On the 91/17 pattern in the brackish ichnofabric

From previous research, I have identified what ichnotaxa dominate each ichnofabric unit and how many ichnotaxa are taking up a share of the total observed ichnofabric units. The contribution of this paper is the 91/17 pattern. The point of this pattern is that only 17% ichnotaxa (6 of 34 ichnotaxa) had 91% of the total ichnofabric units that were observed. In addition, the six ichnotaxa tend to be more monospecific. These are the most effective strategies for the animal to survive by constructing the most essential structures. These strategies are common in brackish paleoecology in the fluvial-marine transition zone.


INTRODUCTION
The fluvial-marine transition zone is a depositional system in the study area (e.g., Bachtiar, 2004;Arifullah, 2005). Most authors interpreted it based on sedimentological evidence only. Although ichnofossil is relatively easy to observe in outcrop, it is lacking use in understanding the paleo fluvialmarine transition zone.
In terms of geomorphic units, ichnofossil is an indirect proxy for the fluvial-marine transition zone. However, it is a direct proxy of paleoecology (Arifullah, 2019). The common paleoecology in the study area is brackish (Bachtiar, 2004). In this place the temperature, salinity and turbidity of water is a fluctuating and biologically stressed environment (Dalrymple and Choi, 2007). Thus, I hypothesize that only a few types of ichnofossil will dominate and emerge as monospecific variants.

METHODOLOGY
I had observed 640 ichnofabric units in 20 outcrops located in Samarinda area, Kutai Basin, East Kalimantan, Indonesia (Figure 1). The outcrops are also part of the Serravallian-Tortonian interval. General morphology (in three dimensions), orientation to bedding surface and branching, burrow fill and burrow lining were scrutinized. This approach is a taxonomic requirement for ichnofossil to address either ichnotaxon or ichnotaxa (Arifullah, et al. 2016).
The predominant ichnotaxon in ichnofabric units is the basis for the naming of the ichnofossil association. An ichnotaxon is dominant if its present is over 50% than other ichnotaxon/ichnotaxa in ichnofabric units (Pickerill and Narbonne, 1995). I put the code to the ichnofabric unit. Each code describes (1) outcrop location, dominant ichnotaxon and secondary ichnotaxon (see Arifullah, 2019). With codification, I can calculate what ichnotaxa dominates and know the tendency of those ichnofossil associations to appear in ichnofabric units, whether they tend to be monospecific or polyspecific. I did that by statistical analysis. The Pareto histogram was used to visualize the probability of ichnofossil association from highest to lowest. Another histogram is presented to compare between monospecific and polyspecific trends of the ichnofabric unit.

RESULTS
Thirty-four ichnotaxa had been identified and twenty of them are dominant in the ichnofabric unit. Based on the Pareto histogram, there are only six ichnotaxa that stand out (Figure 2), which is about 17% of the total identified ichnotaxa. I call it the elite ichnofossil (Arifullah et al. 2020). There are Ophiomorpha (27%), Skolithos (23%), Paleophycus (13%), Planolites (10%), Thalassinoides (10%) and Chondrites (8%) with a cumulative frequency of 91.09%. Thus, the actual pattern is 91/17. It means that only a small portion of ichnotaxa obtains a major share of the total ichnofabric units. In order to understand the ichnotaxa, brief descriptions of the ichnotaxa are presented in the following paragraphs.

Figure 1
The geological map of Samarinda Area (modified after Supriatna, 1995) Ophiomorpha can show structures that are either tunnels, shafts, or both (complex). It appears as full relief and/or epirelief. Its burrow lining is strengthened by lithic fragments and pellets ( Figure 3A).
Since burrow fill of Ophiomorpha shows passive fill and/or meniscate backfill structure, all Ophiomorpha can be found in fine to coarse sandstones.
Skolithos is a cylindrical shaft burrow. It means vertical orientation to the bedding surface ( Figure 3B). It can show straight or curved-like shaft morphology. No branches are found. It appears as full relief and/or epirelief. There is no indication of burrow lining. Skolithos can be spotted in both mudstone and sandstone.

Figure 2
The Pareto histogram of the ichnofossil, there are only six ichnotaxa that stand out (After Arifullah, 2019 Paleophycus is a cylindrical tunnel burrow ( Figure 3C). It means horizontal orientation to the bedding surface and might present as straight or curved-like. No branches shown. It turns up as full and/or epirelief and always displays thin burrow lining. The burrow fill shows passive fill structure. Paleophycus is often found in fine to medium sandstones.
Planolites is a cylindrical tunnel burrow that shows sub horizontal to horizontal orientation to the bedding surface without branching. (Figure 3D). Planolites do not have burrow lining. Its burrow fill displays active fill. In general, its color contrasts with the surrounding sediment. Planolites can be discovered in mudstone or muddy sandstone. Sometimes Planolites' tunnel is mistaken for Chondrites, however Planolites does not have a shaft like Chondrites has. Therefore, research on something similar to Planolites or Chondrites should be done.
Thalassinoides is a complex structure that shows a gallery ( Figure 3E). It appears as full and/or epirelief.
The burrow fill of Thalassionoides shows meniscate backfill and passive fill. The morphology may be similar to Ophiomorpha, but the burrow lining is in stark contrast. Thalassinoides have smooth burrow lining and often appear in mudstone or muddy sandstone.
Chondrites is a complex structure that shows similarities to the morphology of the roots (Figure 3F). Therefore, it must have exposed the shaft and tunnel. It can turn up as full relief and/or epirelief. In epirelief feature, Chondrites shows its tunnel structure only. Chondrites are often found in both mudstone and fine sandstone. Observations on 640 ichnofabric units show that Ophimorpha, Skolithos, Paleophycus, Planolites, Thalassinoides and Chondrites have a more dominant performance as monospecific variants than polyspecific (Figure 4). It means that elite ichnotaxa tend to perform as single ichnotaxon in ichnofabric units.

DISCUSSION AND CONCLUSION
Based on the complexity of the elite ichnofossil, Ophiomorpha, Thalassinoides and Chondrites are permanent structures, while Skolithos, Palaephycus and Planolites are incidental structures. Permanent structures imply longer window colonization than incidental ones. (see Pollard et al. 1993;Taylor et al. 2003). Furthermore, paleoecological significance of the elite ichnofossil is presented in Table 1. It shows how the animal gets its food, substrate stability, the richness of organic matter, sedimentological processes and water turbidity. • Clear water. (Bromley, 1996;Howard, 1978;Pollard et al. 1993) • Suspension feeder.
• Deposit and suspension feeder.
• Dilatancy substrate. (Kotake, 1991) In this ichnological study, the 91/17 pattern indicates that some ichnotaxa are very important while most of the others are of insignificant background. Animals are more effective at making less variety of ichnotaxa. In order to survive, animals construct ichnotaxa by considering factors such as substrate type and access to food. It is the optimal effort to deal with that biological stress environment such as unstable conditions that are always repetitive, unpredictable and possibly reworking existing communities. I think this condition is an established indicator of brackish paleoecology. This interpretation is consistent with the result of benthic studies of Bachtiar (2004).
Brackish paleoecology can occur in any depositional environment located in the fluvial-marine transition zone (Dalrymple and Choi, 2007) which includes deltas, estuaries, tidal flats, stranded plains (Boyd et al. 1992). Therefore, elite ichnofossils cannot be used as an indicator for a specific depositional environment in the fluvial-marine transition zone. Other parameters of ichnofossil data should be observed and their distribution should be mapped.