MUDSTONE “CRYPTOBIOTURBATION” IN THE JURASSIC-CRETACEOUS VACA MUERTA FORMATION: USING TUFFS AS TAPHONOMIC WINDOWS TO UNDERSTAND THE EXTENT OF ANOXIA

PAZ, MAXIMILIANO; MÁNGANO, MARÍA GABRIELA; BUATOIS, LUIS ALBERTO; DESJARDINS, PATRICIO RAFAEL; RODRIGUEZ, MAXIMILIANO NICOLÁS; PARADA, MARTIN NAZARENO; GONZÁLEZ TOMASSINI, FEDERICO; MINISINI, DANIEL; PEREIRA, EGBERTO

La Plata

Congreso; XVIII Reunión Argentina de Sedimentología y IX Congreso Latinoamericano de Sedimentología; 2023

Asociación Argentina de Sedimentología

In the last decade, sedimentologic studies have shown that bioturbation by small organisms may produce laminated intervals in muddy successions, which has been regarded as analogous to cryptobioturbation in modern sands. However, compaction and lack of discrete trace fossils in mudstones preclude a clear differentiation between lamination generated by abiotic processes, from lamination generated by “cryptobioturbation". A database of 1093 m of cores retrieved from the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation (Neuquén Basin) constitutes a special case scenario where the interbedding of tuff and mudstone provides a unique taphonomic window to study bioturbation in ancient muddy substrates. Ca-rich tuffs prevent compaction due to their carbonate early cementation, facilitating the preservation of discrete trace fossils. Bioturbation in the tuffs reveals insights into the fabric of the enclosing mudstone: (1) parallel-laminated (unbioturbated) fabrics have submillimeter-thick tuff laminae without trace fossils, (2) irregular-laminated (“cryptobioturbated”) fabrics show tuffs with trace fossils associated with reworking by meiofauna and small macrofauna (Alcyonidiopsis and the pellet Coprulus), and (3) massive (bioturbated) fabrics record macrofaunal trace fossils (Alcyonidiopsis, Teichichnus, Phycosiphon, and Coprulus). These fabrics are interpreted as different oxygen-controlled biofacies based on currently accepted ichnologic models. Quantification of biofacies along the cores reveals that the Vaca Muerta Formation bottomset (909 m total) comprises 75% of anaerobic, unbioturbated intervals, 16% of quasi-anaerobic and dysaerobic, “cryptobioturbated” intervals, and 8% of aerobic, bioturbated intervals, whereas the foreset (184 m total) is 5% anaerobic, 45% quasi-anaerobic and dysaerobic, and 49% aerobic. This study supports the idea that not only meiofauna, but also small macrofauna produce “cryptobioturbation”. Moreover, it indicates that a great volume of this formation may have been deposited under dysoxic and oxic conditions (not anoxic). However, the high extent of anaerobic biofacies in bottomset deposits (75%) suggests a high influence of anoxic conditions during deposition, calling for caution when uncritically invoking “cryptobioturbation” in mudstone successions. Our analysis based on ichnologic data should be supplemented with future studies based on body fossil data (specially foraminifera) to generate more comprehensive and refined oxygenation proxies.