Evaluating bioturbation as an indicator of colonisation windows in deltaic systems,Kachchh Basin,India

Abstract

The concept of colonisation windows—intervals during which benthic organisms can actively bioturbate before being interrupted by sedimentation or environmental shifts—offers a high-resolution tool for palaeoenvironmental reconstruction. The present study investigates the bioturbation patterns of the Lower Cretaceous, Ghuneri Member (Bhuj Formation) within the wave-dominated deltaic system of the Kachchh Basin. Through ichnofabric analysis, ichnodiversity indices, and ichnonetwork modelling across 26 stratigraphic sections, we delineate the spatial and temporal dynamics of trace fossil assemblages from proximal to distal settings. Results reveal a distinct asymmetrical bioturbation cyclicity, with higher diversity and prolonged colonisation windows concentrated in the distal, stable substrates, and reduced diversity in proximal, high-energy environments. Six ichnofabric types were documented, reflecting variability in sedimentation rates, storm influence, and substrate consistency. Network analysis identified core, bridging, and peripheral ichnotaxa, validating ecological structuring through metrics of centrality and co-occurrence. The lower Ghuneri Member is characterised by well-developed, multi-tiered ichnocoenoses with Glossifungites ichnofacies, while the upper part exhibits increased heterogeneity and ecological restructuring. These ichnological patterns, embedded within the broader deltaic progradation–transgression–regression cycle, underscore the utility of colonisation windows in deciphering facies transitions and depositional energy gradients in pericratonic rift basin settings.

INTRODUCTION

The term ‘colonisation window’ refers to the time interval available for organisms to bioturbate bedforms before renewed sedimentation or a shift in environmental conditions. Introduced by Pollard et al. (1993), the concept highlights the distinction between continuously colonised sediments and those subjected to brief colonisation events. Several factors influence the colonisation window, including sedimentation rates, erosional-depositional cycles (Buatois et al., 2005; MacEachern et al., 2007), and the composition of the benthic ecosystem.

The analysis of colonisation windows through ichnofabric studies provides valuable insights (Bromley, 1996; Pemberton & Frey, 1984) into the temporal succession of biological activity, integrated with sedimentary dynamics. This, in turn, enhances our understanding of bedform migration, depositional processes, and sediment stability. Consequently, the colonisation window becomes a critical tool for interpreting specific depositional environments, identifying sequence stratigraphic surfaces, and reconstructing palaeoenvironmental conditions with higher resolution (Pollard et al., 1993).

In the context of the Cretaceous shallow to marginal marine settings of the Kachchh Basin, the preservation and characteristics of colonisation windows reveal important patterns in the frequency and temporal distribution of biogenic activity. The Mesozoic sedimentary evolution of the Kachchh Basin is marked by three distinct episodes of deltaic sedimentation following the climax rift phase: (a) the Wagad Delta (Fürsich et al., 2012), (b) the Jhuran Delta (Desai & Biswas, 2018), and (c) the Ghuneri Delta (Desai & Chauhan, 2021). These deltaic systems provide an excellent framework for examining colonisation windows in relation to sedimentary dynamics and bioturbation processes.

GEOLOGICAL SETTINGS

The Bhuj Formation, part of the early Cretaceous Mesozoic succession in the Kachchh Basin, (Figure 1) represents a post-rift deltaic phase, comprising three distinct members: Ghuneri, Ukra, and upper (Biswas, 1977, 1982; Desai et al., 2021). Each reflects changes in depositional regimes and palaeoenvironmental conditions. The Ghuneri Member, belonging to the Bhuj Formation and unconformably overlying the Jhuran Formation, exhibits distinct cyclic bioturbation patterns. These cycles typically commence with non-bioturbated shale and culminate in highly bioturbated, coarse-grained sandstone, reflecting shifts in energy conditions and sediment supply. The Ghuneri Member, the lowermost unit, is key to understanding wave-dominated delta systems in a pericratonic rift basin (Chauhan et al., 2022; Desai, 2016; Desai & Chauhan, 2021). When traced east-west, the Ghuneri Member thickens significantly from 168 metres in the east to nearly 535 metres in the west (Ghuneri Dome) (Biswas, 1977). Hence, the outcrops for studying the ichnological variations were selected considering the proximal to distal facies variation within a sedimentary basin. The Ghuneri Member is a highly bioturbated sedimentary unit showing asymmetrical bioturbation cycles. The member is mainly unfossiliferous, except for a few shell lenticels (Rudra et al., 2007; Shukla & Singh, 1993) and plant-rich shales. The member’s asymmetrical bioturbated cycles make it easy for its identification in the outcrops (Chauhan et al., 2022; Desai, 2016; Desai & Chauhan, 2021).

Figure 1.

Geological Map of the Kachchh Basin showing Bhuj Formation outcrops along with the locations of studied sections (Modified after Chauhan et al., 2022).

It is composed of laminated shales, siltstones, and cross-bedded sandstones in coarsening- and thickening-upward cycles (Figure 2), formed by deltaic progradation and interrupted by storm events (Biswas, 1982; Desai et al., 2021). Sedimentary structures include large-scale trough cross-bedded features that characterise fine-to-coarse sandstones. Hummocky cross-stratified fine to medium-grained sandstones (Dott & Bourgeois, 1982; Walker & Plint, 1992), bed tops with medium and large-scaled amplitude wave ripples, linear wave ripples with crest bifurcations or flat crests, and substantial erosion with or without wave-winnowed lags are typical features of the formation (demonstrating the strong influence of wave and storm activity) (Biswas, 1982; Bose et al., 1986; Shukla & Singh, 1993). Lateral facies variations from east to west (Figure 2) indicate westward progradation toward the basin’s depocentre (Desai, 2016).

Figure 2.

Stratigraphic correlation of the Ghuneri Member (Bhuj Formation, Early Cretaceous) across three key sections—Ghuneri Type Section (Distal), Hill Garden Section (Intermediate), and Nadappa Section (Proximal)—from west to east (After Chauhan et al, 2022). The figure illustrates facies variation, ichnofabric development, and Glossifungites Ichnofacies Surfaces (GIFS) across ~530 metres of succession. Note the transition from low to high bioturbation indices (BI) and the dominance of GIFS in middle to upper intervals, particularly in the proximal sections. Trigonia shell beds and amalgamated sandstones are also marked, indicating high-energy depositional episodes and colonisation windows.

