Introduction

Chalks have been differently defined in the literature1,2. The lithologic (diagenetic) definition considers chalk those firm, partly indurated oozes (calcareous nannofossils-rich sediments) that are friable1,3,4 and readily deformed by a fingernail or knife-blade1,3. The compositional definition, in contrast, characterizes chalks as carbonate deposits (of variable depositional texture, mudstone to packstone sensu Dunham5 classification6,7,8, when lithified), which are largely composed of calcareous nannofossils (mostly coccoliths) and micrite, with wide-ranging subordinate amounts of calcispheres, foraminifera, mollusks, echinoderms, and radiolarians, among other calcareous and siliceous components, irrespective of the induration2,6,7,8,9. Though most chalks have a dominant coccolith fraction, some chalks may exhibit dominant portions of micrite (or “micarbs”10) and planktonic foraminifera, with coccolith concentrations even below 20%11,12. For the purposes of this paper and acknowledging that (1) calcareous nannofossil-rich deposits undergo progressive lithification as a function of depth of burial and other factors, thus, any clear-cut distinction between soft, intermediate, and hard deposits is somewhat arbitrary9; and (2) the mixture of compositional and diagenetic terms in the lithological definition may lead to unnecessary complications and confusion2, the compositional definition of chalk is adopted.

Coccoliths are micron-sized calcitic plates secreted by unicellular phytoplankton, called coccolithophores13. Coccolithophores emerged during the Late Triassic and radiated during the Early Jurassic14. They originated in epeiric seas but during the Late Jurassic/Early Cretaceous migrated to deeper open-oceanic settings, where they became the dominant rock-forming carbonate particles until the present15,16. Nevertheless, under the warm/hot climate and high sea level that typified the Late Cretaceous17,18, massive chalk flourished in relatively shallow-water settings19,20, such as epeiric basins and continental shelves21, including marginal lagoonal environments22. This widespread chalk sedimentation was likely a response to a combination of factors, including (1) terrigenous sediment-starvation linked to high sea levels and/or arid climates23, (2) enlargement of areas of shelf sedimentation due to rising sea levels2,9, and (3) warm seawaters featuring low Mg/Ca ratios and high Ca concentrations that promoted coccolithophore production24,25.

Ichnology, the study of organism-substrate interactions, has proved to be highly successful in refining paleoenvironmental interpretations, although not all depositional environments have received the same degree of attention26,27. In particular, numerous ichnologic studies have focused on chalk19,20,21,28,29, and the ichnologic content of shelf-sea and deep-sea chalks has been compared19,30. However, no prior studies have been dedicated to the ichnology of chalks deposited under lagoonal conditions (i.e., in partially to almost fully enclosed shallow shelf settings).

The west-central Texas Buda Formation (commonly referred to as the Buda Limestone; Supplementary Fig. S1) comprises a lower Cenomanian chalk31,32,33 deposited on the broad Cretaceous Comanche Shelf of the Texas Gulf Coast Basin31,32,33,34. This chalk is characterized by a matrix which is visibly dominated by coccoliths debris, partly cemented coccoliths, and well-preserved coccoliths (see Supplementary Fig. S2). The calcite cement, where abundant, may substantially obscure the coccolith particles in this unit (Supplementary Fig. S2), which is a common phenomenon during the diagenetic evolution of some chalks35,36. The matrix of the Buda Formation can be classified either as “Microtexture 3” or as “Dispersed Clay Microtexture” sensu the “Pure” or “Impure” chalk microtexture classification of Saïag et al.8, depending on the CaCO3 content (higher or less than 96 wt.%, respectively). Valencia et al.32,33 reported an average CaCO3 content of 85 wt.% for the west-central Texas Buda Formation, with values ranging from 60 to 99 wt.%. As a comparison, lower Cenomanian chalk units from the Paris Basin has an average CaCO3 content of 75 wt.%, with values ranging from 52 to 95 wt.%8.

