It is widely understood that Earth's stratigraphic record is an incomplete record of time, but the implications that this has for interpreting sedimentary outcrop have received little attention. Here we consider how time is preserved at outcrop using the Neogene–Quaternary Red Crag Formation, England. The Red Crag Formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata, as ancient depositional environments fluctuated between states of deposition, erosion and stasis. We use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600–800 kyr of ‘Crag-time’. Much of the apparently missing time may be accounted for by the parts of the formation at subcrop, rather than outcrop: stratigraphic time has not been lost, but is hidden. The time-completeness of the Red Crag Formation at outcrop appears analogous to that recorded in much older rock units, implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary-stratigraphic record as a truthful chronicle of Earth history.
Supplementary material: Further details of the regional geology and specific information on outcrops are available at https://doi.org/10.6084/m9.figshare.c.4561001
It has been recognized for over a century that Earth's stratigraphic record is time-incomplete, and that vertical successions of sedimentary strata are punctuated historical chronicles (Barrell 1917; Dott 1983). Unconformities and diastems riddle the rock record at a variety of scales (Miall 2016) and such gaps, of often unknown extent and duration, have implications for considering strata as a record of elapsed geological time. They can skew estimates of ancient rates of sedimentation or climate change (Sadler 1981; Kemp et al. 2015; Miall 2015; Toby et al. 2019), can mean that allogenic signals have been shredded by autogenic processes (Jerolmack & Paola 2010; Foreman & Straub 2017; Hajek & Straub 2017), and can add a further layer of incompleteness to a fossil record already rendered lacking by taphonomic filters (Kowalewski & Bambach 2003; Holland 2016; Saraswati 2019). Furthermore, the overwhelming proportion of ‘missing time’, relative to preserved stratigraphic time, has long raised fundamental questions about the veracity of strata as a historical archive and whether they can truly represent ancient processes and environments.
A recent upsurge of developments in understanding the time-completeness of the stratigraphic record, particularly from a stratigraphic modelling perspective, has been comprehensively discussed by Miall (2015) and Paola et al. (2018). Recent advances can be summarized as pointing to three recurring themes, namely: (1) ancient strata are dominantly a record of commonplace sedimentary processes and not exceptional events (Jerolmack & Paola 2010; Paola 2016); (2) the time-dominant sedimentation state under which the sedimentary-stratigraphic record accumulated was stasis; that is, ‘neither deposition nor erosion’, rather than ‘either deposition or erosion’ (Tipper 2015; Davies et al. 2017; Foreman & Straub 2017); (3) any time-gaps in the deposition of one vertical stratigraphic section of basin fill can have been contemporaneous with deposition of strata elsewhere within the same depocentre (Runkel et al. 2008; Reesink et al. 2015; Gani 2017).
These emerging understandings have potentially major implications for the way we interpret the geological record (e.g. Miall 2014; Hampson et al. 2015; Durkin et al. 2017; Davies & Shillito 2018; Kocurek & Day 2018). A better understanding of time-length scales that present at rock outcrop is needed because it is common practice for geologists to focus attention at the scale of an individual outcrop or group of outcrops, which provide the most tangible point of contact for understanding the physical sedimentary records of ancient environments, and their intensive properties (e.g. palaeontological or geochemical signatures).
The purpose of this contribution is to investigate time at outcrop by describing field observations that act as proxies for time-completeness, using examples from the Neogene–Quaternary Red Crag Formation of eastern England, a sub-tidal sedimentary succession that is known from a number of discrete, small outcrops.
The Red Crag Formation
The Red Crag Formation is the second oldest unit of the late Cenozoic Crag Group, which crops out in eastern England (McMillan et al. 2011; Lee et al. 2015; Mathers & Hamblin 2015) and consists of four discrete transgressive formations (the others, from oldest to youngest, being the Coralline Crag, Norwich Crag and Wroxham Crag formations) (Fig. 1). Each of the formations is separated by regional unconformities and all were deposited in open marine settings near the landward head of the Crag Basin, a localized embayment in the SW corner of what is now the North Sea. Although there is some uncertainty in the Red Crag Formation's precise age, the oldest parts of the unit are agreed to be latest Pliocene (Piacenzian) and the youngest are earliest Pleistocene (Gelasian), and the duration of Red Crag deposition is consistently reported to be between 600 and 800 kyr (e.g. Zalasiewicz et al. 1988; Hallam & Maher 1994; Gibbard et al. 1998; Head 1998; Maher & Hallam 2005; Williams et al. 2009; Wood 2009; Wood et al. 2009; McMillan et al. 2011; Riches 2012; Mathers & Hamblin 2015). Additionally, it has long been recognized that the unit is diachronous and becomes older southwards (Riches 2012); its oldest strata (an outlier at Walton-on-the-Naze, Essex) may be separated from the rest of the unit by an unconformity (e.g. Wood et al. 2009).