The Ghuneri Member exhibits moderate to high ichnodiversity, with over 20 ichnogenera and nearly 30 ichnospecies (Chauhan et al., 2022; Desai, 2016; Desai & Chauhan, 2021). Trace fossils show vertical and lateral tiering, reflecting changes in energy conditions and colonisation patterns. These features indicate a depositional setting between fair-weather and storm-wave bases. Glossifungites ichnofacies at multiple stratigraphic levels suggest episodic firmground conditions following sediment consolidation (Desai & Chauhan, 2021).

The overlying Ukra Member records a brief marine transgression, marked by glauconitic shales and marine fossils in the Kachchh Basin (Bansal et al., 2017; Desai, 2016). The upper member marks the return to fluvial dominance during regression. Collectively, the three members record a complete depositional cycle: deltaic progradation (Ghuneri), transgression (Ukra), and regression (upper), providing insights into Lower Cretaceous basin evolution.

A detailed taphonomic framework for the Ghuneri Member (Desai & Chauhan, 2021) identifies six distinct ichnofabric types:

Low bioturbated ichnofabric;

Structureless sand ichnofabric;

Incomplete bioturbated top ichnofabric;

Bioturbated units with Glossifungites ichnofacies;

Firmground bypass ichnofabric;

Amalgamated bioturbated units with resumed colonisation.

These are primarily influenced by colonisation window duration, sedimentation rates, and storm-induced substrate changes (Buatois et al., 2005; MacEachern et al., 2010). Short colonisation windows in high-energy settings led to low bioturbation (BI-1 to 2), often confined to bed tops. Prolonged windows during calmer intervals produced intensely bioturbated units (BI-5 to 6). For instance, Type-3 ichnofabrics show strong bioturbation limited to upper bed surfaces, indicating opportunistic colonisation during pauses in sedimentation. Substrate consistency changes, especially the formation of firmgrounds, resulted in Glossifungites ichnofacies with trace fossils such as Balanoglossites and Gastrochaenolites (Figure 1). These mark periods of sediment starvation and reflect the end of bioturbation cycles. For example, an excellent site of Balanoglossites occurs at the base of the Ghuneri Member (Desai & Saklani, 2012).

Amalgamated ichnofabrics (Type-6) developed through repeated sedimentation and colonisation, where storm events reworked previous ichnofabrics (Figure 2). Trace fossil suites like Skolithos and Ophiomorpha dominate these composite layers, especially in proximal deltaic settings.

This study involves ichnological mapping across 26 stratigraphic sections of the Ghuneri Member, which is extensively exposed throughout the Kachchh Basin. For regional analysis, the study area was divided into four sectors, namely (a) Sector 1: Mundhan–Ghuneri; (b) Sector 2: Nakhatrana; (c) Sector 3: Bhuj (including southern Bhuj) and (d) Sector 4: Kaswali (eastern Kachchh) (Figure 1). Section selection was based on the following criteria: (a) Exposure of type and reference sections; (b) Ease of stratigraphic correlation, and (c) Strike-aligned orientation to facilitate facies analysis.

MATERIALS AND METHODS

Ichnological data from selected Ghuneri Member outcrops from across the basin were collected following quadrat sampling techniques (Figure 1). Field observations were systematically recorded and subjected to statistical analysis. Sections were selected to represent the complete east-to-west (proximal to distal) transect of the basin. Ichnological data were encoded using a binary presence-absence system, where ‘1’ denotes the presence and ‘0’ denotes the absence of trace fossils at a given site (Table 1). Quadrat sampling involved placing a 0.25 m² square frame at each site and counting all trace fossils within the frame. Trace fossils within each quadrat were counted systematically, with sampling repeated along both strike and dip directions in key sections. The compiled data were analysed for standard statistical analysis, especially ichnodiversity. This index includes (a) Taxa abundance, (b) Diversity and Evenness indices (Shanon, Dominance, Alpha Diversity, Beta Diversity), for the workflow followed refer to Figure 3. To interpret diversity patterns, the study adopts Whittaker’s (1972) hierarchical biodiversity framework, distinguishing among alpha, beta, and gamma diversity. Alpha diversity refers to the species diversity within a specific area or ecosystem, typically quantified as species richness. In this study, alpha diversity is defined as the species diversity of a local community (e.g., per hectare or within a discrete natural habitat patch), representing within-habitat diversity. In contrast, beta diversity captures the variation in species composition between distinct communities, reflecting between-habitat diversity and species turnover across spatial gradients. Further, the collated ichnological data were analysed for diversity, assemblage, and ichnonetwork analysis following Baucon and Felletti (2013a, b and 2015a, b). Fundamentally, each block of information is referred to as a ‘node’ that is linked by connections, ‘edges’ (Brandes & Erlebach, 2005; Dale, 2017). These nodes and edges are analysed for their connectivity measures, node metrics, linkages within the network, and indicators of system dynamics based on graph adjacency matrices (Dale, 2017). In ichnology, each node in the network represents a distinct ichnotaxon, while the edges denote instances of co-occurrence within bedding planes across the outcrops (Baucon & Felletti, 2013a). The direction and weight of edges reflect the strength and asymmetry of these relationships, which may relate to ecological succession, facilitative colonisation, or preservation biases (Dale, 2017; Wulff et al., 1989). Centrality metrics such as degree, betweenness, and eigenvector centrality were used to identify ichnotaxa with the highest structural importance within the assemblage. They describe this as an association matrix that displays the strength and structure of interactions between trace fossils.

The methodology (Figure 3) combines capturing, managing, analysing, and displaying geographically referenced ichnological data and integrating it with network theory for characterising the environmental significance of trace fossils.

Figure 3.

Workflow illustrating the methodology adopted for ichnotaxonomic diversity and network analysis of trace fossils from outcrop data. The process involves field-based trace fossil data collection, data tabulation and curation, diversity analysis using PAST software (presence-absence matrix and Shannon diversity index), and subsequent network analysis using GEPHI with Yifan-Hu layout, degree ranking, and edge weight visualisation.

