Sandstone petrography and provenance of the Los Molles and Las Lajas Formations (Cuyo Group-Jurassic) in the Arroyo La Jardinera Region, Neuquen Basin, Argentina

Abstract

The Cuyo Group (Pliensbachian-Early Callovian) in the Arroyo La Jardinera area (southwestern Neuquén basin) comprises about 2000m of siliciclastic strata. This area is crossed by the 46 Route and is located about 60 km from Aluminé. It offers a good example of a regressive succession characterized by slope and basin floor gravity flow strata (Upper Los Molles Fm) overlain by shelf and nearshore facies (Las Lajas Fm) and, finally, by fluvial (Challacó Fm) deposits. Seven vertical sections have been described in the area (Paim et al, 2008) and more than 120 thin sections of medium-grained sandstone have been studied under the microscope by classical point counting techniques. This information was plotted in the Dickinson provenance diagrams and analyzed together with paleocurrent and conglomerate composition data. In general terms, the Upper Los Molles Formation records transitional and dissected arc source areas while the Las Lajas Formation dissected arc and recycled oro - gen provenance. Taking into account the transitional relationships between both units, the following upward trends are apparent: increasing grain size, porosity and plagioclase, microcline and chert content and decreasing matrix content. The connection between source areas, stratigraphy and paleogeography is analyzed bearing in mind the presence of widespread Jurassic andesitic source areas to the west, granitoids to the south, and acid volcanic rocks to the east. The Lajas Formation has a higher maturity – quartz content – than both the Los Molles and Challacó formations. This higher quartz content was assigned to specific sedimentary processes rather than to changes of the source area, i.e., fluvial (Challacó Fm) and gravity (Los Molles Fm) flows towards the depocentre, their strata hence recording a primary source located to the WSW, whereas parallel to the coastline, alongshore drift to SSE associated with strong wave-reworking would easily explain the higher degree of textural and compositional maturity of the Lajas Fm sandstone beds.