Lithologically, the Red Crag Formation consists of poorly sorted, semi-consolidated, coarse-grained shelly quartz and carbonate sands that are dark green and glauconitic at depth but have been weathered to an iron-stained orange–red colour at outcrop (Humphreys & Balson 1985; McMillan et al. 2011). The sediment usually has an extremely high content of aragonitic and calcitic shell debris, although at some locations the upper part of the unit has been decalcified to pure quartz sand as a result of later Pleistocene soil development (Kendall & Clegg 2000). Variable palaeocurrent indicators, large-scale cross-bedding, bioturbation, and sedimentary structures including flaser bedding and bidirectional cross-strata indicate that the unit was primarily deposited by migrating large-scale subtidal sandwaves (Figs 2 and 3) (Dixon 1979, 2005, 2011; Mathers & Zalasiewicz 1988; Zalasiewicz et al. 1988; Balson et al. 1991; Hamblin et al. 1997).
Outcrops of the Red Crag Formation
Outcrops of the Red Crag Formation are typically of limited extent, but of good quality for discerning its internal sedimentary architecture (Fig. 4). No single outcrop approaches the full 40 m thickness of the unit, but this can be ascertained from some of the hundreds of boreholes that have been made across unexposed parts of the regional outcrop belt (Fig. 5; British Geological Survey 2018). Two primary types of exposed outcrop exist and have formed the focus of this study: (1) crag pits (six outcrops; Fig. 4a), which are static inland exposures formerly quarried for agricultural and aggregate purposes (O'Connor & Ford 2001); (2) coastal outcrops (two outcrops; Fig. 4b and c), which comprise dynamic natural exposures of small cliffs that are frequently reworked by wave activity along a highly erodible and recessive coastline (Environment Agency 2015).
Significantly for later discussion in this paper, the vertical cliff faces exposed in both types of outcrop are of limited extent: crag pits have a mean height and lateral extent of 5.5 m and 93.5 m respectively, whereas coastal outcrops have equivalent dimensions of 9 m and 1555 m.
Further details of the regional geology and specific information on outcrops are available in the supplementary material.
Sedimentation states and the preservation of time
The sedimentary-stratigraphic record has long been considered to be an archive of elapsed time: put simply, deposited sediment ‘preserves time’ and erosion of that sediment ‘removes time’. A time interval is generally considered preserved when a sedimentary deposit representing any time from that interval remains in the stratigraphic column at the location of interest (Strauss & Sadler 1989; Paola et al. 2018). However, this definition of preserved time is complicated by the recognition that not all time at a given location would have equated to a period of deposition or erosion; in fact, many sedimentary systems will have existed in a condition of sedimentary stasis for the majority of the time they were active (Dott 1983; Tipper 2015; Foreman & Straub 2017; Paola et al. 2018).
Tipper (2015) has suggested that time spent in stasis cannot be preserved because there is nothing to be preserved. Yet although this may be conceptually true for understanding how synthetic vertical stratigraphic columns record time, it is unsatisfactory for explaining real-world sedimentary rock outcrops. If a sedimentary surface, persisting for a duration of sedimentary stasis in an active environment, is not eroded, then that surface has the potential to accrue information generated by processes and events occurring as time passes during the stasis interval: for example, as multiple generations of surficial ichnological, microbial and abiotic sedimentary structures, or as distinct geochemical or pedogenic vertical profiles (Miall & Arush 2001; Barnett & Wright 2008; Christ et al. 2012; Davies et al. 2017; Davies & Shillito 2018; Paola et al. 2018; Shillito & Davies 2019). Where such signatures can be identified alongside signatures of erosion and deposition, it becomes possible to broadly estimate the duration of accrual of a package of sedimentary strata as preserved at a given outcrop, with implications for how representative that outcrop may be of ancient sedimentary environment.