RESULTS

Ichnological diversity analysis

Thirty-one recurring ichnotaxa data are recorded at selected outcrops of the Cretaceous Ghuneri Member (Table 1). The Ghuneri Member was informally divided into upper and lower parts based on the strength of the bioturbational cycles. All the diversity indices were generated and compared for the upper and lower parts of the Ghuneri Member. The data were compiled by coding the absence of ichnotaxa at a particular site as (0) and its presence as (1). The data is compiled in Table 1. The Shannon diversity index (H) is another index that is commonly used to characterise species diversity in a community. Shannon’s index accounts for both abundance and evenness of the species present. From the Figure 4, it can be envisaged that the Shanon Index shows higher values for the Western Ghuneri and Ugedi sections, while it is comparatively lower for the central sections. Similarly, results are also seen from Dominance (D), which indicates that as the index value increases, diversity decreases. This analysis also supports the view that most of the diversity is seen in the distal part of the basin, and the proximal part shows a reduction in diversity.

Table 1.

Occurrence matrix of trace fossil ichnogenera across selected localities of the Ghuneri–Bhuj region, Kachchh Basin. The table records the presence (1) and absence (0) of major ichnogenera in various outcrops representing the Lower Cretaceous Ghuneri Member of the Bhuj Formation. Each row corresponds to a studied locality (e.g., Ghuneri-W, Ghuneri-S, Bhuj-Quarry), while each column lists a distinct ichnogenus observed in the succession.

	Arenicolites	Balanoglossites	Chondirtes	Conichnus	Diplocraterion	Gyrochorte	Gyrolithes	Lockeia	Ophiomorpha	Paleophycus	Polykladichnus	Psilonichnus	Taenidium	Teichichnus	Thalassinoides
Guneri-W	1	0	0	0	1	0	0	0	0	1	0	0	0	1	1
Guneri-S	1	0	0	0	1	0	0	0	0	1	0	0	0	1	0
Guneri-T	0	1	0	0	0	0	0	0	1	1	1	0	0	0	1
Guneri-E	0	0	0	0	1	0	0	0	0	0	0	1	0	1	1
Siyot	0	0	0	1	1	0	0	0	1	1	0	1	0	0	1
Katesar	0	0	1	1	0	0	0	1	1	1	0	1	1	1	1
Atada	0	0	0	0	1	0	0	0	0	0	0	1	0	0	1
Ugedi	1	0	0	0	0	1	0	0	1	1	0	1	0	0	0
Bhuj-Hill	0	0	0	0	0	1	1	0	0	0	1	1	0	0	0
Bhuj-Quarry	0	0	0	1	0	1	0	0	0	0	0	0	0	0	1
Bhuj-By-pass	0	0	0	0	0	1	0	0	1	1	1	0	0	1	1
Ghodpur	1	0	0	0	0	0	1	0	0	0	1	1	0	0	0
Nagor	0	0	0	0	0	0	1	0	0	0	1	1	0	0	0
Nadappa-1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Nadappa-2	1	0	0	0	0	0	0	0	0	0	0	1	0	0	0
Kodki	0	0	0	0	1	0	0	0	0	0	1	0	0	0	0
Rampar	0	0	0	0	0	0	0	0	0	1	0	1	0	0	1
Jhawar­nagar	1	1	0	0	0	0	0	0	1	1	1	1	0	1	1

Figure 4.

Ichnofossil diversity across the studied outcrops in the lower part of the Cretaceous Ghuneri Member. (a) Shannon Diversity Index (H) of trace fossil assemblages. Vertical bars represent standard deviation, and dots indicate outlier values. Sites such as Ghuneri-T and Jhawarnagar exhibit relatively higher ichnodiversity, whereas Nadapa-1 shows the lowest. (b) Dominance Index (D) of trace fossil assemblages, reflecting the relative abundance distribution of ichnotaxa. Vertical bars indicate standard deviation, and dots represent outliers. Nadapa-1 exhibits the highest dominance, suggesting a community structure skewed toward fewer ichnotaxa, while Ghuneri-T and Jhawarnagar show more even distributions.

In the lower part of the Ghuneri Member (Figure 4), Shannon-H values range from approximately 0.8 to 2.6, with the highest diversity recorded at the Katesar and Jhawarnagar sections. While Nadappa-1 section exhibits the lowest diversity alongside the highest dominance (~0.5), most other outcrops maintain moderate diversity (H’ ≈ 1.5–2.3) and low-to-moderate dominance values.

In the upper part of Ghuneri Member (Figure 5), Shannon-H remains relatively high at Ghuneri-W, Ghuneri-S, and Bhuj-Hill Garden (H’ > 2.0), with a notable drop at Ugedi and Bhuj-Quarry. Ugedi shows the highest dominance value (~0.35), while other outcrops like Bhuj-Bypass and Ghodpur exhibit moderate diversity (H’ ≈ 2.0) and low dominance (~0.15). Across both datasets, dominance and diversity tend to vary inversely.

Figure 5.

Ichnofossil diversity across the studied outcrops in the upper part of the Cretaceous Ghuneri Member. (a) Shannon Diversity Index (H′) across outcrops in the Kachchh Basin. Vertical bars show standard deviation. Higher H′ values in Ghuneri-W, Ghuneri-S, and Ugedi reflect greater ichnodiversity, while lower values in Bhuj-Quarry and Kodki indicate reduced diversity, possibly due to environmental stress or limited colonisation. (b) Simpson’s Dominance Index (D) for trace fossil assemblages across Kachchh Basin outcrops. Lower D values in Ghuneri-W, Ghuneri-S, and Ugedi suggest higher evenness and stability, while higher values in Bhuj-Quarry and Kodki reflect dominance by a few ichnotaxa, indicating stress or restricted colonisation. Error bars represent variation in community structure.

The observed variations in Shannon diversity and dominance indices (Magurran, 2004) across the Ghuneri Member outcrops reflect differences in ecological stability and substrate conditions. High-diversity and low dominance, as seen at sections like Katesar, Jhawarnagar, Ghuneri-W, and Bhuj-Hill, suggest well-oxygenated, stable substrates that supported a balanced and diverse ichnofaunal community, likely under lower environmental stress and prolonged colonisation windows.

In contrast, low-diversity and high dominance at sections such as Nadappa-1 and Ugedi indicate environmental stress or episodic disturbance, possibly related to rapid sedimentation, reduced oxygenation, or ecological filtering that favoured a few opportunistic trace-makers. The upper part of the Ghuneri Member shows more fluctuation in both indices, suggesting shifting depositional settings and variable colonisation dynamics across time and space. These patterns are consistent with a deltaic to marginal marine environment, where energy levels, sediment supply, and substrate consistency frequently change, shaping the composition and dominance structure of the ichnoassemblages.

