Assessing the expansion of the Cambrian Agronomic Revolution into fan-delta environments

The intensity, extent, and ecosystem-level impact of bioturbation (i.e. Agronomic Revolution) at the dawn of the Phanerozoic is a hotly debated issue. Middle Cambrian fan-delta deposits in southwestern Saskatchewan provide insights into the paleoenvironmental extent of the Agronomic Revolution into marginal-marine environments. The studied deposits reveal that several environmental stressors had direct impact on trace-fossil distribution and bioturbation intensities in Cambrian fan deltas. Basal and proximal subaerial deposits are characterized by very coarse grain size and absence of bioturbation. Mid-fan and fan-toe deposits were formed under subaqueous conditions and are characterized by rapid bioturbation events in between sedimentation episodes when environmental stressors were ameliorated, providing evidence of a significant landward expansion of the Agronomic Revolution. Transgressive marine deposits accumulated after the abandonment of the fan-delta system display high levels of bioturbation intensity, reflecting stable environmental conditions that favored endobenthic colonization. The presence of intense bioturbation in both subaqueous fan delta and transgressive deposits provides further support to the view that Cambrian levels of biogenic mixing were high, provided that stable environmental conditions were reached. Our study underscores the importance of evaluating sedimentary facies changes to assess the impact of environmental factors prior to making evolutionary inferences.


Results
These deposits were divided into fifteen facies and grouped into two broad facies associations: fan-delta and open-bay deposits (SI Appendix, Tables S2 and S3). Fan-delta deposits were characterized following conventional terminology 25,26 . Subdivisions used for open-bay deposits follow previously proposed schemes for this environment 33,42 . Facies association 1 (FA1): proximal to distal fan-delta deposits. Description. This facies association is dominated by very coarse-to medium-grained sandstone and granule to boulder conglomerate. It constitutes the Basal Sandstone Unit at the flanks of Precambrian highs. This facies association forms 3-22 m thick upward-fining intervals and tends to be abruptly overlain by FA2. FA1 is composed of facies Cgb, S-Cg, Sg, Sg-bio, Sd, and locally facies Sh (Fig. 2). Granule to boulder conglomerate (Cgb) (Fig. 2a), generally ungraded, is commonly found at the base of the succession, forming dm-to m-thick beds, and resting on top of metamorphic and igneous Precambrian rocks. Basal conglomerate beds are gradationally overlain by interbedded sandstone and pebble conglomerate (S-Cg) ( Fig. 2b) with rare mudstone drapes. Locally, basal conglomerate beds are sharply overlain by massive gravelly sandstone (Sg) (Fig. 2d). Bioturbated sandstone (Sg-bio) (Fig. 2e) and argillaceous sandstone (Sd) (Fig. 2c,f) are particularly common in the upper intervals of FA1. Parallel-laminated shale (Fig. 2g) is locally observed forming intervals up to one meter thick.
Interpretation. Ungraded boulder conglomerate represents individual erosive and non-cohesive debris-flow deposits accumulated at the top of a fan delta with prevalent subaerial conditions. Sg represents a transition between the subaerial and subaqueous sections of the fan delta, evidenced by rare vertical burrows (Diplocraterion isp.) at bed tops. Facies S-Cg and Sg-bio are genetically related and represent the mid-fan delta. These facies were deposited in subaqueous conditions with sediments probably being reworked by weak tidal currents as suggested by the pervasive presence of mudstone drapes forming flaser and wavy bedding. Facies Sd records deposition at the toe of the fan and its finer-grained sandstone evidences waning flow conditions. Facies SH records suspension-fallout deposition in low-energy sheltered areas, probably formed between fan wedges. The low ichnodiversity and sparse bioturbation in FA1 indicate stressful conditions for benthic colonization. In particular, the lack of bioturbation in basal conglomerate and gravelly sandstone probably is due to a combination of subaerial conditions, high energy, and sedimentation rate. In turn, absence of bioturbation in local parallel-laminated mudstone intervals suggests stressful brackish-water conditions in a low-energy, protected area. The presence of linguliform brachiopods in these deposits is also consistent with brackish-water conditions. Deposit-feeding structures (Teichichnus rectus, Planolites isp.) dominate the assemblage, suggesting that organic particles were more abundant in the sediment. Horizontal structures of deposit feeders are common in mudstone-draped intervals, recording higher burrowing activity during periods of low energy and suspension fallout. Although rare, the local presence of dwelling traces of suspension-feeders (Palaeophycus isp., Diplocraterion isp.) suggests limited feeding from suspended particles. Passive predation is indicated by Bergaueria isp., typically attributed to sea anemones 43 .