Stratigraphic signatures of sedimentation states in the Red Crag at outcrop
Time spent in different sedimentation states is recorded in different ways in the sedimentary-stratigraphic signatures of the Red Crag Formation at outcrop. By definition, the most obviously recorded sedimentation state is deposition: without deposition there is no sediment accumulation, and so time spent in this state is recorded as the sediment pile itself. Likewise, erosion has left discernible stratal discordances within the sediment pile, which are readily apparent as bounding surfaces (Fig. 6). Erosional surfaces are primarily a negative record of time, recording the erasure of time records that once existed (Sadler 1999). Within the Red Crag Formation, none of the studied outcrops contain major erosional surfaces (i.e. extending the full width of an exposure), so there is little direct evidence of wholesale deletion of depositional records at outcrop scale (Fig. 6).
Sedimentary stasis is revealed in the Red Crag Formation as bounding surfaces that record a synoptic topography from the time of deposition (Paola et al. 2018). These can sometimes be recognized by the preservation of complete bedforms with convex top surfaces, often with evidence that later sediment was draped over the antecedent substrate morphology (Fig. 7).
More commonly, the extensive Red Crag ichnofauna (Fig. 3, Table 1) gives clues to sedimentary stasis. Every burrowed horizon in the unit provides evidence that intervals of stasis punctuated the deposition of the Red Crag Formation, because the colonization of a substrate requires time for organisms to excavate sediment without disturbance from erosion or deposition (Goldring 1960; Buck 1985; Frey & Goldring 1992; Pollard et al. 1993; Davies & Shillito 2018). As complete vertical burrows may be impossible to distinguish from truncated burrows without bedding plane evidence (e.g. Goldring 1960; Hallam & Swett 1966; Buck 1985; Wetzel & Aigner 1986; Nara 1997; Davies et al. 2009) (lacking in the unconsolidated Red Crag Formation), burrows can be determined to be complete only when they intersect with synoptic topographies (e.g. inclined burrows intersecting with foresets or dune lee slopes: Fig. 8; Pollard et al. 1993). However, even where they are only preserved in truncated form, they are direct evidence that deposition was not continuous, and instead alternated with a state of stasis (± erosion) (Fig. 8).
Sedimentation states cannot be maintained in perpetuity so, at any given location, states of deposition (D), erosion (E) and stasis (S) will be in spatial and temporal flux while the sedimentation system is active. Compound sedimentation states reflect this variability (i.e. D–E–D, D–S–E–D, D–S–D and D–E–S–D) and can be deduced by close scrutiny of signatures that mark the transition between two strata, which by definition must each record deposition (D). Such signatures of compound sedimentation states are common and highly variable within the Red Crag Formation (Fig. 9), as a direct result of their depositional environment and the narrow frame of reference provided by outcrop.
Why are signatures of compound sedimentation states common and variable in Red Crag outcrops?
Tipper (2015) introduced the concept of ‘point sedimentation systems’ to explain time-completeness in vertical synthetic stratigraphic columns, referring to a specific point (in a mathematical sense) within the space of a sedimentary environment, variably subject to erosion, deposition and stasis. For the purpose of studying real-world strata, this concept can be practically extended to be applicable to the narrow spatial focus offered by all or part of an outcrop, which provides a sedimentary record of a ‘point’ within a much wider sedimentary environment, across which stasis, erosion and deposition can be happening simultaneously (Runkel et al. 2008; Davies & Shillito 2018). Within a depositional environment, the spatial frame occupied by a future outcrop could witness multiple compound sedimentation states over the time that it took to accrue vertically, resulting in high variability in signatures of compound sedimentation states. This variability is particularly pronounced in the Red Crag Formation, because it was deposited in a tidal setting, where intervals of deposition and erosion are very often punctuated by stasis. For example, in modern shallow tidal sediments, Reineck (1960) calculated that <0.0001% of time was recorded as deposited sediment layers at any given point. Intertidal settings experience frequent stasis: following tidal stillstand (of as little as 10–20 min duration) an interval of erosion (or further stasis, if the reversed current is weak) can generate pause planes within bedforms (Boersma 1969; Boersma & Terwindt 1981; Allen et al. 1994). Subtidal sandwaves are also variable: at any spatial point on the seafloor, a substrate may aggrade, degrade or remain in stasis over short timescales, even while the underlying sandwave remains in a net migratory state. In a survey of 25 very large dunes at 26–30 m water-depth in the modern North Sea, Van Dijk & Kleinhans (2005) monitored the change in elevation of the seafloor substrate over the course of a year. They found that eight sampling locations (all on the upper lee slope of dunes) saw a decrease in elevation (i.e. experienced erosion), 33 saw an increase in elevation (i.e. experienced deposition), but six saw no change in elevation (i.e. stasis).