The Buda Formation was deposited under varying semi-restricted, slightly restricted to nearly open lagoonal conditions. The latter reflects a higher (but still partial) degree of connection with the open sea, largely controlled by the interplay of a wide and low-gradient paleotopography, together with topographic highs (e.g., Stuart City/Sligo paleo-reefal complex, Terrel Arch, San Marcos Arch) and fluctuating relative sea levels, under possible eustatic control32,33. Evidence includes (1) the thinning or pinching out of the Buda Formation in the Lower Cretaceous Stuart City/Sligo paleo-reefal complex37; (2) the broad and very low-gradient of the Buda Formation paleo-shelf38,39; (3) the endemic nature of the Buda Limestone ammonites (Budaiceras and Faraudiella40); (4) the occurrence of rapid salinity- and oxygen-fluctuating facies, e.g., the brackish-water Cribatina texana-bearing facies (WF1) rapidly transitioning to a normal-marine facies (WF232); (5) the highly-variable macrobenthos diversity and bioturbation intensity32,33,34; (6) the occurrence of a benthic fauna dominated by mollusks (largely oysters), echinoderms, calcispheres, and green-algae (dasycladales and bryopsidales)32,33,34; (7) a foraminiferal fraction dominated by benthonic species, with subordinated unkeeled planktonic communities, such as heterohelicids, favusellids and hedbergellids32,33; and (8) the occurrence of proximal tempestites32, low-angle cross-bedding and oolitic facies33; among other features32,33,34,39. This line of evidence not only testifies its lagoonal character and shallow-water nature but reflects a different extent of interconnection with the open ocean32,33.

The Buda Formation represents a unique opportunity to understand how ancient endobenthic communities responded to changing environmental parameters inherent to lagoonal circulation. Therefore, the aims of this study are to (1) document the trace-fossil content of the Buda Formation chalk, (2) compare the chalk ichnofauna of this unit with the trace-fossil content of chalk from shelf- and deep-sea settings, and (3) provide a characterization of lagoonal chalk trace-fossil assemblages that allows differentiating this more restricted setting from those present in chalk formed in the open sea. The underlying broad objective of this research is to expand the applications of ichnology to depositional environments that remain underexplored. Additionally, the outcome of this study illustrates the importance of integrating ichnologic and sedimentologic datasets in elucidating physicochemical parameters during deposition of ancient chalks in particular and carbonates in general.

Results

Sedimentologic characteristics

The west-central Texas Buda Formation consists of variably bioturbated chalky (coccolith-rich) mudstone to packstone that comprises nine main sedimentary facies (see Supplementary Table S1) interpreted to have been deposited in seven subenvironments, within a generalized lagoonal shelf setting32,33. These subenvironments, listed from lower to a higher grade of connection with the open sea, include (1) a shallow-subtidal, high-energy, brackish-water, semi-restricted lagoon; (2) a shallow-subtidal, low- to moderate-energy, oxygen-deficient (dysoxic), normal marine, semi-restricted lagoon; (3) a shallow-subtidal, moderate-energy, normal marine, slightly-restricted lagoon; (4) a shallow-subtidal, low-energy, normal marine, slightly-restricted lagoon; (5) a deep-subtidal, low-energy, normal marine, slightly-restricted lagoon, with episodic storm deposition; (6) a shallow-subtidal, moderate- to high-energy, normal marine, nearly open lagoon; and (7) a deep-subtidal, low energy, normal marine, nearly open lagoon32,33.

Trace-fossil content

Chalk facies in the Buda Formation contain a total of twenty ichnotaxa (Figs. 1, 2, 3, 4). These include seventeen bioturbation structures, namely Asterosoma isp. (Fig. 4b), Bergaueria isp. (Fig. 2h), Bichordites isp. (Fig. 2c), Chondrites isp. (Figs. 2b, d, 3c, 4b), ?Conichnus isp. (Fig. 3f), Gyrolithes isp. (Fig. 2i), Lockeia siliquaria (Fig. 2g), Palaeophycus isp. (Fig. 2e), Planolites isp. (Figs. 2e, 3c), Protovirgularia isp. (Fig. 2g), Rhizocorallium isp., Rosselia socialis, ?Sinusichnus isp. (Fig. 4e), Taenidium isp. (Fig. 2d), Teichichnus rectus (Fig. 2f), Thalassinoides isp. (Figs. 2a, i, 3a–g, 4c), and an unknown vertical trace fossil (Fig. 4c). In addition, three bioerosion structures, namely Gastrochaenolites isp. (Fig. 4d), Trypanites isp. (Fig. 2g) and indeterminate microborings (Fig. 4f), have been identified. Overall, the assemblage is dominated by Thalassinoides isp. and Chondrites isp., which are pervasive in most of the bioturbated deposits (Fig. 1, Supplementary Figs. S3–S5). In terms of subenvironments, the identified ichnotaxa are differentially distributed within the above-mentioned seven main lagoonal depositional settings, as detailed in the following sections.

