Architecture and controls of thick, intensely bioturbated, storm-influenced shallow-marine successions: An example from the Jurassic Neuquén Basin (Argentina)
Introduction
The deposition and preservation of individual storm-related event beds in shallow-marine settings have been reported and extensively discussed in the literature (Niedoroda et al., 1989; Wheatcroft, 1990; Snedden and Nummedal, 1991; MacEachern and Pemberton, 1992; among many others). Facies models for wave- and storm-dominated shoreline and shallow-marine systems are relatively well established (e.g., Walker and Plint, 1992; Reading and Collinson, 1996; Johnson and Baldwin, 1996; Clifton, 2006; Plint, 2010), and they are recently incorporating two-dimensional, quantitative studies for refining shoreline reconstructions (e.g., Isla et al., 2020a, Isla et al., 2020b). MacEachern and Pemberton (1992) characterized three types of shorefaces based on the intensity and frequency of storms: intense, moderate, and weak (low-energy) shorefaces. It is typically assumed that a thoroughly bioturbated succession with little or not preserved storm-event beds within a storm-influenced shoreface-offshore system would represent weakly storm-affected shorefaces dominated by fair-weather deposits (MacEachern and Pemberton, 1992; MacEachern et al., 1999; Pemberton et al., 2012).
More than 100 m thick successions of storm-influenced, shallow-marine deposits characterized by highly bioturbated strata are not frequent in the stratigraphic record. However, they tend to be unusually common in rift to early post-rift stages of the North Sea Central Graben (Fraser et al., 2003; Gowland, 1996; Howell and Flint, 1996; Baniak et al., 2014), in rift stages of the North Sea Viking Graben (Ravnås et al., 1997; Løseth et al., 2009), and in early post-rift stages of the South American Neuquén Basin (Bardas Blancas Formation, Veiga et al., 2013). Other unusual examples of highly bioturbated, storm-influenced successions include the Bridport Sand Formation in the extensional Wessex Basin (Morris et al., 2006) and the Late Cretaceous Emery Sandstone Member of the Mancos Shale in the Western Interior foreland basin (Edwards et al., 2005). However, a thorough analysis of all these examples to test if they can be simply placed in the low-energy shoreface end-member of the MacEachern and Pemberton (1992) spectrum, or if there are other controlling factors that contribute to produce thick bioturbated storm-influenced successions, has not yet been attempted.
In this study, we present a detailed sedimentological study of a thick, highly bioturbated succession exposed in the northern Neuquén Basin (Lower-Middle Jurassic, Bardas Blancas Formation) with the following objectives: a) to describe and analyse an intensely bioturbated, storm-influenced shallow-marine succession, b) to compare the stratigraphic record of the Bardas Blancas Formation with thick, highly bioturbated units from other basins, c) to discuss the combination of several depositional controls that contribute to the complete destruction of original sedimentary structures and storm-event beds during several million years.
Section snippets
Geologic and stratigraphic setting
The Neuquén Basin is located on the eastern side of the Andes in west-central Argentina, between latitudes 32° and 40° South, covering an area of over 150,000 km2 (Fig. 1A). It comprises a nearly continuous stratigraphic record of up to 6000 m thick strata from the Upper Triassic to Lower Cenozoic, and it is one of the most important petroleum provinces of South America (e.g. Legarreta and Uliana, 1991). The sedimentary record of the Neuquén Basin includes continental and marine siliciclastics,
Study area and previous work
Veiga et al. (2013) provided a detailed architectural and sequence stratigraphic analysis of the Bardas Blancas Formation in the Sierra de Reyes study area, integrating outcrop and subsurface information from a 3000 km2 large area. They included two outcrop sections in the western and eastern sectors of the Sierra de Reyes anticline and several wells in the eastern subsurface region (Fig. 2). That study provides a framework in which to place the detailed sedimentological and ichnological
Facies associations and depositional model
The facies and facies associations of the Bardas Blancas Formation and its transition to Los Molles Formation are presented in Table 1. Six facies associations (FA) have been defined for the study interval including: FA1 - Delta front, FA2 - Upper shoreface, FA3 - Lower shoreface, FA4 - Offshore transition, FA5 - Proximal offshore, and FA6 - Distal offshore. The definition and interpretation of these facies associations is broadly in agreement with the proposed by Veiga et al. (2013). Hereby we
Architecture of an intensely bioturbated succession
The shallowing-upward units identified in the Bardas Blancas are parasequences bounded by flooding surfaces (Fig. 4, Fig. 8A), uncommonly demarcated by shell beds. These stratigraphic units are internally composed of bedsets with subtle stratigraphic boundaries (Fig. 8A). In the lower interval of the unit, parasequences show a complete transition from mudstones of FA6 (distal offshore) to clean, trough cross-bedded sandstones of FA2 (upper shoreface) (Fig. 4). In the middle and upper intervals
Discussion
The preservation potential of individual storm-related event beds (or tempestites) in shallow-marine settings and the lam-scram textures resulting from partial to total biogenic reworking of these event beds have been extensively reported and discussed (Wheatcroft, 1990; MacEachern and Pemberton, 1992; among many others). Three types of shoreface settings are distinguished based on the intensity and frequency of storms: intense, moderate, and weak or low-energy (MacEachern and Pemberton, 1992).
Conclusions
- 1.
The Lower-Middle Jurassic Bardas Blancas Formation represents an up to 220 m thick, highly bioturbated, storm-influenced shallow-marine succession developed during the early post-rift stage of the Neuquén Basin.
- 2.
Most of its stratigraphic record is dominated by muddy sandstones and sandy to silty mudstones deposited in offshore-transition to proximal-offshore settings, in which benthic- fauna efficiency to rework individual storm-event beds was persistently close to 100% during a time span
Declaration of Competing Interest
None.
Acknowledgments
E.S. would like to thank CONICET and Universidad Nacional de La Plata for partially supporting this project. M.P. and I.M. acknowledge Aker BP, sponsor of the ShelfSed project (University of Oslo). We thank J. Cuitiño and A. Weztel for their constructive reviews and specifically the latter for his dedicated revision of the English writing. An anonymous reviewer also provided additional comments. We are also grateful to Francisco Rodriguez-Tovar for the invitation to participate in this special
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