Estimating the duration of sedimentation states
Red Crag Formation outcrops are amalgams of signatures of different sedimentation states and compound sedimentation states. By estimating how long each recorded sedimentation state lasted, it is possible to estimate the time it took to accrue the sedimentary strata that constitute a particular outcrop.
Many Red Crag outcrops record the deposits of sandwaves, which, in modern tidal settings, can migrate a distance equivalent to their average height within a single tidal cycle of c. 12 h (Dalrymple 1984). The average height of ancient Red Crag sand waves is not directly discernible owing to erosional truncation and limited outcrop size. However, the minimum height (i.e. the vertical distance between the top and bottom of a foreset) of different bedforms is calculable. For the largest cross-bedded units known, the rate of migration was probably in excess of 3 m every 12 h, meaning that the time taken to deposit the layer that extends for half the width of the present outcrop (Fig. 4a) would have been at most 15 days.
Tipper (2016) suggested that herringbone cross-strata was another sedimentary structure that could be used to constrain instantaneous sedimentation rates, illustrating this by suggesting that such bidirectional cosets are deposited during one flood–ebb tidal cycle. This is flawed because there is no guarantee that such opposite-directed cross-strata were deposited during the same single semi-diurnal cycle (Kvale 2012): deposition and erosion may occur only during stronger tides, and substrates may be in stasis for considerable intervals of a tidal year (Allen et al. 1994). However, the bidirectionality seen in the Red Crag Formation does imply that the agents of deposition (i.e. flood and ebb tidal currents) could feasibly have deposited paired sets over intervals of hours to months (e.g. see examples in Figs 2b, c and 8c).
Whereas stratigraphic time lost to erosion may be unknowable, the duration of erosive events can be estimated. Certain erosional surfaces in the Red Crag Formation appear to be intrinsically linked to tidal timescales; for example, internal erosional surfaces within cosets of cross-bedding (Fig. 10) most probably reflect erosional pause planes (Boersma & Terwindt 1981). Like reversing cross-strata, the frequency of repetition of these could be semi-diurnal or longer term, but the duration of erosion for individual surfaces would have been accommodated within one tidal reversal (i.e. an interval of hours).
We know very little about time represented by ordinary surfaces with no signs of stasis (Dott 1983). As these are typically the most common surfaces, it is only possible to estimate the minimum duration of stasis for an outcrop succession. In the Red Crag Formation, this is allowed by the consideration of burrowed surfaces. The burrowing rate of different individual tracemakers in modern shallow marine settings has received only limited attention (Dafoe et al. 2008; Gingras et al. 2008b), but a selection of quantified rates is shown in Table 1. These can be used to estimate the minimum time that the system was in stasis, by calculating how long it would take for the fastest-burrowing potential tracemaker to excavate the largest burrow along a given stasis surface, where the internal volume of the burrows can be roughly approximated as one or more cylinders (πr2h, where r is burrow radius and h is burrow length).
The time taken to excavate particular individuals of the known ichnogenera (Table 2) provides a very approximate and conservative minimum estimate of the time spent in stasis. The most important conclusion here is that burrow formation is a geologically rapid process that occurs only during sedimentary stasis (Table 3), but there a number of caveats to these estimates, namely: (1) it is impossible to unravel the temporal sequence of the generation of a suite of individual burrows along the same stasis surface; (2) the speed at which burrows are excavated depends on factors such as grain size; (3) estimates are made with reference to the limited data published on burrowing rates; (4) very large dwelling burrows (e.g. Psilonichnus) are problematic because stasis is most likely to have persisted for an unknown interval after the burrow was excavated, and while the tracemakers were continuing to use the burrows as domiciles. Burrowing rates of modern crabs (the suspected Psilonichnus tracemakers; Humphreys & Balson 1988) have been calculated only as the time taken for an individual to fully bury their carapace in sediment (e.g. McLay & Osborne 1985; Lastra et al. 2002), and estimates of excavation rates at depth (where overburden and compressive force chains of packed grains impede burrowing speed; Dorgan et al. 2006) have not been reported. As an approximation of excavation speed for the largest Psilonichnus, we here use the maximum invertebrate rate reported in Table 2 of 10 cm3 h−1, although the margins of error here may be large.