Figure 1
figure 1

Composite stratigraphic column of the Buda Formation in the Dryden-Comstock transect (west Texas), based on the DR1, DR16, DR17, DRD-B, and DR33-B outcrop sections, showing sedimentary facies, ichnotaxa and ichnofacies distribution, as well as their respective Bioturbation Index (BI) sensu Taylor and Goldring154 and percentual bioeroded area. Logs drawn by Fernando L. Valencia using Adobe Illustrator 2022 software version 26.2.

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Figure 2
figure 2

Dominant ichnotaxa identified in chalk from the Buda Formation: (a) basal view of Thalassinoides isp. [Th], A-1 locality; (b) cross-sectional view of an interval with pervasive Chondrites isp. [Ch], “Shanklin, W.R.2” core; (c) cross-sectional view of abundant Bichordites isp. (Bi) and its preservational variant “Laminites” [La], DR33-B locality; (d) cross-sectional view of polished slab showing Taenidium isp. cross-cut by ?Chondrites isp., DR1 locality; (e) cross-sectional view of Palaeophycus isp. [Pa] and Planolites isp. [Pl], “Shanklin, W.R.2” core; (f) cross-sectional view of Teichichnus rectus [Te], DR-33B locality; (g) basal view of deposits showing partially preserved Protovirgularia isp. [Pr] and adjacent Lockeia siliquaria [Lo] cross-cut by Trypanites isp. [Try], SA-1 locality; (h) cross-sectional view of Bergaueria isp. [Be], A-1 locality; (i) cross-sectional view of a passively filled (firmground) Thalassinoides isp., showing intergradation with Gyrolithes isp. [Gy] at the bottom, DR16 locality.

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Figure 3
figure 3

Representative photographs of trace fossils from the main lagoonal deposits: (a) and (b) show basal views of sparse Thalassinoides isp. [Th] in brackish-water lagoonal facies (WF1; BI = 2), DR16 locality; (c) cross-sectional core view showing the transition (base to top) from an oxygen-depleted subfacies (CF2b) to a more oxygenated deposit (CF2c) of a semi-restricted lagoonal setting; see the upward-increase in both bioturbation degree and the size of the deep-tier Chondrites isp. [Ch], “Shanklin, W.R.2” core; (d) cross-sectional view of highly bioturbated facies (WF2) in slightly restricted lagoonal subenvironment (BI = 4–5), DR16 locality; (e) basal view of moderately bioturbated deposits showing poorly preserved Thalassinoides isp. in a slightly restricted lagoonal facies (WF4b, BI = 4), DR33-B locality; (f) and (g) extremely bioturbated (BI = 6), nearly open lagoonal facies (CF4b) displaying by three-dimensional networks assigned to Thalassinoides isp., accompanied by minor ?Conichnus isp. [Co], A-1 locality.

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Figure 4
figure 4

Additional ichnotaxa from the Buda Formation: (a) cross-sectional view of Rosselia isp. [Ro] in WF4c, DR1 locality; (b) cross-sectional view of proximal tempestites (WF5) showing Asterosoma isp. [As] cross-cut by deeper-tier Chondrites isp. [Ch], DR16 locality; (c) cross-sectional view of a proximal tempestite (WF5) showing an unknown vertical trace [Uv] near the base of the erosive deposit (WF5a), followed upwards by a hummocky-cross-bedded and more bioturbated sub-facies at the topmost (preserved) tempestite (WF5d); see Thalassinoides isp. [Th] cross-cut by Chondrites isp. [Ch] at this upper interval, DR16 locality; (d) cross-sectional view of locally abundant Gastrochaenolites isp. [Ga] in WF1, DR16 locality; (e) top view of a passively-filled (firmground) ?Sinusichnus isp. in WF2, DR16 locality; (f) SEM picture of clay-filled microboring [Mi] cross-cutting the recrystallized chalk matrix and a calcite-cemented bioclast.

