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 has 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

True substrates are defined as sedimentary bedding planes that demonstrably existed at the sediment-water or sediment-air interface at the time of deposition, as evidenced by features such as ripple marks or trace fossils. Here we describe true substrates from the Silurian Tumblagooda Sandstone of Western Australia, which have been identified by the presence of the surficial trace fossil Psammich nites. The examples are unexpected because they have developed along erosional internal bounding surfaces within a succession of cross-bedded sandstones. However, their seemingly counterintuitive preservation can be explained with reference to recent advances in our understanding of the time-incomplete sedimentary-stratigraphic record (SSR). The preservation of true substrates seems to be an inevitable and ordinary result of deposition in environments where sedimentary stasis and spatial variability play important roles. We show that the true substrates developed during high-frequency allogenic disturbance of migrating bedforms, forcing a redistribution of the loci of sedimentation within an estuarine setting, and subsequently permitting an interval of sedimentary stasis during which the erosional bounding surfaces could be colonized. These observations provide physical evidence that supports recent contentions of how sedimentary stasis and the interplay of allogenic and autogenic processes impart a traditionally underestimated complexity to the chronostratigraphic record of geological outcrop. CHRONOSTRATIGRAPHY AND TRUE SUBSTRATES It has been recognized for over a century that unconformities and sedimentary breaks riddle Earth's stratigraphic record at a variety of scales, such that two-dimensional (2-D) stratigraphic sections are fragmentary chronicles of elapsed geological time (Barrell, 1917; Sadler, 1981; Dott, 1983). Recently, a number of largely model-driven studies have explored the previously underappreciated causes and effects of this time-deficient SSR (Miall, 2015; Paola et al., 2018). Three recurring themes are: (1) Ordinariness: The SSR preferentially records mundane rather than dramatic events (Jerolmack and Paola, 2010; Paola, 2016). (2) Sedimentary stasis: The dominant sedimentation state under which the SSR accumulated was stasis; i.e., 'neither deposition nor erosion', rather than 'either deposition or erosion' (Ganti et al., 2011; Tipper, 2015; Straub and Foreman, 2018). (3) Spatial variation: Any time gaps in one 2-D stratigraphic section of a basin fill were likely compensated by contemporaneous deposition of strata elsewhere within the same basin (

The landscapes and seascapes of Earth's surface provide the theatre for life, but to what 12 extent did the actors build the stage? The role of life in the long-term shaping of the 13 planetary surface needs to be understood to ascertain whether Earth is singular among known 14 rocky planets, and to frame predictions of future changes to the biosphere. Modern 15 geomorphic observations and modelling have made strides in this respect, but an under-16 utilized lens through which to interrogate these questions resides in the most complete 17 tangible record of our planetary history: the sedimentary-stratigraphic record (SSR). The 18 characteristics of the SSR have been frequently explained with reference to changes in 19 boundary conditions such as relative sea level, climate, and tectonics. Yet despite the fact that 20 the long-term accrual of the SSR was contemporaneous with the evolution of almost all 21 domains of life on Earth, causal explanations related to biological activity have often been 22 overlooked, particularly within siliciclastic strata. This paper explores evidence for the ways 23 in which organisms have influenced the SSR throughout Earth history and emphasizes that 24 further investigation can help lead us towards a mechanistic understanding of how the 2 25 planetary surface has co-evolved with life. The practicality of discerning life signatures in 26 the SSR is discussed by: 1) distinguishing biologically-dependent versus biologically-27 influenced sedimentary signatures; 2) emphasizing the importance of determining relative 28 time-length scales of processes and demonstrating how different focal lengths of observation 29 (individual geological outcrops and the complete SSR) can reveal different insights; and 3) 30 promoting an awareness of issues of equifinality and underdetermination that may hinder the 31 recognition of life signatures. Multiple instances of life signatures and their historic range 32 within the SSR are reviewed, with examples covering siliciclastic, biogenic and chemogenic 33 strata, and trigger organisms from across the spectrum of Earth's extant and ancient life. 34 With this novel perspective, the SSR is recognised as a dynamic archive that expands and 35 complements the fossil and geochemical records that it hosts, rather than simply being a 36 passive repository for them. The SSR is shown to be both the record and the result of long-37 term evolutionary synchrony between life and planetary surface processes. 38 42 precipitation from solution. Where it is tractable, in exposed outcrop, cores or seismic 43 sections, the SSR has immense value as a record of ancient surface processes. It is the 44 primary repository of deep time geochemical and fossil evidence, and the only tangible 45

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Rock outcrops of the sedimentary-stratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sediment-water or sediment-air interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentary-stratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these short-term, transient, small-scale features are counter-intuitively abundant within a 3.8 billion year-long sedimentary-stratigraphic record that is known to be inherently time-incomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the time-length scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional by-products of the accrual of the sedimentary-stratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentary-stratigraphic record. They should be recognized as providing high-definition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentary-stratigraphic record is unrepresentative of the geological past.