New Biostratigraphic Proposal for the Lower Palaeozoic Tucunuco Group (San Juan Precordillera, Argentina) Based on Marine and Terrestrial Palynomorphs

Abstract

This is an exhaustive analysis of the palynomorph assemblages, organic-walled marine phytoplankton and miospores from the Tucunuco Group. This group includes the La Chilca and Los Espejos formations and is exposed in the Central Precordillera of San Juan. Four localities were included in this study, i.e., Río Jáchal, Río de las Chacritas, Cerro La Chilca and Quebrada Ancha. Previous data concerning several fossil groups, such as graptolites, conodonts and brachiopods, suggest a mainly Llandovery-Wenlock? age for the La Chilca Formation and a Wenlock to locally Lochkovian age for the Los Espejos Formation. Marine palynomorphs are predominant in the two formations in all the studied sections. Although terrestrial palynomorphs are the minor component, trilete spores are abundant, diverse and biostatigraphically relevant in the Los Espejos Formation. Correlations using marine palynomorph assemblages and miospore biozones recognized worldwide allow constraining the age of the two stratigraphic units. Based on marine phytoplankton, the middle-upper part of the La Chilca Formation is assigned to the Wenlock. In the Los Espejos Formation, palynomorph assemblages allow the recognition, from the base to the top, of the Wenlock with doubts, the Ludlow, and the Pridolí. The Lochkovian has been documented exclusively in the northern part of the basin. This confirms the presence of the Silurian/Devonian boundary in the upper part of the Los Espejos Formation. A north-south correlation panel of the studied sections is presented.

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

“…The presence of palynomorphs in these deposits of the La Chilca Formation is reported by García‐Muro and Rubinstein () from the Ancha Creek, which indicates a Telychian age.…”

“…The uppermost Ordovician/Silurian sedimentary succession in the Central Precordillera is represented by the Tucunuco Group (Figure d), comprising the La Chilca Formation (Hirnantian to Wenlock) and the Los Espejos Formation (Wenlock? to Lochkovian; Cuerda, , ; García‐Muro & Rubinstein, ; Garcias, ), and its lateral equivalent, the Tambolar Formation, close to the San Juan River (Peralta, ).…”

“…The lowest represents residual deglaciation deposits where the dropstone‐bearing sandstone facies are sediments deposited during the slack phases from floating icebergs in the lower subtidal environments (Figure b). This section of the LCH Formation cropping out at the Ancha Creek contains palynomorphs and graptolites (García‐Muro & Rubinstein, ; Lenz, Cuerda, & Peralta, ; Peralta, ) of Hirnantian–Rhuddanian age.…”

“…García‐Muro and Rubinstein () mentioned the lack of fossils recorded in these beds. However, the stratigraphic relationships with the underlying and overlying deposits allows assigning, with doubts, an Aeronian?–Telychian age.…”

First documentation of the Polygnathoides siluricus conodont Zone (Ludfordian) in South America (Argentina) and the stratigraphic significance of the younger species of Kockelella (Conodonta)

“…The La Chilca Formation culminates with a level of oolitic sandstone followed by green shale of the Los Espejos Formation (Cuerda, ). The age of this unit extends from the Hirnantian to the late Llandovery–early Wenlock (Benedetto, ; García‐Muro & Rubinstein, ; Lehnert et al, ; Pöthe de Baldis, ). This formation was interpreted as a typical shallow‐water, storm‐dominated sequence (Astini & Maretto, ; Astini & Piovano, ; Peralta, ), possibly related to a global sea‐level change.…”