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Softground trace-fossil assemblages in semi-restricted lagoonal settings (Figs. 1, 2a,b,e, 3a–c, 4d and Supplementary Figs. S3–S5)

The semi-restricted lagoonal subenvironment is represented by brackish-water packstone (WF1) and oxygen-deficient (dysoxic) mudstone to packstone (CF1a-b and CF2a-c)32,33. In general, these facies are the least bioturbated and show the softground trace-fossil assemblages with the lowest ichnodiversity (richness in ichnotaxa41,42) and ichnodisparity (variability of morphological plans in the biogenic structures41,42) of the studied strata. The oyster-rich packstone (WF1), representative of the brackish-water lagoonal setting, is characterized by low to moderate bioturbation degree (BI = 2–3), comprising monospecific suites of decapod burrows43,44 represented by Thalassinoides isp., which is interpreted as a mid-tier burrow cross-cut by a subsequent and deeper-tier generation of Thalassinoides isp. (Fig. 3a,b). Likewise, the calcisphere-rich wackestone–packstone (CF1a–b) and wispy-laminated mudstone–wackestone (CF2a–c), characteristic of the dysoxic lagoonal setting, display impoverished but slightly more diverse suites, with highly variable bioturbation intensity (BI = 1–4). Sparse burrowing activity (BI = 1–2) is recorded in the pyrite- and glauconite-richer subfacies (CF2b) linked to the more oxygen-depleted periods33 (Fig. 3c). The trace-fossil suites occurring in these deposits are largely composed of deep-tier Chondrites isp. of variable sizes (Figs. 2b, 3c), made by either lucinid bivalves or vermiform animals45, which are cross-cutting mid-tier common to rare Thalassinoides isp. and subordinated occurrences of shallow-tier worm-like trace fossils46 assigned to Planolites isp. (Figs. 2e, 3c) and Palaeophycus isp. (Fig. 2e). In addition, rare Rhizocorallium isp., likely produced by crustaceans47, is present locally.

Softground trace-fossil assemblages in slightly restricted lagoonal settings (Figs. 1, 3d,e, 4b,c)

The slightly restricted lagoonal setting comprises both the clay-/quartz-richer mudstone to wackestone facies from the low-energy, shallow- to deep-subtidal setting (WF3, WF4b) and the coccolith-richer wackestone to packstone from the more energetic shallow-subtidal subenvironment (WF2)32,33. These deposits are characterized by a more intense bioturbation degree and by trace-fossil suites with an increase in ichnodiversity in comparison with the more enclosed subenvironments (semi-restricted lagoon). The trace-fossil content in slightly restricted settings is largely limited to Thalassinoides isp. (Fig. 3d,e), Chondrites isp., Planolites isp., Palaeophycus isp., Taenidium isp. (typically attributed to worm-like animals48), and Bichordites isp. (including its preservational variant49,50 “Laminites”, attributed to sea urchins51). The bioturbation intensity varies from BI = 2–4, with the lower burrowing activity recorded in the terrigenous sediment-richer facies. Proximal tempestites (WF5a–d) formed in the deep-subtidal, slightly restricted lagoonal subenvironment show reduced bioturbation intensity (BI = 1–3) and ichnodiversity when compared with the background sedimentary facies (WF4b). These event beds are also characterized by the presence of worm-like trace fossils52,53 assigned to Asterosoma isp. (Fig. 4b) and Teichichnus rectus, as well as by Bichordites isp. and an unknown vertical trace fossil (Fig. 4c).

Softground trace-fossil assemblages in nearly open lagoonal settings (Figs. 1, 2d–f, 2h,i, 3f,g and Supplementary Figs. S3–S5)

The nearly open lagoonal deposits comprise the shallow-subtidal, moderate- to high-energy, wackestone to packstone facies of central Texas (CF3, CF4a–c), and the deep-subtidal, low-energy, mudstone to wackestone from western Texas (WF4a, WF4c). These more ocean-flushed chalk deposits of the Buda Formation are characterized by the highest bioturbation degree, as well as by the highest ichnodiversity and ichnodisparity in the softground trace-fossil suites. Stenohaline fauna-rich wackestone–packstone facies, representative of the shallow-subtidal, moderate- to high-energy, nearly open marine lagoonal setting (CF3, CF4a–c), are characterized by pervasive bioturbation (BI = 4–6), high ichnodiversity, and high ichnodisparity. Burrows include pervasive three-dimensional boxwork-like networks assigned to mid-tier Thalassinoides isp. (Fig. 3f,g), intergrading with rare spiral vertical burrows (Gyrolithes isp.), cross-cutting shallow-tier Planolites isp., and rare bivalve trace fossils54 attributed to Lockeia siliquaria and Protovirgularia isp. (Fig. 2g). These networks are overprinted by rare mid-tier burrowing anemone structures55 included in Bergaueria isp. (Fig. 2h) and ?Conichnus isp. (Fig. 3f), or more commonly by abundant deep-tier Chondrites isp. Mudstone and skeletal wackestone, from similar nearly open marine lagoonal but deeper-subtidal and quieter-water settings (WF4a, WF4c), are also characterized by pervasive bioturbation (BI = 4–6) with increased ichnodiversity and ichnodisparity. These assemblages are typically composed of shallow-tier Planolites isp. and Palaeophycus isp., cross-cut by mid-tier Thalassinoides isp., which in turn, is overprinted by deeper mid-tier Taenidium isp. (Fig. 2d), as well as by subordinated Bichordites isp. and its preservational variant “Laminites” (Fig. 2c). Accessory mid-tier burrows include worm trace fossils52,53,56 assigned to Teichichnus rectus (Fig. 2f), Asterosoma isp., and Rosselia socialis (Fig. 4a). All these shallow- to mid-tier ichnotaxa are cross-cut by deep-tier ?Chondrites isp. (Fig. 2d).

