Journal of South American Earth Sciences, Vol. 4, No. 3, pp. 215-229, 1991 Printed in Great Britain 0895-9811/91 $3.00 + 0.00 © 1991 Pergamon Press plc & Earth Sciences & Resources Institute Discontinuity surfaces in the Lower Cretaceous of the High Andes (Mendoza, Argentina): Trace fossils and environmental implications M. G. MANGANO and L. A. BUATOIS CONICET, Facultad de Cieneias Naturales e InstitutoMiguel Lillo, Universidad Nacional de Tucum&n, Casilla de Correo 1,Correo Central, 4000 San Miguel de Tucumhn, Argentina (Received November 1989; Accepted March 1991) Abstract---The paleoecologic and paleoenvironmental significance of trace fossilsrelated to discontinuity surfaces in the Lower Cretaceous marine depositsof the Aconcagua area are analysed here. Carbonate- evaporiteshoaling-upward cycles, developed by high organicproductionin a shallow hypersalinerestricted environment, make up the section. Two types of cyclesare defined,being mainly distinguishedby their subtidal unit. Cycle I begins with a highlydolomitizedlower subtidalunit(FaciesA), followedupward by an intenselybioturbated upper subtidalunit (FaciesB). The nodular packstone facies(BI)is capped by a discontinuitysurface(firmground or hardground) and occasionallyoverlainby an oystreidbed (FaciesC). Cycle IIischaracterizedby a pelletoidal subtidalunit (FaciesB 2)with an abnormal salinityimpoverished fauna. Both cyclesend with intertidalto supratidal evaporite deposits (Facies D and E, respectively). Attentionisparticularly focusedon cycleIdue to itsichologic content. The mode ofpreservationand the distribution oftracefossils in nodular packstone facies are controlled by originalsubstrateconsolidation. Thalassinoid~s paradoxicus (pre-omissionsuite)representscoloniza- tion in a softbottom, while Thalassinoides suevicu8 (omission suite pre-lithification) is apparently re- strictsdto firm substrates. When consolidationprocessesare interruptedearly,only an embryonic hard- ground that representsa minor haltin sedimentation was developed. Sometimes, consolidationprocesses continued leadingto an intraformationalhardground. Colonization by Trypanites solitarius (omissionsuite post-lithification) and Exogyra-like oystreidspossiblycharacterizeshard substrate stage. When two dis- continuitysurfacesfollowclosely, a post-omissionsuitemay be defined in relationto the lower cemented surface. As trace fossils are so closely related to changes in the degree of bottom lithification, they prove to be very useful as indicators of substrate evolution. The presence of discontinuity surfaces, evidenced by trace fossil association, suggests changes of sedimentary rate and environmental conditions that should be taken into account in future studies seeking to erect depositional models for these Cretaceous deposits. INTRODUCTION ICHNOLOGY IN MARINE CARBONATES is similar in some aspects to ichnology in clastic environments. This is particularly true in carbonate sequences with an important extrabasinal terrigenous supply in temperate climates. On the other hand, when the clastic supply is scarce, especially in warm shallow carbonate seas, other processes and factors, like early diagenesis and intense biotic action, become impor- tant (cf Ekdale et al., 1984). Several authors pointed out the importance of bottom consolidation as a con- trolling factor that regulates organism colonization (e.g., Goldring and Kazmierczak, 1974; Bromley, 1975; Gruzczynski, 1979; Bromley and Ekdale, 1986). According to its classic definition, a "hardground" is a stratigraphic discontinuity in calcareous sedi- ments where lithification takes place before the deve- lopment of a permanent sedimentary cover (synsedi- mentary lithification). A series of intraformational discontinuity surfaces has been identified in the ana- lysed sequence. Many of these horizons represent Address all correspondence and reprint requests to Dr. M. G. M~ingano (tel: [54](81)330-633; fax: [54] (81) 311-462;telex: 61143 BUTUC-AR;E-mail: postmaster~nture.eduiar). minor breaks in sedimentation that attained only partial lithification (firmgrounds); however, some of them formed true intraformational hardgrounds. Distribution and mode of preservation of trace fossils provide evidence for the evolution of these peculiar surfaces. Different benthic organisms inhabited the substrate as it progressed through the successive stages of softground, firmground, and hardground, giving as a result a palimpsest ichnofabric (Ekdale et al., 1984). The aim of this paper is threefold: (i) to document the existence of arid shallow marine deposits and dis- continuity surfaces in the Lower Cretaceous rocks of the Aconcagua area; (ii) to present a new example of trace fossils related to discontinuity surfaces in very shallow water carbonates; and (iii)to outline the importance of trace fossil analysis, especially in cer- tain marginal environments where they represent an important clue to paleoenvironmental evolution. The examples studied here are located in the Quebrada Agua Blanca, west-southwest of the village of Puente del Inca in the Aconcagua area, High Andes (Mendoza province, western Argentina; Fig. 1). The first studies in this sector of the High Andes dealt particularly with regional and structural geo- logy (Darwin, 1846; Steltzner, 1873, 1878, 1923; 215