Abstract

SANCHEZ, María Lidia  and  ASURMENDI, Estefanía. Depositional model for the Cerro Lisandro Formation: mouth lobes and Gilbert-type deltas. Upper Cretaceous, central region of the Neuquén basin, Argentina. Rev. mex. cienc. geol [online]. 2014, vol.31, n.2, pp.141-162. ISSN 2007-2902.

The geometry of a foreland basin is mainly a product of a complex dynamic balance between orogenic loading, erosion and sedimentation, and lithospheric flexural response. To improve our understanding of the relationships between these processes and the Andean foreland basin stratigraphy, a detailed stratigraphic and sedimentologic study of the Late Cenomanian - Early Turonian Cerro Lisandro Formation in the surface and subsurface of western Neuquen basin is presented in this paper. The Cerro Lisandro Formation shows a cyclic alternation of lacustrine and Gilbert-type delta deposits, with lacustrine-dominated intervals up to 152 m thick. Flood-dominated Gilbert-type deltas represent a volumetrically significant component of the Cerro Lisandro Formation, on a foreland basin margin that developed during the Late Cenomanian - Early Turonian in central-western Neuquen basin (Argentina). Excellent exposures of the vertical and lateral relationships among facies assemblages reveal a suite of deltaic and lacustrine depositional environments within elongate, highly subsiding troughs (foredeeps) that developed in front of the advancing Andean thrust systems. The entire succession is around 370 m thick and consists mainly of lacustrine clay and siltstone, and deltaic clinostratified and mouth bar lobe sandstone. The Gilbert-type deltas are organized into numerous vertically-stacked successions that display well-developed tangential foresets, poorly-preserved topsets, toesets and bottomset. The internal depositional architectures consisting of alternating progradational, aggradational-progratacional and retrogradational geometries were controlled by high-frequency, climatic events in a relatively rapidly subsiding basin. The common element that characterizes all the depositional architectures detected within the main studied sections is the constant influence of high rates of subsidence and sediment supply, associated with orogenic uplift and loading on the deltaic systems during sediment accumulation. The tectonic control of the basin margin from which the deltas were sourced forced a forward-stepping (basinward) arrangement of stacked Gilbert-type deltas, and these produced clinoforms that become progressively younger toward the basin depocentre. The concept of stratigraphic base level, or the ratio between accommodation and sediment supply (A/S ratio), has been used to analyse the Cerro Lisandro Formation. The lacustrine/Gilbert-type deltas cyclicity is interpreted as recording repeated uplift of the Agrio fold and thrust belt to the west, and a subordinate influence of paleoclimate. Cerro Lisandro Formation was deposited in underfilled conditions; during the early underfilled basin period, the basin geometry is mainly controlled by the orogenic loading, and a forebulge zone with an approximately fixed location is formed. A qualitative model for the migration and stratigraphic fill of an underfilled Andean foreland basin is proposed. This model provides a qualitative pattern of the cratonward migration of the forebulge zone within a several-million-year tectonically-driven cycle. It is demonstrated that the uplifting forebulge zones migrated toward the craton during the Cenomanian - early Turonian.

Keywords : Gilbert type delta; mouth lobes; Lisandro Formation; Cretaceous; Neuquén basin; Argentina.

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