Cambrian?Ordovician tide- to wave-dominated shallow-marine clastic environments from Sierra de Cajas, northwest Argentina

VAUCHER, ROMAIN; VACCARI, N. EMILIO; MUÑOZ, DIEGO F.; BUATOIS, LUIS A.; WAISFELD, BEATRIZ G.

Congreso; 20th International Sedimentological Congress; 2018

Well-exposed Cambrian?Ordovician clastic sedimentary rocks fromnorthwest Argentina have been the subjectof intense paleontologic work aiming to unravel the evolutionary dynamics of the Great Ordovician Biodiversification Event (GOBE). However, detailed sedimentological studies in order to reconstruct paleoenvironmental conditions are less common. This study represents one of the first attempts to evaluate the depositional framework of the Cambrian?Ordovician transition in Sierra de Cajas, Jujuy Province with the aim of evaluating the potential impact of environmental factors on biodiversityat the onset of the GOBE. The studied interval ranges from Stage 10 (Furongian; late Cambrian) to early Tremadocian (Early Ordovician), which corresponds to the Parabolina(N.) frequens argentina, J. keideli andKainella andina trilobite biozones. This section is ca. 350 m thick and ismainly composed by siltstone and sandstone. From base to top,this sedimentary succession has been subdivided intothe PadriocFormationandthe Guayoc Chico Group (GCG; Lower, Middle, and Upper members). ThePadrioc Formation mostly consists of cross-bedded sandstone with associated mudstonedrapes on foresets alternating with rhythmic intercalations of sandstone and siltstone. This heterolithic interval comprises hummocky cross-stratified(HCS) and wave-ripple cross-laminated sandstonedisplaying ladderback ripples.The Lower Member of the GCG consists of massive siltstone intervals interbedded with cm-thick micro HCS sandstone (cm to dm-scale wavelength). The Middle Memberof the GCG is characterized by a higher proportion of thicker (more than 1 m) HCSsandstone, either amalgamated or forming discrete layers, as well as wave-ripple cross-lamination and low-angle cross-bedding associated with keystone vugs. The Upper Member of the GCG is composed by a siltstone interval with thin sandstone layers displaying cm-to dm-scale micro HCS.The Padrioc Formation seems to reflect subtidal to intertidal environmentsin a tide-dominated to mixed tide-and wave-influenced embayment, whereas the GCG records deposition within a wave-dominated shallow-marine environnment, with theLower Membercorresponding dominantly tooffshore settings,the Middle Member representing offshore transition toforeshoreenvironments. The Upper Member represents a return to offshore transition-offshoreconditions. The stratigraphic succession clearly highlights relative sea-level changes and coastal physiography controlling parasequence and parasequence set architecture and reflecting an overall passage from tide-dominated to wave-dominated conditions. The Lower Member of the GCG records the highest trilobitediversity in the basin for this period, most likely as a result of the interplay between normal marine salinity and low energy conditions.