Ichnonetwork analysis and matrix association

Following ichnonetwork analysis, ichnological data from the Ghuneri Member were analysed using a network-based model. The resulting system is represented by a Yifan-Hu algorithm layout (Hu, 2011) with degree range and edge weight optimisation, along with a default circular preview. Here, nodes are evenly distributed with minimal edge crossing (Hu, 2011). In this model, each ichnotaxon functions as a node, and positive entries in the association matrix indicate a link (or edge) between two nodes. These connections represent co-occurrence relationships, with stronger associations visualised by thicker lines.

A corresponding association matrix, which shows the probability of co-occurrence of two ichnotaxa on the Jaccard similarity index (Figure 6), supplements this graph and serves as a powerful tool for identifying trace fossil associations and potential environmental drivers. The graph was constructed using ClustVis, a web-based open-source software visualisation tool (https://biit.cs.ut.ee/clustvis/). This heatmap represents (Figure 6) an ichnotaxa co-occurrence association matrix, where both the rows and columns denote individual trace fossil taxa. The colour scale indicates the strength of co-occurrence between ichnotaxa, ranging from dark teal for weak associations to beige/orange for stronger co-occurrences. High values suggest that a given pair of ichnotaxa frequently co-occur across the sampled outcrops or stratigraphic units. The hierarchical clustering shown by the dendrograms groups ichnotaxa based on the similarity of their co-occurrence patterns. Core clusters such as Psilonichnus, Gyrochorte, and Thalassinoides represent a central ichnocoenosis, commonly observed in stable, moderately energetic marine settings. In contrast, taxa like Schaubcylindrichnus or Quebecichnus display fewer and more selective associations, possibly reflecting ecological specialisation or occurrence in restricted settings. The matrix provides insight into ichnological assemblage structures and ecological relationships within the trace fossil record. The clustering patterns reflect likely behavioural or environmental affinities such as tiering overlap, shared sediment consistency preferences, or similar colonisation windows. Strong co-occurrences between shallow-tier forms (e.g., Palaeophycus, Planolites) and deep-tier structures (e.g., Diplocraterion, Teichichnus) may indicate prolonged colonisation intervals or reduced physical disturbance. Highlighting of Arenicolites isp. Along the axis suggests its use as a reference taxon for evaluating ecological linkages, showing frequent association with taxa like Lockeia, Rosselia, and Chondrites. Overall, this matrix is a powerful tool for detecting ichnological sub-communities, reconstructing paleoenvironmental gradients, and identifying key bridging or central ichnotaxa within a benthic assemblage. These ichnonetwork insights are instrumental in reconstructing depositional environments and tracing ichnodiversity gradients within the Kachchh Basin, offering a nuanced understanding of the spatial and temporal dynamics of trace fossil assemblages.

Figure 6.

Correlation matrix of trace fossil genera based on co-occurrence patterns across studied outcrops. The colour scale represents the strength and direction of Pearson correlation coefficients, with blue indicating positive correlations and red indicating negative correlations. Larger ellipses represent stronger associations. Notable positive correlations are observed between Arenicolites–Diplocraterion, Gyrochorte–Gyrolithes, and Teichichnus–Thalassinoides, suggesting similar environmental preferences or colonisation behaviours.

Network analysis of the lower part: Ghuneri Member

The lower part of the Ghuneri Member comprises cyclic bioturbation units, each starting with an unbioturbated unit and ending with highly bioturbated units. There are up to seven cycles in the lower part. The sections are selected as per the proximal-distal nature of the basin. Figure 3 shows the matrix association of the ichnotaxa of the lower part of the Ghuneri Member. Based on this data following ichnoassemblages are delineated. Three Ichnoassemblages are validated through ichnonetwork analysis (Figure 7a and 7b). Figure 7.

Network analysis of trace fossil associations in the (a) Lower and (b) Upper parts of the Ghuneri Member, Kachchh Basin. Node size indicates ichnotaxa centrality; edge thickness reflects co-occurrence strength. Core, bridge, and peripheral ichnotaxa are identified based Figure 8. on their network roles, highlighting shifts in ichnological interactions through the stratigraphic succession.

1. Core ichnotaxa assemblage (High Centrality and Connectivity).

2. Transitional or Bridging Taxa (Moderate Centrality, High Betweenness).

3. Peripheral or Specialised Taxa (low Centrality, High Clustering).

Core ichnotaxa assemblage (High Centrality and Connectivity)

The core ichnotaxa in the lower Ghuneri ichnonetwork include Psilonichnus (PS), Arenicolites (AR), and Polykladichnus (PO) (Figure 7a; Plate 1). These taxa exhibit the highest eigenvector centralities—1.0 (PS), 0.9488 (AR), and 0.8867 (PO)—as well as high weighted degrees, ranging from 4.85 (PS) to 5.25 (AR). Their degree values (6, 6, and 5, respectively) also suggest extensive direct connections with other ichnotaxa. Furthermore, they all show a clustering coefficient of 1.0, indicating their strong participation in fully interconnected local sub-networks.

PLATE 1.

Core Ichnotaxa Assemblage (High Centrality and Connectivity) from the Lower Ghuneri Member. This plate illustrates key ichnotaxa that form the structural backbone of the Lower Ghuneri ichnonetwork. (A) Densely colonised Polykladichnus showing slender, vertically oriented burrows with characteristic upward Y-shaped bifurcations. (B) Moderately dense Psilonichnus, characterised by large, subvertical, thickly lined burrows suggestive of crustacean dwelling structures. (C) Typical J-shaped burrow of Arenicolites, representing vertical U-tubes formed by stationary suspension feeders. (D) Bedding plane view showing paired burrow openings of Arenicolites, indicating intense bioturbation and high trace maker activity in the substrate.

Psilonichnus (PS) remains the most central ichnotaxon, forming dense and highly influential connections throughout the network. It likely represents a widespread coloniser in moderately to high-energy environments with firm substrates, characteristic of shoreface zones. Arenicolites (AR), while slightly lower in eigenvector centrality than PS, holds the highest weighted degree (5.25), pointing to its ecological importance as a vertical dwelling trace typical in wave-influenced sandy substrates. Polykladichnus (PO), with both high eigenvector and strong edge integration (degree = 5), suggests a role in deeper-tier colonisation under stable substrate conditions. Collectively, these ichnotaxa form a tightly knit, highly connected core module, indicative of long colonisation windows, sustained environmental stability, and fully developed benthic communities in shallow marine to shoreface settings.

