Architecture and controls of thick, intensely bioturbated, storm-influenced shallow-marine successions: An example from the Jurassic Neuquén Basin (Argentina)

Highlights

• We investigate a thick, biorturbated, storm-influenced shallow-marine succession

• We compare several other examples develop in different basin styles

• We challenge that deposition is controlled by frequency and magnitude of storms

• We propose long-term biogenic reworking efficiency is related to basin-scale depositional factors

Abstract

Thick (>100 m-thick), highly bioturbated storm-influenced shallow-marine deposits are not frequent in the stratigraphic record, but they tend to be common in aggradational to retrogradational successions. Individual storm-event beds have typically low preservation potential in these successions, yet depositional settings are characterized on the basis of storms processes. Here we present a sedimentological study of a thick, bioturbated exhumed succession deposited during the early post-rift stage of the Neuquén Basin (Argentina) and compare its stratigraphic record with examples worldwide, in order to discuss the potential factors controlling the total overprint of storm-event beds during several million years. The Bardas Blancas Formation being 170–220 m thick in the study area is dominated by muddy sandstones and sandy mudstones, and it also includes subordinate proportions of clean sandstones and pure mudstones, collectively representing different environments of a storm-influenced shoreface-offshore system. The offshore transition and proximal offshore strata invariably comprise intensely bioturbated deposits, with only a few preserved HCS-sandstone beds. The unit shows for most of its thickness a long-term aggradational pattern spanning 7–10 Myr and is associated with low riverine influence.

By combining the observations and interpretations of the Bardas Blancas Formation with other subsurface and exhumed intensely bioturbated, shallow-marine successions, we dispute the general assumption that these are associated with low frequency or low magnitude of storms. Alternatively, we argue that the long-lived efficiency of benthic fauna on overprinting most if not all the storm-event beds that reached the offshore-transition sector, results from the combination of several factors: deposition in relatively confined marine depocentres, persistent low riverine influence, and long-term aggradational stacking pattern. As these conditions can develop in a variety of basin styles, such as rift, early post-rift, and foreland settings, the recognition of thick, bioturbated successions as the ones discussed here can be used to infer more realistic constrains for depositional models and better predict facies distribution in such storm-influenced systems.

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.