Firmground and hardground trace-fossil assemblages (Figs. 1, 2g,i, 4d–f and Supplementary Figs. S3–S5)

In addition to the softground ichnotaxa, overprinting firmground and hardground trace-fossil suites occur in the Buda Formation. Firmground trace fossils are abundant and distributed in the different depositional subenvironments, as well as within major (e.g., sequence boundary, maximum flooding surface) and minor (e.g., flooding surface) stratigraphic surfaces. Firmground trace fossils include Thalassinoides isp. with rare Gyrolithes isp. terminations (Fig. 2i), and ?Sinusichnus isp. (Fig. 4e). The hardground trace fossils, on the other hand, consist of bivalve borings57 assigned to Gastrochaenolites isp. (Fig. 4d), the worm-produced boring58 Trypanites isp. (Fig. 2g), and an indeterminate microboring only visible under back-scattered electron microscopy analysis (Fig. 4f). These bioeroding structures are usually sparse (yet locally covering up to 40% of the rock fabric) and largely linked to major stratigraphic surfaces, such as sequence boundaries and maximum flooding surfaces. As in the case of the firmground burrows, the bioerosion trace fossils are not linked to a specific lagoonal subenvironment but are more commonly distributed through all the shallower-water chalk deposits from central Texas.

Discussion

Lagoons are shallow-neritic water bodies developed on platforms that are generally protected by wide shallow seas, by reef trends, by sand-shoal barriers, or by islands59,60. Several studies in lagoons and nearshore settings have shown that physicochemical stress factors, such as freshwater discharge, hydrodynamic energy, sedimentation rate, water turbidity, and bottom-water oxygenation, have a considerable impact on the benthos, therefore influencing trace-fossil composition61,62,63. Substantial freshwater discharge and the resultant brackish-water conditions have been widely acknowledged as major stressors in marginal-marine systems61,64,65. In the Buda Formation, the brackish-water chalk interval is characterized by a depauperate Cruziana Ichnofacies (Fig. 1), with extremely low ichnodiversity and ichnodisparity (monospecific suite of Thalassinoides isp.), and low to moderate bioturbation (BI = 2–3), in agreement with the basic tenets of the brackish-water trace-fossil model64,65. In fact, ichnodisparity and ichnodiversity increase parallel to an increase in the degree of connection with the open sea.