Transitional or bridging ichnotaxa (Moderate Centrality, Moderate Connectivity)

The bridging ichnotaxa include Teichichnus (TE), Ophiomorpha (OP), Diplocraterion (DI), and Conichnus (CO) (Figure 7a; Plate 2). These trace fossils possess moderate degrees (4–5) and moderate eigenvector centralities, ranging from 0.501 (TE) to 0.763 (CO), with weighted degrees between 2.41 and 4.35. Each has a clustering coefficient of 1.0, reinforcing their role in well-integrated micro-networks.

PLATE 2.

Transitional or Bridging Ichnotaxa (Moderate Centrality and Connectivity) from the Ghuneri Formation. This plate highlights ichnotaxa with moderate centrality and connectivity in the ichnonetwork, which act as functional bridges between core and peripheral taxa. (A) Diplocraterion colonised sandstone bed from the Hill Garden outcrop, showing deep U-shaped vertical spreite structures indicative of repeated retraction and reburrowing. (B) Oblique bedding plane view of Teichichnus from the Ghuneri Dome, exhibiting gently curved, stacked spreiten suggestive of deposit-feeding activity. (C) Cross-sectional view of Teichichnus showing inclined laminae formed by systematic upward movement through sediment. (D) Thick sandstone bed with dense colonisation by Ophiomorpha, recognised by its lined burrow walls and pelletal structures, indicating crustacean activity. (E) Long, vertically oriented Conichnus conicus escape trace, approximately 2.3 metres in length, representing the response of an organism to rapid sedimentation.

Teichichnus (TE), with degree three and eigenvector centrality of 0.501, appears to mediate between core traces and peripheral niches, often associated with episodic sedimentation and recolonisation surfaces. Ophiomorpha (OP), with a degree of five and an eigenvector centrality of 0.913, reflects robust modular connectivity, commonly found in firm, sandy substrates shaped by both biogenic and hydrodynamic influences. Diplocraterion (DI) (eigenvector: 0.6076) and Conichnus (CO) (0.7638) act as tier-spanning facilitators—linking shallow and deeper-tier ichnofabrics. They occupy strategic roles where energy and sediment supply fluctuate, especially during colonisation resets or in environments with varying substrate compaction. These bridging taxa promote cross-tier interaction and ecological redundancy, enabling resilience in transitional settings like lower shoreface or interdistributary bays.

Peripheral or specialised ichnotaxa (Low Centrality, High Clustering)

Peripheral ichnotaxa include Palaeophycus (PA), Gyrochorte (Gy), Gyrolithes (GYO), Chondrites (CH), Taenidium (TA), Thalassinoides (TH), Balanoglossites (BA), and Lockeia (LO) (Figure 7a; Plate 3). These taxa consistently exhibit low eigenvector centralities (ranging from 0.288 to 0.659), lower weighted degrees (typically 2.4 to 3.6), and low-to-moderate degrees (2–4), yet maintain high clustering coefficients (1.0), indicating strong local community structure despite peripheral network position.

PLATE 3.

Peripheral or Specialised Ichnotaxa (Low Centrality, High Clustering) from the Lower Ghuneri Member. This plate illustrates ichnotaxa with low network centrality but often forming localised high-density clusters, indicating ecological specialisation or niche partitioning. (A) Horizontal, unlined Palaeophycus burrow showing passive fill, indicative of dwelling traces. (B) Bedding plane view of meandering Gyrochorte in siltstone, suggesting locomotion of surface-feeding organisms. (C) Oblique view of Taenidium, characterised by a meniscate backfill structure formed by deposit feeders. (D) Chondrites preserved in fine-grained siltstone, displaying fine, dichotomously branching burrow networks linked to oxygen-poor conditions. (E) Helical vertical burrow of Gyrolithes, possibly indicating deeper-tier bioturbation. (F) Three-dimensional burrow network of Thalassinoides, representing complex crustacean dwelling structures. (G) Firmground surface colonised by Balanoglossites, commonly associated with reduced sedimentation rates. (H) Almond-shaped Lockeia resting trace, typically formed by bivalves in quiet-water settings.

Chondrites (CH), Taenidium (TA), and Thalassinoides (TH) all share the lowest eigenvector centrality (0.288), indicating limited influence on the broader network. These may reflect specialised trace-makers in dysoxic, fine-grained, or restricted substrates. Gyrolithes (GYO), with the lowest degree (2) and eigenvector centrality (0.380), appears to be highly localised, possibly forming under niche conditions such as low-energy, firm but organic-rich substrates. PA and

Gy, with moderate degree and eigenvector values, may represent opportunistic grazing or dwelling traces that do not influence the broader ichnonetwork but reflect microhabitat variability. BA, although structurally peripheral, exhibits a slightly higher eigenvector (0.715) and weighted degree (4.11), suggesting occasional bridging influence—possibly in deeper tiers or during environmental shifts. These peripheral ichnotaxa are crucial for capturing environmental patchiness, ecological specialisation, and episodic colonisation in more restricted depositional settings.

Ichnology of the upper part of the Ghuneri Member

The matrix association and network analysis of the upper part of the Ghuneri Member shows drastic differences from the lower part of the Ghuneri Member (Figure 7b). The following Ichnoassemblages are identified:

1. Core Ichnotaxa (High Influence, Central Nodes).

2. Bridge Ichnotaxa (High Betweenness Centrality).

3. Peripheral or Specialised Ichnotaxa.

Core Ichnotaxa (High influence, central nodes)

The core ichnotaxa of the assemblage of Psilonichnus, Gyrochorte, and Polykladichnus (Figure 7b; Plate 4) exhibit the highest centrality values and extensive edge connectivity within the ichnological network with high values of closeness and eigenvector centrality. From a network topology perspective, these taxa occupy hub-like positions at the centre of multiple modularity classes, forming a highly cohesive subnetwork. In terms of network metrics, all three taxa display elevated closeness centrality (ranging from 0.714 to 0.833), indicating their proximity to other nodes and efficient access throughout the network. Their eigenvector centrality values are equally significant, with Gyrochorte reaching the maximum score of 1.000, followed by Psilonichnus (0.973) and Polykladichnus (0.905), signifying a strong association with other highly connected ichnotaxa. In the present ichnoassemblage, Psilonichnus (closeness: 0.769; eigenvector: 0.973) emerges as the most central and influential taxon, functioning as a core coloniser. While Gyrochorte (closeness: 0.833; eigenvector: 1.000) serves as a key sediment-feeding burrow, deeply integrated within the assemblage. Polykladichnus (closeness: 0.714; eigenvector: 0.905) represents a structurally important deep-tier burrow that bridges multiple ichnotaxonomic clusters. Their frequent co-occurrence with both generalist and transitional ichnotaxa reflects a well-integrated ichnocoenosis. Collectively, their network dominance supports the interpretation of long colonisation windows, moderate sedimentation rates, and well-oxygenated substrates.

