member of the Asterozoa acterized by the transition from a low energy sub- without distinguishing between asteroids and ophi- tidal unit to high energy subtidal deposits up to uroids (e.g., Santos and Campanha 1970; Brito inter- and supra-tidal deposits (see Monaco and 1977; Crimes and Jiang 1986). However, Mángano Giannetti 2002; Giannetti and Monaco 2004). et al. (1999) convincingly showed that an ophiuroid In the studied section, the whole sedimentary tracemaker is the most reliable interpretation in the succession is developed through the vertical stack- majority of cases. Mikuláš (1990) and West and ing of cyclic, meter-scale, shallowing-thickening- Ward (1990) have also provided evidence of Aste- upward packages showing a characteristic asym- riacites lumbricalis preserved with ophiuroids metric profile due to the different weathering pro- remains. files of their component lithofacies. At its base, marking the beginning of the marine transgression, each cycle starts with a deci- 2 PALAEO-ELECTRONICA.ORG metre-thick transgressive basal lag comprised of erential trapping of mud by microbial mats during reworked intraclasts and bioclasts and/or intrafor- storm flooding and then deposition of sand as the mational breccias with flat pebbles ripped from the current wane (Damicco and Hardie 1994). The top stromatolitic levels at the top of the underlying of the peritidal cycle shows a 30 cm thick inter- to cycle. supra-tidal sequence made of a interlaying of thinly Above this lag, the main part of the subtidal laminated dark peloidal mudstone and reddish mil- portion of the cycle is alternatively made of oolitic- limetre-thick sandy-dolostone layers. The presence bioclastic calcarenites that, in some cases, occur of thin stromatolite layers with subaerial exposure as graded storm layers or of grey, peloidal pack- evidence, makes reasonable that in our case the stone/wackestone. The subtidal parts of the cycles tidal flat, after the accumulation of the peloidal may include gastropod and bivalve lags both in pri- mud, became exposed to the subaerial environ- mary position and as reworked, disarticulated ment with pedogenetic rubefaction, mud cracking, valves. and dolomitisation. Among these finely laminated The inter-supratidal, upper portion of each layers and immediately below the supratidal inter- cycle is represented by thin reddish marly levels or val, a monospecific Asteriacites lumbricalis assem- sometimes by stromatolitic dolostone that includes blage was recognised (Figure 4). a wide range of depositional structures and diage- The lower part of the subsequent cycle netic modification produced during subaerial expo- (deposited in the lower-middle subtidal environ- sures of the sediments, from sheet and polygonal ment of a carbonate lagoon) is intensely burrowed. cracks, tepee and birdseye structures to microsta- Centimetre-sized elliptical spots and vertical tun- lactictic cements, and other products of vadose nels are infilled by peloidal grainstones and mixed diagenesis. skeletal debris. These tubular concentrations may Microfossils analysis allowed the recognition be related to open burrow networks of crustaceans of various taxa such as Paleodasycladus mediter- burrowed in a firm substrate (Monaco and raneus, P. fragilis, Siphovalvulina spp., Pseudop- Garassino 2001). In other overlying parase- fenderina butterlini, Ammobaculites sp., quences, burrowing of this basal portion of the Spiraloconus sp. This microassemblage strongly cycle is evidenced by turnarounds at nodes and indicates a middle-late Sinemurian age that seems branched tunnels indicating deposit feeding activity coherent with the stratigraphical location of the by decapod crustaceans (Thalassinoides and Oph- studied levels (Petti et al. 2009b). iomorpha). Commonly, cylindrical branches Y to T shaped show diameters up to 5 cm. Mazes in gen- SEDIMENTOLOGY AND ICHNOLOGY OF THE eral are regularly tiered, and burrowing can be rec- TRACE BEARING CYCLE ognised as a primary factor in the nodularity of this basal part of the cycles. Here follows a detailed ichnological descrip- The middle and upper part of the cycle is tion of two shallowing upward cycles including both much less burrowed. Small trace fossils (0.5 to 1 Asteriacites and dinosaur traces (Figure 3). cm in diameter) include Chondrites, Planolites and The base of this sedimentary portion is repre- Skolithos. These traces are associated to ripple sented by a peloidal mud followed by sandy layers marks and reworked bivalve and brachiopod skele- with parallel to slightly irregular upper and lower tal fragments. Among taphonomic characters abra- contacts and internal laminae varying from parallel sion, reorientation, disarticulation, winnowing, and to undulatory. This heterolitic stratification (sensu shelter porosity suggest the dominance of recur- Damicco and Hardie 1994), or flaser-lenticular bed- rent physical agents on the biological ones ding (sensu Reineck 1960) was considered by (Monaco 1999). These taphonomical features are Reineck and Wunderlich (1968) as quite common related to a middle subtidal environment with a in shallow water carbonate platform deposits. shallowing trend from the bottom