Overall, the characteristics of the Buda Formation trace-fossil assemblages indicate that endobenthic colonization of this chalk occurred during low-energy periods, as indicated by the generalized scarcity of vertical dwelling burrows and escape trace fossils, the abundance of horizontally oriented endobenthic structures, and the overall high bioturbation intensity66,67. Nevertheless, this was not the case for the proximal tempestites facies. Therein, (1) the dominant vertical components of the Thalassinoides isp. networks, (2) the occurrence of unknown vertical trace fossils, and (3) the relatively low bioturbation intensity of these event layers (BI = 1–3) compared to the slowly settled background deposits (BI = 2–5), likely reflect that, despite most endobenthic colonization occurred during a waning stage of the flow, energy was persistently high near the bed67. The rate of sediment accumulation is considered to have a major impact on the colonization window and the consequent bioturbation intensity29. Rapid sedimentation frequently reduces or inhibits bioturbation68, as in the case of the tempestites, whereas slow rates of sedimentation represent the ideal scenario for intense burrowing activity69. In stratigraphic sequences, sediment accumulation rates are controlled by the interplay between sediment supply and accommodation70. Unlike in siliciclastic sediments, sediment supply in carbonate systems may not only be controlled by changes in physical energy (e.g., storms, turbidity currents) but by the productivity of the carbonate factory71. In the Buda Formation, the carbonate factory was largely controlled by changing climate conditions, sea level, and variable water turbidity32. The more productive times (i.e., periods with increased sedimentation rate) were characterized by high sea levels, higher temperatures, and quieter and less turbid waters32. Interestingly, however, the most bioturbated facies are those associated with the most carbonate productive and less clay-diluted cycles (Lechuguilla and Love Station members32) (i.e., cycles characterized by increased sedimentation, thus somehow contradicting the influence of sedimentation rate on the colonization window). Nevertheless, the relatively reduced bioturbation intensity in the slowly settled and clay-richer depositional cycle (Red Light Member32) may not have been a response to changes in sedimentation rates but a consequence of other significant coeval operating stress factors, such as elevated water turbidity, which is known as an important stress factor for the benthic communities72.

Oxygen content has been widely regarded as a major environmental stress factor on the benthos29,73. From an ichnologic perspective, progressive dysoxia is generally reflected by decreasing (1) ichnodiversity, (2) bioturbation intensity, (3) burrow size, and (4) burrow penetration74. These organism responses, however, may have been induced by several other physicochemical parameters (e.g., substrate consistency) or taphonomic factors, rather than decreasing oxygenation levels75. In the Buda Formation, dominantly oxic bottom-water conditions are expressed by the overall abundant bioturbation intensity and ichnodiversity. Nevertheless, facies with reduced bioturbation intensity and ichnodiversity, and concomitant occurrences of (locally) abundant pyrite and small-sized Chondrites (e.g., CF2b; Fig. 3c), might reflect an endobenthic response to times of oxygen-depleted (dysoxic) bottom-waters73,76.

Some of the above-mentioned factors (e.g., dysoxia) may also operate in open shelf- and deep-sea environments. Hence, it is not surprising that shelf-sea chalks (epicratonic) and deep-sea chalks (bathyal–abyssal)19,30 show some similarities in ichnofaunal content with the Buda Formation21,28,29,30,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143 (Table 1; Supplementary Table S2). Shelf- and deep-sea chalks are composed of trace-fossil suites largely attributable to the Seilacherian Zoophycos Ichnofacies, with abundant occurrences of Chondrites, Zoophycos, and Planolites, as well as common Teichichnus (Fig. 5a,b; Table 1, Supplementary Table S2). However, the deep-sea chalk deposits differ from their shelf counterparts by the lower occurrences of decapod burrows, such as Ophiomorpha and Thalassinoides; the latter being the dominant ichnogenus in shelf-sea settings (Fig. 5a,b; Table 1; Supplementary Table S2). Moreover, among the recurring chalk ichnogenera, Palaeophycus, Phycosiphon, Taenidium, and Trichichnus are consistently absent or rare in deep-sea chalk deposits (Fig. 5a,b; Table 1; Supplementary Table S2). Skolithos, on the other hand, seems to be more common in deep-sea chalks (Fig. 5a,b). Shelf-sea chalks also show some differences between their bathymetrical end-members (Supplementary Table S2). For example, chalks accumulated in shallow-water environments (e.g., inner shelf) contain trace-fossil suites dominated by Thalassinoides with very rare or absent Zoophycos, attributable to the Cruziana Ichnofacies. Deeper-water shelf-sea chalks (e.g., outer shelf, distal epeiric basins), in contrast, exhibit trace-fossil suites with abundant occurrences of Zoophycos that illustrate the Zoophycos Ichnofacies. Taking into consideration the abundance of Thalassinoides in both shallow- and deep-shelf settings (Supplementary Table S2), the scarcity/absence of Zoophycos seems to be critical in discriminating between these two environments on ichnologic grounds.

Table 1 Distribution of recurrent ichnogenera in chalk, based on this study (for lagoonal chalk) and literature compilation (for shelf and deep-sea chalk21,28,29,30,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143; see Supplementary Table S2 for references). The categories of abundance follow the percentage of occurrence in the different chalks: Very common (≥ 75%), common (25–74%), uncommon (15–24%), rare (1–14%), absent (0%).
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Figure 5
figure 5

Distribution of the most recurring chalk ichnotaxa in shelf-sea (a) versus deep-sea (b) environments. Ch = Chondrites; Cy = Cylindrichnus; Gy = Gyrolithes; Op = Ophiomorpha; Pa = Palaeophycus; Ph = Phycosiphon; Pl = Planolites; Sk = Skolithos; Ta = Taenidium; Te = Teichichnus; Th = Thalassinoides; Tr = Trichichnus; Zo = Zoophycos.