PLATE 4.

Core Ichnotaxa Assemblage (High Influence, Central Nodes) in the Upper Part of the Ghuneri Member. This plate highlights the dominant ichnotaxa exhibiting high influence and centrality within the upper Ghuneri ichnonetwork, including Psilonichnus, Gyrochorte, and Polykladichnus. (A) Bedding plane view of Gyrochorte burrow showing biserially arranged oblique pads, elongated along the axis of the ribbon-like trace. (B) Fan-like divergent expression of Gyrochorte variabilis, indicative of variable locomotory behaviour. (C) Ferruginous-lined vertical burrow of Psilonichnus in shale, with coarse-grained fill, suggestive of crustacean dwelling structures. (D) Densely colonised Polykladichnus displaying upward-branching, slender burrow networks in fine-grained sediment, signifying intense infaunal activity.

Peripheral or specialised ichnotaxa

The ichnotaxa Arenicolites, Thalassinoides, Rhizocorallium, and Skolithos represent peripheral or specialised elements within the ichnological network (Figure 7b; Plate 5). Characterised by low closeness centrality (ranging from 0.526 to 0.625) and minimal betweenness centrality (0.0 for AR, TH, and RH; 1.67 for SK), these trace fossils exhibit limited connectivity within the network, suggesting more specialised ecological roles or occurrence in environmentally restricted settings. Arenicolites and Rhizocorallium are marked by low centrality and occupy peripheral positions within Modularity Class 1, indicative of taxa that are functionally and structurally isolated. Arenicolites likely represents a shallow-tier, short-lived dwelling structure, while Rhizocorallium, as a deep-tier, slow coloniser, may reflect low-energy or firmground substrates. Thalassinoides appears as a background ichnotaxon, showing no structural influence in the network and suggesting a low ecological impact despite its presence across facies. In contrast, Skolithos displays a slightly elevated betweenness value (1.67), hinting at a minor bridging role. However, its overall peripheral location and low clustering imply a stress-tolerant, opportunistic coloniser typical of high-energy, rapidly shifting environments. From a network analysis standpoint, the low centrality and clustering coefficients of these ichnotaxa reveal their limited interaction with the core and bridging nodes, reinforcing their interpretation as either facies-end members or specialist trace-makers adapted to episodically disturbed, deltaic, or tidally influenced settings. Thus, the occurrence of these peripheral ichnotaxa may point to short colonisation windows, rapid sedimentation events, or environmentally constrained substrate conditions.

PLATE 5.

Peripheral or Specialised Ichnotaxa from the Upper Part of the Ghuneri Member. This plate illustrates ichnotaxa considered peripheral or specialised within the ichnological network of the upper Ghuneri Member, often indicative of specific environmental conditions or tiering strategies. (A) U-shaped vertical burrow of Arenicolites, representing stationary suspension-feeding activity. (B) Thalassinoides burrow system showing branching architecture with expanded nodes, typically formed by crustaceans in well-oxygenated, firm substrates. (C) Long, straight Rhizocorallium trace extending laterally, reflecting spreite structures produced by deposit feeders. (D) High-density vertical burrows of Skolithos, indicative of high-energy, shifting substrate conditions such as foreshore to upper shoreface environments.

Bridge ichnotaxa (High Betweenness Centrality)

The ichnotaxa Diplocraterion Polykladichnus and Gyrochorte function as key bridge nodes within the ichnological network (Figure 7b; Plate 6), connecting otherwise separate clusters and facilitating interaction across different ecological or tiering regimes. From a network perspective, these bridge nodes are essential for maintaining structural connectivity. Their roles are underscored by their high betweenness centrality values—Diplocraterion (9.0), Gyrochorte (6.75), and Polykladichnus (4.42)—indicating their critical positions on the shortest paths linking other nodes. Diplocraterion, with the highest betweenness, serves as both a topological and ecological bridge, linking trace fossils from distinct modularity classes and suggesting an affinity for transitional environments, such as the interface between shoreface and offshore settings. In several sections of Ghuneri Member, Diplocraterion forms post-depositional colonisation events in the bedforms (Plate 2a). Polykladichnus while central to the core assemblage, also plays a bridging role by facilitating connectivity between deep-tier ichnotaxa and more opportunistic or surface-dwelling ichnotaxa, reinforcing its significance in long-lived, stable colonisation regimes. Gyrochorte uniquely functions as both a core taxon and a bridge, suggesting it plays a dual role in maintaining internal cohesion within sediment-feeding—dominated assemblages and enabling broader ecological integration. From the perspective of edge connectivity, all three taxa exhibit broad and cross-modular linkages, forming edges with ichnotaxa from various tiering groups. These edge patterns indicate that these bridge nodes occupy interface zone environments characterised by facies gradients, tiering transitions, or ecological fluctuation.

Thus, these bridge ichnotaxa form distinct ichnoassemblages, indicating ecological flexibility, adapting to episodic shifts in environmental parameters such as energy level and oxygen availability. Their presence highlights the complexity and heterogeneity of the depositional system, likely characterised by facies transitions and mixed colonisation strategies.

PLATE 6.

Bridge Ichnotaxa (High Betweenness Centrality) within the upper part of the Ghuneri Ichnological Network. This plate displays ichnotaxa with high betweenness centrality, acting as critical connectors between different ichno-assemblages and facilitating ecological and spatial transitions within the trace fossil network. (A) Deep-tier Diplocraterion showing prominent vertical U-shaped spreiten structures, indicative of repeated reoccupation and sediment displacement. (B) Moderately colonised Polykladichnus with slender, bifurcating burrows, representing flexible infaunal strategies and network connectivity. (C) Gyrochorte with sinuous, biserially arranged pads on a horizontal plane, reflecting complex locomotory behaviour and high functional bridging capacity.