Discussion
Cored intervals sampled at the flanks of Precambrian highs reveal a succession that at its base is dominated by deposits representing the subaerial portion of fan deltas. No deposition is envisaged in the study area during forced regression (i.e. falling stage systems tract). The first stage of sedimentation is represented by lowstand systems tract deposits (SI Appendix, Figs. S4, S5 and S6A). Initial base-level rise over the craton in southwestern Saskatchewan resulted in the creation of accommodation space, rapidly filled by prograding to aggrading fan deltas during normal regression. This lowstand stage is recorded in the succession by stacked conglomeratic beds of the inner fan, forming bedsets up to 16 m thick. www.nature.com/scientificreports/ Subsequent transgression led to landward migration of the fan-delta system and its final abandonment once completely flooded during the middle Cambrian. With continuous transgression, the rate of accommodation eventually exceeded the rate of sedimentation causing the shift of the shoreline in a landward direction; this is reflected in the gradual upwards deepening of fan-delta facies. The onset of the transgressive systems tract is represented by sandy, bioturbated, subaqueous mid and toe fan deposits gradually overlying unbioturbated subaerial proximal fan deposits, forming an overall fining-upward succession (SI Appendix, Figs. S4 and S5). Sustained sea-level rise led to the establishment of an embayment depositional setting, and proximal to distal bay deposits accumulated (SI Appendix, Fig. S6B). As transgression continued and elevated areas were flooded, the rate of sediment supply was greatly diminished, and both storm and fair-weather waves reworked the outer delta deposits, forming sandy proximal embayment complexes. Wave action is revealed by the presence of symmetrical and combined-flow ripples. A decrease in the salinity stress and a transition to near-normal marine conditions are evidenced by more continuous bioturbation, abundant body fossils, and the formation of glauconite in areas of low energy and low sedimentation rates. Once formed, glauconite grains were transported and accumulated by storm and wave currents producing a distinctive green facies 48 . Glauconitic sandstone commonly lies sharply on top of outer fan-delta facies and overlain in turn by distal bay to shelf facies. Locally, transgressive lag deposits separate proximal bay from fan-delta facies, indicating pronounced transgressive erosion. As the transgression continued, the shoreline migrated landwards, and a deepening of facies occurred. Well cores in a more distal position (101/11-23-005-26W3/00) from the source area show shelf mudstone erosively overlying proximal fan-delta facies. In these areas, mid and toe fan facies were completely removed due to wave ravinement as the sea transgressed. Thick mudstone accumulated in the shelf area, below the storm wave base (SI Appendix, Fig. S6B,C). Warm and relatively shallow-water conditions were favorable for carbonate precipitation, as indicated by the presence of limestone and dolostone.
Fan-delta deposits in southwestern Saskatchewan are gravelly and characterized by thick conglomerate containing boulder-size clasts, with no apparent grading, that transition downflow into conglomeratic sandstone and mudstone-draped sandstone (SI Appendix, Fig. S3). These deposits are associated with steep and tectonically active scarps, and commonly developed a conical-to wedge-shape geometry 25 . Tidal amplification could have been promoted by the development of irregular and protected semi-enclosed areas at the front of and between fan wedges (SI Appendix, Fig. S6A). In subsurface Saskatchewan, trace fossils were key to differentiate between the subaerial and subaqueous segments of fan deltas. The Cambrian interval records the initial forays of benthic organisms into shallow-and marginal-marine settings. Colonization of continental settings by animals did not occur until Silurian times, therefore bioturbation in fluvial and subaerial settings was most likely excluded 23,[49][50][51] . In contrast, subaqueous fan-delta deposits in southwestern Saskatchewan are characterized by the presence of bioturbation structures, recording short-term to even more continuous colonization windows for the burrowing endobenthos (Fig. 4).
Cambrian alluvial plains were essentially devoid of plant cover and typified by poorly constrained, low-sinuosity sheet-braided fluvial systems 52 . The absence of vegetation cover was detrimental for fine-grained sediment www.nature.com/scientificreports/ retention and alluvial plains were mud-deficient 53 . This configuration of the alluvial landscape promoted episodic sediment delivery to coastal and shelfal areas which was mostly controlled by discharge variations in fluvial catchment areas 52 . Cambrian fan-delta strata in Saskatchewan present some common points with pre-vegetation settings described in previous works [54][55][56][57][58] , such as very thick (1-3 m) sand-to gravel-rich deposits, overall low mud content, and abundant sandy bedforms. Subaqueous mid-fan coarse-grained deposits are characterized by low to absent bioturbation, reflecting stressful conditions resulting from a combination