Time taken to deposit individual outcrops of the Red Crag Formation
Precisely determining the time taken to deposit an individual outcrop of the Red Crag Formation is impeded by (1) the inability to accurately determine sequences of events during stasis from their physical records, (2) the inability to confidently calculate original dune height from preserved foresets, and (3) a reliance on potentially imperfect modern analogues. Despite this, the lack of major erosional surfaces at outcrop suggests that little time has been destroyed and lost at outcrop-scale and is instead missing owing to stasis. Equally, that time spent in stasis appears to have been relatively short because there is a lack of complete bioturbation reworking of primary sedimentary structures or shell material, and no evidence for palimpsesting of multiple generations of burrows at the same horizon, despite the Red Crag seas supporting an abundant infauna. The lack of evidence for prolonged bioturbation implies that horizons were probably in stasis on timescales no longer than hours to days (Table 2), and probably reflect tidal current quiescence on semi-diurnal or synodic timeframes (Kvale 2012).
Figures 10 and 11 show how the entire sediment piles that form pit outcrops of the Red Crag Formation can reasonably be estimated to have accumulated over time intervals of days to months. Thus, when we encounter the unit as an individual outcrop, we are dealing with sediment accrued over very minor time intervals, well within the range of human experience. This observation seems counterintuitive when we consider that the time taken to deposit the Red Crag Formation, as an entire stratigraphic entity, was 600–800 kyr, equating to average sedimentation rates of 0.5–0.66 cm ka−1 to deposit the unit's full 40 m thickness. However, as noted by Miall (2015), such quantified average sedimentation rates are essentially meaningless: it is an understanding of the instantaneous sedimentation rate (taken to deposit a particular bedform or sedimentary component) that informs most on the nature of deposition. The small spatial scale of the Red Crag Formation outcrops has discretized the time-length scale of visible strata to focus only on those features deposited over sub-annual sedimentation rate scales (Miall 2015).
Where does the time go? Discussion
As every individual outcrop of the Red Crag Formation reveals only a maximum of a few months in the life of the active sedimentary environment, they provide vanishingly small windows into the 600–800 kyr of total ‘Crag-time’. Two obvious questions arise from this understanding: (1) Where did the vast majority of Crag-time end up, if not preserved at outcrop? (2) If we have such small windows, how can we trust them to be representative of what was really happening during Crag-time?
With respect to the first question, part of the answer lies in the time-length scale of the exposed outcrops that we are viewing: the window on time that we have is miniscule, but so is the window on space. For example, the outcrop at Capel Green (Fig. 10) may reveal as little as 35 days out of 292 million days (800 kyr) of Crag-time, but then the spatial area of the outcrop is only 156 m2 out of c. >4 billion m2 of Crag Group (as mapped onshore). When we consider that stratigraphic time is smeared laterally over an outcrop belt (Runkel et al. 2008; Reesink et al. 2015; Gani 2017; Davies & Shillito 2018), the null hypothesis is that it is highly improbable that any two outcrops record the exact same time interval: they are all floating pockets of preserved time with no hope of being accurately chronostratigraphically anchored within the 600–800 kyr boundaries of net Crag-time (Fig. 12). This opens the possibility that the fraction of preserved Crag-time may not be negligible after all: we simply cannot access the majority of the mapping unit as it is concealed as subcrop. In other words, time is not lost, but hidden. We can see strata only from the vantages of outcrop or core, but these are tiny windows relative to the bulk volume of sediment that is still preserved today (e.g. Fig. 5). It is simply impossible to see all of the internalized physical strata hidden behind cliff faces and between outcrop exposures or cores. We contend that those ancient sedimentary products that can be witnessed today do not record temporally rare events, but rather that the observable outcrop exposure of sedimentary product is a spatially rare phenomenon: relative to the extent of (1) the ancient depositional environment and (2) the full extent of its unexposed lithostratigraphic corollaries.
The outcrops of the Red Crag record exposures of ‘days’, but are they representative of ‘every day’ process during the interval of deposition? The null hypothesis must be that they are, because of the strong similarity between the different exposed outcrops, which all contain a comparable array of tidal sedimentary structures and trace fossils (Figs 2 and 3). This attests to the likelihood that mundane, non-unique conditions were persistent for most of Crag-time (i.e. the ‘strange ordinariness’ discussed by Paola et al. 2018). Exposed outcrops are random samples of the net volume of a succession: if they are all telling the same story, despite having been deposited on month-timescales that are separated by unknowable intervals of time, then it is highly probable that they are preserving the ‘norm’ rather than exceptions.