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In general, and somewhat unsurprising given their close bathymetric proximity, the trace-fossil assemblages of the Buda Formation are like those of the shallow-water shelf-sea chalks (Table 1, Supplementary Table S2), as indicated by trace-fossil suites dominated by the ichnogenera Thalassinoides and Chondrites, and lacking Zoophycos, therefore illustrating the Cruziana Ichnofacies. Likewise, the common occurrences of the substrate-controlled Glossifungites and Trypanites ichnofacies reflect similarities with ichnofaunas recorded in shallow-water shelf-sea chalks. Nevertheless, the lagoonal Buda Formation chalk shows ichnologic features that differ from those of (open) shallow-water shelf-sea deposits, which may be attributable specifically to its lagoonal character. These features include (1) overall higher ichnodiversity and ichnodisparity levels in lagoon deposits, and (2) abrupt vertical changes in trace-fossil content, reflecting different expressions of the Cruziana Ichnofacies. These differences in trace-fossil content between chalk formed under open marine conditions and chalk formed in a lagoonal setting reflect a complex interplay of ecologic and taphonomic factors.

In comparison with chalk formed under open marine conditions, the studied lagoon chalk trace-fossil assemblages typically show higher ichnodiversity and ichnodisparity. Average ichnodiversity and ichnodisparity in shelf chalks are 9 and 7, respectively (n = 24; Supplementary Table S2). Of all these units, the highest ichnodiversity and ichnodisparity have been recorded in the Upper Cretaceous Austin Chalk (19 bioturbation ichnotaxa and 14 categories of architectural design), which is the only one showing similar ichnodiversity and ichnodisparity levels to those of the Buda Formation (20 ichnospecies and 17 categories of architectural design, respectively). In addition, the Buda Formation contains the bivalve trace fossils Lockeia siliquaria and Protovirgularia isp., and the burrowing anemone trace fossils Bergaueria isp. and ?Conichnus isp., all previously unrecorded in chalk anywhere. Though the Buda Formation deposits are dominantly normal marine in character (except by the brackish-water WF1), the fact that ichnodiversity and ichnodisparity levels are higher in lagoonal settings than in the open sea seems to be inconsistent with current ideas regarding ichnology of marginal-marine environments. However, this apparent anomaly can be explained by invoking an interplay of taphonomic and ecologic controls. Ichnodiversity in shelf and deep-sea chalk is typically relatively low. However, this low level may reflect taphonomic overprint due to the extremely high bioturbation degree in these deposits108, resulting in ichnofabrics of low fidelity showing a preservational bias towards elite trace fossils144. Intense bioturbation may have allowed preferential preservation of deep-tier trace fossils, preventing preservation of those emplaced in shallow tiers, therefore artificially decreasing ichnodiversity145. In other words, intense bioturbation is masking the true diversity levels of animal activity in these open marine settings. Although the degree of bioturbation in the studied lagoonal chalk is overall relatively high, not all intervals have suffered biogenic reworking to the same extent that those formed under fully marine conditions. This allowed keeping open a taphonomic window that promoted the preservation of shallow-tier trace fossils, therefore contributing to a diversity increase that reflects enhanced ichnologic fidelity.

In addition to this taphonomic overprint, spatial heterogeneity may have played a role in promoting overall high ichnodiversity and ichnodisparity levels in the lagoonal Buda Formation. Increased environmental heterogeneity along a broad spectrum of spatial scales is known to enhance diversity146,147. Specifically, carbonate lagoons may have been host to a complex mosaic of habitats contributing to both within- and between-community variability148. A comprehensive study dealing with geographic patterns of marine benthos biodiversity along the European coasts showed that lagoons tend to be considerably higher in species density and diversity than open coast systems149. Furthermore, the relatively high depositional depth variance within the Buda Formation, which include very-shallow subtidal to deep subtidal lagoonal settings (above SWB)32,33, maybe also responsible for increased diversity in this unit150.