DISCUSSION

Several lines of evidence suggest a primary thick sequence of deltaic body related to one diachronous mega delta system existed during upper Jurassic to Cretaceous times (Desai & Biswas, 2018). This thick deltaic sequence initiated during the Mid-Kimmeridgian and ended up in the Albian during the rift inversion phase (Desai & Biswas, 2018; Desai et al., 2021). This thick deltaic sequence is interrupted by two transgressive events, which demarcate different environmental episodes of the delta. The present bhuj delta is characterised by (Bhattacharya, 2006) (a) an asymmetrical cycle of biotubration, (b) an Increase in ichnotaxa diversity trends from proximal to distal parts. Based on the above-discussed diversity and ichnoassemblages, it can be conveniently interpreted that the Bhuj Delta was wave-dominated (Chauhan et al., 2022).

Comparison between the lower and upper parts of the Ghuneri Member

One of the prime differences between the lower and upper Ghuneri Member is that the lower part comprises (a) a high diversity of the ichnotaxa, (b) shorter cycles of bioturbation, (c) frequent and widespread Glossifunguites ichnofacies surfaces (Figure 2; Desai & Chauhan, 2021). We can interpret this diversity pattern in terms of environmental gradients, facies influence, and colonisation styles:

High-diversity locality includes (S ≈ 11–13) Ghuneri-W, Ghuneri-S, Ghuneri-T, Katesar, Rampur, Jhawarnagar (Figure 8). These localities host the core and transitional ichnoassemblages, indicative of stable, oxygen-rich marine settings with moderate to high-energy (Buatois & Mángano, 2011; Pemberton et al., 2001), often found in shoreface to offshore transition zones. Important assemblages include Palaeophycus–Thalassinoides–Planolites; Palaeophycus–Skolithos–Thalassinoides and Psilonichnus–Palaeophycus–Ophiomorpha ichnoassemblages. These are well-developed, multi-tiered bioturbated occurrences (Droser & Bottjer, 1986; Pemberton & MacEachern, 2006) indicative of high oxygenation and soft, sandy substrates with long colonisation windows. They represent fully marine conditions associated with the Cruziana–Skolithos ichnofacies transition. The dominance of three-dimensional, walled, dwelling burrows, including Thalassinoides and Ophiomorpha suggests the presence of longer duration of colonisation windows without frequent ecological resets (Pollard et al., 1993).

Figure 8.

Variation in ichnogeneric richness (S) across outcrops arranged along a proximal-distal gradient in the Kachchh Basin. Vertical bars represent the range of observed diversity at each locality. Localities are colour-coded by diversity class: high-diversity (yellow), moderate-diversity (green), and low-diversity (pink), as indicated by the legend. A general trend of decreasing diversity from distal to proximal settings is observed, reflecting environmental stress and substrate variability influencing trace maker colonisation patterns.

Moderate-diversity localities (S ≈ 7–10)—such as Ghuneri-E, Siyot, Atada, Bhuj-Hill, Bhuj-Quarry, Ugedi, and Bhuj-Bypass—represent heterogeneous, energy-variable settings typically located at the transition between open marine and marginal marine environments (Figure 8). The ichnoassemblages identified in these sections—Rhizocorallium–Arenicolites–Palaeophycus, Diplocraterion–Teichichnus–Psilonichnus, and Conichnus–Rhizocorallium–Taenidium—reflect mixed energy and salinity conditions, and are dominated by stress-tolerant and opportunistic trace-makers. Similar ichnotaxa are well-documented in wave-dominated deltaic systems; for instance, in the Cretaceous strata of the Western Interior Seaway, Alberta, trace fossils such as Diplocraterion, Arenicolites, and Conichnus are common in delta-front deposits. At the same time, Rhizocorallium and Teichichnus are more typical of fair-weather colonisation. In the Kachchh Basin, such mixed ichnotaxa assemblages are often the result of fair-weather suites being overprinted by deeper-tier, storm-influenced assemblages. This pattern is characteristic of the Type-2 ichnofabric of structureless sand described by Desai and Chauhan (2021) and Buatois and Mángano (2002). It is interpreted as evidence for short-term colonisation windows following episodic high-energy events.

Low-diversity localities (S < 6) sites include Ghodpur, Nagor, Nadapa-1, Nadapa-2, Kodki. They represent stress-prone or highly restricted environments, characterised by salinity fluctuations, short colonisation windows and low substrate stability or high sedimentation rates (Figure 8). They are dominated by peripheral and episodic colonisers ichnoassemblage, notable trace fossils include Gyrochorte, Polykladichnus Solemyatuba, Quebecichnus and Conichnus. Most of these burrowing taxa are limited by substrate stability and dominated by specialist trace-makers that have adapted for colonising in firmgrounds under high ecological stress. These localities align with the peripheral and episodic colonisers group, dominated by facies-restricted ichnotaxa with low recurrence and ecological specialisation.

These diversity pattern validates the ichnonet-based assemblage classification (Baucon & Felletti, 2013), confirming that network centrality correlates with environmental stability and substrate openness, while peripheral assemblages mark stressed, facies-controlled settings.

Colonisation window as a tool for palaeoenvironmental reconstruction

Colonisation windows, as defined by the temporal and substrate conditions that permit benthic colonisation, have proven to be reliable indicators of palaeoenvironmental dynamics across a range of deltaic and wave-influenced systems. In the Cretaceous successions of the Western Interior Seaway—particularly in Alberta—lower shoreface to offshore transition zones host well-developed, diverse ichnoassemblages (e.g., Thalassinoides, Diplocraterion, Planolites) formed during prolonged periods of substrate stability (Buatois & Mángano, 2011; Pemberton et al., 2001). In contrast, delta-front and distributary mouth-bar deposits are typically characterised by shallow-tier traces (Skolithos, Arenicolites) constrained to bed tops, reflecting shorter colonisation windows due to rapid sedimentation and frequent hydrodynamic disturbance.