The bias of the present
A present-to-past vantage point can skew and bias our perspective of a variety of geological phenomena (e.g. Budd & Mann 2018), and this is particularly true when considering the Red Crag Formation from a time perspective: a relatively young formation, with relatively transient outcrops (i.e. owing to coastal retreat). In this regard, the active reworking of coastal outcrops is informative, and sheds light on the remanié fossils that are common throughout the Crag Group as a whole.
Remanié fossils are those that are reworked from significantly older deposits and end up forming a fraction of a population of much younger fossils within a given stratum (Craig & Hallam 1963; Kowalewski & Bambach 2003), and are common within the Red Crag Formation (Riches 2012). However, remanié assemblages of Red Crag fossil fauna are also being formed at present as cliff collapse along the Suffolk coast mixes Pliocene–Quaternary sediment and fossils into modern beach sands (Environment Agency 2015): complete shells of the distinct, left-handed gastropod Neptunea contraria are commonly found in loose sediment of beaches such as Bawdsey (Fig. 13), but are definitively reworked from the Red Crag Formation as the modern range of this species is much further south (Bay of Biscay to Morocco) (Nelson & Pain 1986). Once on the beach, many of the reworked fossil shells re-enter circulation as sedimentary particles and, if fortuitously buried with sand, could, like recent shell debris, have thousand-year plus longevity within the active beach environment (e.g. Flessa 1993). Significantly, the environments into which these future remanié fossils and their host sediment are being eroded are extant nearshore and shallow marine environments (Fig. 14), with direct analogy to the ancient crag environments. This appears to be the latest stage in a historical continuity of fossil-recycling between crag units: the Red Crag contains reworked shelly fossils from the Pliocene Coralline Crag, and the Norwich and Wroxham crags contains reworked shelly fossils from the Plio-Pleistocene Red and Coralline crags. If the Red Crag Formation is being reworked at present into extant crag-like environments, this begs the question: are we still living in Crag-time?
We propose a perspective where the present coastline is considered to be the boundary of an active ‘North Sea Crag’ (Fig. 15), stacked on top of the extent of the depositional environments of the previous crags. Each of the older units is separated by an unconformity that marks intermittent disruption in the continuity of shallow marine deposition: in the case of the unconformity between the most recent crag (Wroxham) and the present, this is associated with glacioeustatic relative sea-level fall. The unconformities, corresponding to the Group 2 unconformities (104–105 years) of Miall (2016), reflect only retreat of crag deposition away from our present-biased frame of reference (i.e. the onshore outcrop belt), and offshore parts of the North Sea will have seen continuous deposition throughout some of the unconformity intervals. As such, the received perspective that we are now ‘post-crag’ may be a bias from living in the present, and onshore, and could be no different from the apparent post-crag conditions that would have been perceived had we been undertaking geological investigations on land during the interval of unconformity generation between, say, the Red and Norwich crags. The extensive, inter-formation unconformities are distinguished from the intensive, intra-formation discontinuities (e.g. Fig. 6) because (1) their trigger was external to the depositional system (e.g. sea-level change rather than autogenic recyling of sediment piles within a sedimentary environment) and (2) they diminish time-completeness regionally, whereas intensive unconformities remain most important in diminishing time-completeness within individual outcrops.
One marked difference between the interval between the Wroxham Crag and today (compared with the unconformity intervals bounding the Red Crag) is that sediments that post-date the Wroxham Crag Formation currently exist onshore, most notably the 0.4 Ma Anglian glacial deposits (Lee et al. 2015). However, a large fraction of the Crag Group outcrop belt has negligible or patchy cover from younger sediments, so a future rise in relative sea-level could theoretically transplant subtidal sandwave deposits immediately on top of similar facies of the ancient crag formations. The extensive erosional unconformity separating the ‘future crag’ from the ancient crag would, in many places, be indistinguishable in its character from the preceding unconformities that separate the formations of the Crag Group (although it would potentially be marked in places with spatially restricted alluvial and glacial ‘members’).