The sharp changes in trace-fossil content in the Buda Formation chalk are evidenced, for example, by the abrupt vertical replacement of the depauperate, monospecific suite of the Cruziana Ichnofacies occurring in the brackish-water deposits (WF1) by the more diverse expression of this ichnofacies that characterizes the overlying more marine and pervasively bioturbated deposits formed in less restricted lagoonal settings (WF2; Fig. 1), therefore most likely reflecting a lower incidence of the salinity stressor. Another example is noted with the sharp switch from deposits (CF2c) having the impoverished proximal Cruziana Ichnofacies to the overlying highly bioturbated deposits (CF4b) containing the archetypal Cruziana Ichnofacies (Supplementary Figs. S3–S5), due to a substantial upward increase in bottom-water oxygenation. This vertical variability in the different expressions of the Cruziana Ichnofacies in the Buda Formation may represent another distinguishing feature of lagoonal chalks, as a reflection of the rapidly fluctuating physicochemical conditions in this marginal marine setting151.

Conclusions

The ichnologic content of the lagoonal chalk deposits from the Buda Formation is similar to those of the (open) shallow-water shelf-sea chalks, as represented by ubiquitous Thalassinoides isp. and Chondrites isp., together with the absence or scarcity of Zoophycos isp. Nevertheless, the studied lagoonal chalk displays higher both ichnodiversity and ichnodisparity than its open ocean counterpart. This relatively richer endobenthic community in the Buda Formation chalk may be the result of increased environmental variability in the lagoonal realm and the effect of differential taphonomic processes in comparison to the open ocean chalks.

Methods

Material

This ichnologic study is based on Buda Formation sections previously assessed in sedimentologic and stratigraphic studies by Valencia et al.32,33 in the west-central Texas region. It comprises the analysis of seven outcrops (A-1, SA-1, DR1, 16, DR17, DRD-B, DR33-B), and one cored-section (“Shanklin, W.R. 2”), where nine main sedimentary facies were described32,33 (see Supplementary Table S1 and Supplementary Fig. S1 for facies summary and outcrops/well-core locations, respectively). A total of ten rock slabs representative of most sedimentary facies were collected and polished at the facilities of the University of Saskatchewan. Material lacking a good trace fossil-host rock color contrast was photographed and digitally treated using Adobe Photoshop software (version 21.2.3) to enhance trace-fossil recognition, by using techniques developed by Dorador and Rodriguez-Tovar152,153, where adjustments of image levels, contrast/brightness, and vibrance were implemented. In addition, selected rock samples were prepared for back-scattered electron microscopy analysis (SEM), using the Phenom-World Phenom XL scanning electron microscope at the Texas A&M University at College Station.

Ichnologic analysis

The ichnologic characterization of the studied material (outcrops, cores, and polished rock-slabs) included ichnotaxonomic descriptions, tiering analysis, and semi-quantitative estimation of the bioturbation intensity via the Bioturbation Index (BI) of Taylor and Goldring154. This index is a widely applied bioturbation intensity scheme that comprises seven categories. BI = 0 indicates no bioturbation (0%). BI = 1 is represented by sparse bioturbation (1–4%) with few discrete trace fossils. BI = 2 represents low bioturbation (5–30%). BI = 3 indicates moderate bioturbation (30–61%), with rare overlapping of the traces. BI = 4 indicates high bioturbation (61–90%), with common overlapping of the traces. BI = 5 typified sediment with intense bioturbation (91–99%), with abundant overlapping a limited reworking. BI = 6 illustrates fully bioturbated (100%) and totally reworked sediment, due to repeated overprinting of the biogenic structures. For the degree of bioerosion, a percentage of area occupied by borings was quantitatively assessed in photographed material, using a combination of the Adobe Photoshop and ImageJ software for boring-area calculations following the method in Cao et al.155. This method includes (1) delineation of the boring/bioeroded area using Adobe Photoshop’s “lasso tool” and painting it in black; followed by the (2) measurements of the “total area” and “bioeroded area” (in pixels) using the “wand (tracing) tool” in ImageJ; and (3) the final calculation of the bioerosion percentage in the studied image. SEM observations, on the other hand, were limited to the identification of biogenic structures non-visible under macroscopic analysis. In addition, we have compiled a comprehensive global dataset of trace fossils in chalk based on literature evaluation (Supplementary Table S2). Each of the recorded trace-fossil occurrences has been checked and the ichnotaxonomy revised to use a consistent classification framework.