of high-energy, high sedimentation rates, and possible salinity dilution during episodes of increased fluvial discharge. It was only during brief periods of quiescence that endobenthic activity took place, evidenced by the presence of sparse trace fossils associated with mudstone drapes, reflecting short-term colonization windows. In contrast to the more stressful conditions of the mid fan delta, argillaceous and tide-influenced outer fan facies display an increase in bioturbation intensity, indicating a more continuous colonization window. Despite an increase in the intensity of bioturbation in fan-toe deposits, ichnodiversity remained relatively low, suggesting that reduced salinity was a significant stressor. Transgressive glauconitic deposits overlying fan-delta strata show higher levels of endobenthic activity, higher ichnodiversity, and common marine body fossils, reflecting an increase in salinity and less stressful conditions (Fig. 5).
The studied deposits allow assessing the paleoenvironmental extent of the Agronomic Revolution in marginalmarine settings. In particular, the impact of bioturbation during the early Paleozoic has been a topic of debate with contrasting views regarding the intensity of biogenic reworking in Cambrian deposits (see discussion in 4 ). The implications of our study are four-fold. First, integration of ichnologic and sedimentologic datasets within a stratigraphic framework suggests that bioturbation intensity reached high levels under stable environmental conditions. This is illustrated by the fact that transgressive deposits representing accumulation in open-bay and shelf environments, formed under near-normal marine salinity conditions, low energy, and well-oxygenated conditions, tend to be intensely bioturbated. Second, even short-term periods of amelioration in environmental stressors allowed for moderate intensities of biogenic reworking to occur. These rapid bioturbation events are clearly displayed in the fan-delta toe deposits, recording endobenthic colonization during times in between sedimentation episodes. These two points taken together support the view that Cambrian levels of bioturbation intensity were high provided stable environmental conditions were met. Third, the occurrence of trace fossils in both mid fan delta and fan-delta toe deposits provides evidence of significant landward expansion of the Agronomic Revolution. This is consistent with several studies that have documented trace fossils in a wide variety of Cambrian marginal-marine environments, such as bays, deltas, and estuaries 23,24,56,[59][60][61][62][63][64] . We note that fan deltas, given their coarse grain size, rapid deposition, and very high hydrodynamic energy, may be regarded as representing an end member in terms of stressor intensities if compared with other marginal-marine environments, such as interdistributary bays or estuarine basins. Fourth, our study underscores the importance of carefully www.nature.com/scientificreports/ evaluating subtle changes in the nature of sedimentary facies (e.g. heterolithic deposits) in order to assess the impact of environmental factor prior to making inferences in evolutionary terms. Many studies of fan deltas focus on the sedimentary facies, depositional processes, and geometry of such systems, but only a few integrate sedimentologic and ichnologic observations 32,34,65,66 . Notably, most of these studies deal with post-Paleozoic fan deltas, being the study by 66 on a Pennsylvanian system a notable exception. Expanding the fan-delta ichnology dataset would be essential to track secular changes in style of bioturbation in these environments through geologic time. Ideally, this would require documentation of the trace-fossil content of the various fan-delta subenvironments and a large dataset including systems of different ages and geologic contexts.

Materials and methods
This study is based on the analysis of seven continuous well cores (SI Appendix, Table S1) drilled at the flanks of basement highs in southwestern Saskatchewan. Cored intervals were logged in detail and the deposits were divided into sedimentary facies based on lithology, bed contacts, bed thickness, color, grain size, mineralogy, physical sedimentary structures, and trace-fossil content and distribution. The facies were grouped into two facies associations.
Classification of sandstone was based on generally accepted schemes 67 . The classic Udden-Wentworth scale 68 was used for classifying grain-size and visual estimations of roundness and sorting followed current practices 69 . Bed thickness was classified after previous terminology 70 . Glauconite content was visually estimated by using percent abundance charts.
Ichnologic data comprise ichnotaxon identification to ichnospecies where possible, trophic types, degree of bioturbation, and cross-cutting relationships. Degree of bioturbation was estimated using the system proposed by Taylor and Goldring 71 , and a bioturbation index grade (BI), ranging from 0 (undisturbed bed) to 6 (complete bioturbation and sediment reworking) was provided.

Data availability
The data that support the findings of this study are available from the corresponding author, A.I., upon reasonable request. www.nature.com/scientificreports/