Lacustrine cyanoids from Cerro Puntudo Formation (Cuyana Basin, Triassic, San Juan, Argentina)

BENAVENTE, CECILIA A.; CABALERI, NORA; MANCUSO, A

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

<!-- /* Font Definitions */ @font-face {font-family:Cambria; mso-font-alt:"Palatino Linotype"; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1073741899 0 0 415 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:Cambria; mso-fareast-font-family:Cambria; mso-bidi-font-family:"Times New Roman"; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> In order to better refine the paleoenvironmental analysis of the lacustrine succession in the Triassic Cerro Puntudo Formation (Anisian) located in the Cuyana rift basin of Argentina, a half-meter thick limestone was studied. This carbonate layer contains two tuff and two silty clay laminae about 1 cm in thickness that allows for the differentiation of three units. All three units are characterized by the presence of cyanoids (oncolites containing cyanobacteria and algae), silty carbonate and siliciclastic matrix, and charophyte remains, and are classified as algal wackestones. The cyanoids have concentric laminae and range in diameter from 0.5 to 8.0 cm. They have irregular margins and multiple laminae in the cortex coating of one or more nuclei. In some cases the nuclei consists of charophyte thalli and reproductive structures (gyrogonites). Transversal sections of the thalli measure between 0.2 and 0.6 mm wide with longitudinal sections of gyrogonites 600 µm long with an approximate diameter of 400 µm. Both structures are replaced by spar and can be seen in longitudinal and transverse sections. Between the laminae algal filaments of cyanophytes have been preserved as tubules of 10 to 20 µm long with micritized external walls. The matrix is composed of carbonate and siliciclastic silt grains as well as cyanoid coated grains. These limestone units are interpreted as deposits of a marginal lacustrine paleoenvironment that has undergone diagenesis within subaqueous conditions, no subaerial exposure or pedogenic features are evident. The process that gave rise to the cyanoids involved the growth of microbial mats above the thalli of charophytes while they were in situ in the littoral zone of the Cerro Puntudo paleolake. The existence of moderate energy currents and agitation may have promoted the formation and growth of the concentric cyanoids in this Triassic nearshore environment.