Implications for the preservation of time in older formations
The notion that crag deposition may be a work in progress is speculative but geologically rational, and has implications for time-preservation in older strata. The duration of crag deposition, whether finished or not, corresponds to the persistence or persistent reappearance of marine conditions in the southwestern North Sea region over the last c. 5 myr (Lee et al. 2018); an interval of time inferior to the duration of many stratigraphic formations in the geological record. Significantly, the sedimentological characteristics of the Red Crag Formation that show that its outcrops were deposited on human timescales are also common in much more ancient strata, deposited in similar sedimentary environments (Fig. 16). Lithified ancient strata may differ from the crag through forming much thicker successions (formed over longer intervals, in tectonic settings more prone to subsidence), and sometimes being exposed at vertical scales in which extensive unconformities are more apparent, but fundamentally they are composed of similar building blocks, with potentially similar spatial extent of outcrop and time significance, to the Red Crag Formation.
This understanding shifts how we understand the chronostratigraphic fidelity of ancient strata. Ager (1986) ended a paper concerning the time significance of a 10 m thick debris-flow deposit in Jurassic strata with the conclusion that ‘it all happened one Tuesday afternoon’. Notwithstanding that Ager's (1986) sentiment has been widely disputed (e.g. Fletcher et al. 1986; Sheppard 2006), in the Red Crag Formation it seems more likely that any individual outcrop all happened one ‘February going into March’, a subtle but critical difference revealing strata not as dramatic events, but as sediment piles deposited both quickly and unexceptionally. This removes a level of perceived incompleteness from the ancient record and suggests that any given outcrop is most probably representative of normal conditions, particularly when similar facies signatures are replicated in multiple discrete outcrops of the same unit.
This ‘bias towards the boring’ in preserved strata means that we can trust the fidelity of the signatures within such outcrops more than is commonly perceived. The intensive properties of even Precambrian strata at outcrop should be just as time-complete as the Red Crag Formation at outcrop: any outcrop can still be a monthly or sub-annual record that is unanchored, within a given time frame (Fig. 17). The implication of this is that, if we bracket Earth's sedimentary record by geological period, the first appearance (followed by persistence thereafter) of sedimentary or ichnological (or, in some instance, palaeontological) features at a number of worldwide outcrops probably reflects genuine evolutionary origin, and cannot be dismissed as being unreliable owing to the ‘incompleteness of the sedimentary record’.
Observing the Red Crag Formation from a temporal perspective supports recent understanding of strata and time and attests that stasis was the dominant sedimentation state, ordinariness is the dominant signature and Crag-time is smeared laterally across exposed and unexposed parts of the Red Crag outcrop belt.
Outcrops of the Red Crag Formation are discrete pockets of sediment that was deposited on monthly to subannual timescales, related to the tidal rhythms of its depositional environment. The duration of outcrop accrual can be estimated with sedimentological and ichnological proxies for rates of deposition, erosion and sedimentary stasis. These reveal that miniscule fractions of elapsed geological time can be seen at outcrop (relative to the 600–800 kyr duration of deposition of the formation), but this is only because outcrops provide windows onto miniscule areas of space (relative to the area of original deposition). We suggest that outcrops of similar sedimentary strata of any age are directly comparable, recording mundane, sub-annual-timescale, sedimentation. In other words, although the rock record is unavoidably incomplete, when it is viewed at outcrop and considered from a time perspective it can be seen to be intricately detailed, surprisingly high-resolution, and, in many instances, there may be no reason to doubt its veracity as a historical chronicle.
This paper was improved by useful reviews from A. Miall and L. Herringshaw, and comments from editor A. Hartley.
A.P.S. was supported by the Natural Environment Research Council (grant number NE/L002507/1).
NSD: Conceptualization (Lead), Data curation (Lead), Formal analysis (Lead), Investigation (Equal), Methodology (Lead), Project administration (Lead), Supervision (Lead), Validation (Lead), Visualization (Lead), Writing – Original Draft (Lead), Writing – Review & Editing (Equal); APS: Conceptualization (Supporting), Formal analysis (Supporting), Investigation (Equal), Visualization (Supporting), Writing – Original Draft (Supporting), Writing – Review & Editing (Supporting); WJM: Conceptualization (Supporting), Formal analysis (Supporting), Investigation (Equal), Methodology (Supporting), Validation (Supporting), Visualization (Supporting), Writing – Original Draft (Supporting), Writing – Review & Editing (Supporting)
Scientific editing by Adrian Hartley