A similar ichnological architecture is documented in the Campanian–Maastrichtian delta systems of Patagonia (Moyano Paz et al., 2020), where ichnofabric asymmetry and vertical trace fossil tiering closely track shifts in energy regime, sedimentation rate, and substrate consolidation. Dafoe and Pemberton (2007) document similar patterns in the Lower Cretaceous McMurray Formation, where ichnological assemblages vary predictably with deltaic sub-environments. Proximal settings show low-diversity suites with shallow-tier traces such as Skolithos and Arenicolites, while distal and abandoned delta lobes host deeper-tier, more diverse assemblages including Diplocraterion, Thalassinoides, and Planolites, reflecting longer and more stable colonisation windows. Of particular relevance is the Early Miocene Tacata delta system of eastern Venezuela (Buatois et al., 2008), where alternating fluvial and storm-wave influences produced a spectrum of colonisation responses. Fair-weather deposits exhibit diverse trace suites formed under prolonged colonisation windows. In contrast, storm-dominated or prograding settings preserve monospecific or stressed assemblages (e.g., Skolithos, Diplocraterion, Teichichnus) confined to thin event beds. These global analogues consistently demonstrate that the expression of colonisation windows through ichnofabric structure and trace fossil diversity is a function of sedimentary energy, substrate firmness, and ecological continuity.

The ichnological and sedimentological framework of the Ghuneri Member in the Kachchh Basin finds strong resonance with these global examples. As documented by Desai and Chauhan (2021), the Ghuneri succession preserves alternating highly bioturbated and sharp-based, non-bioturbated sandstones—reflecting episodic high-energy deposition followed by colonisation windows of variable duration. Chauhan et al. (2022) further delineated ichnoassemblage variation across the wave-dominated delta system, with proximal zones (distributary mouth bars, upper delta fronts) dominated by low-diversity, shallow-tier traces and distal sectors (lower delta fronts, prodelta) marked by more stable ichnofabrics, deeper-tier burrows, and higher diversity.

The lower part of the Ghuneri Member is typified by longer colonisation windows, allowing development of multi-tiered trace fossil suites and frequent Glossifungites surfaces. Here, network analysis highlights a stable ichnoassemblage structure, where core ichnotaxa such as Psilonichnus, Gyrochorte, and Polykladichnus dominate, and bridging forms like Diplocraterion maintain ecological continuity across facies transitions. These well-integrated ichnofabrics align with low-energy, distal deltaic settings seen in global analogues.

In contrast, the upper part of the Ghuneri Member displays greater ichnological heterogeneity and colonisation asymmetry. Frequent storm reworking, increased sediment supply, and shifting delta lobes led to shorter, more fragmented colonisation windows. This is expressed in patchy bioturbation, reduced ichnodiversity, and reorganisation of network structure. Bridging ichnotaxa such as Diplocraterion and Polykladichnus gain prominence, while peripheral forms (Thalassinoides, Rhizocorallium, Skolithos) occur sporadically and often in stressed conditions.

Within this context, Diplocraterion emerges as a key trace fossil in interpreting colonisation dynamics. It occurs widely in both protrusive and retrusive forms across multiple facies, reflecting its opportunistic behaviour. Based on field data and earlier ichnological work (Chauhan et al., 2022), four assemblage types are recognised:

Type A: Scattered Diplocraterion alongside Skolithos, Arenicolites, and Palaeophycus in distal delta-front settings.

Type B: Isolated Diplocraterion in stressed, low-diversity assemblages.

Type C: Diverse Diplocraterion-rich assemblages atop mouth bars, formed under high-energy, late-stage colonisation conditions.

Type D: Crowded Diplocraterion assemblages on firmgrounds, associated with delta abandonment or bypass surfaces.

These types correspond well with colonisation surfaces documented in Figure 9. In Figure 9, Panel (A) shows a classic Balanoglossites firmground from the Ghuneri Dome; Panel (B) depicts a compound surface with softground traces (Ophiomorpha) overlain by post-firmground suites; Panel (C) captures a Glossifungites shale surface with overprinted unlined burrows; Panel (D) presents an erosional contact hosting Polykladichnus; Panel (E) features a diffuse surface with ichnotaxonomic shifts; Panel (F) displays an isolated Diplocraterion bed; and Panel (G) shows a bedding plane view of a mature Glossifungites ichnofacies. These ichnological expressions, supported by network centrality metrics, consistently point to Diplocraterion, Polykladichnus, and Gyrochorte as key trace-makers responding to variable colonisation opportunities across the basin.

Figure 9.

Geological map of the Cretaceous Bhuj Formation displaying the locations of studied outcrops, along with their corresponding surface expressions and morphological characteristics.

Taken together, the ichnofabric architecture, trace fossil diversity, and ichnotaxonomic network structure in the Ghuneri Member validate the colonisation window concept as a robust framework for interpreting palaeoenvironmental shifts in deltaic settings. The parallels with global examples—along with the distinct expression of colonisation patterns under rift-margin controls—underscore the broader applicability of this approach across sedimentary basins.

CONCLUSIONS

The ichnological architecture of the Ghuneri Member within the Cretaceous Bhuj Formation presents compelling evidence for reconstructing palaeoenvironmental conditions in a wave-dominated deltaic system. The identification of six distinct ichnofabric types, coupled with ichnodiversity gradients and ichnonetwork analysis, underscores the significance of colonisation windows as diagnostic tools for understanding sedimentary and ecological dynamics.

The lower part of the Ghuneri Member exhibits higher ichnodiversity, frequent Glossifungites surfaces, and cyclic bioturbation patterns indicative of stable substrates, prolonged colonisation intervals, and moderate sedimentation regimes. In contrast, the upper part reflects increased heterogeneity, transitional ichnoassemblages, and signs of ecological resetting, likely driven by fluctuating energy levels and facies shifts.

Network metrics reveal a structured ichnoassemblage hierarchy, with core taxa such as Psilonichnus, Gyrochorte, and Polykladichnus functioning as ecological anchors, while bridging and peripheral taxa delineate substrate-specific colonisation strategies and environmental constraints. The observed diversity patterns along the proximal-distal transect validate the environmental gradient hypothesis, with the highest ichnodiversity concentrated in distal, low-stress settings.

Overall, the integration of ichnofabric analysis, diversity indices, and network modelling offers a high-resolution framework for interpreting deltaic sedimentary processes and benthic colonisation dynamics. The study affirms the utility of ichnological proxies in characterising depositional energy regimes, substrate stability, and ecological tiering, and positions colonisation windows as a critical parameter in sequence stratigraphic and palaeoenvironmental studies of pericratonic basins.