Exploring Ichnofacies Distribution in Ancient Seaways: The Late Miocene Rian Corridor as a Natural Laboratory

Oceanic gateways have modulated ocean circulation and have inuenced climatic variations throughout the Earth´s history. During the Late Miocene (7.8 - 7.35 Ma), the Atlantic Ocean and the Mediterranean Sea were connected through the Rian Corridor (Morocco). This gateway is one of the few examples of deep ancient seaways with a semi-continuous sedimentary record. Deposits comprise turbidites intercalated between deep-sea ne grain sediments (i.e., hemipelagites and drift deposits), channelized sandstone contourite facies, and shallow marine sandstones. Herein an ichnological analysis was conducted in these upper Miocene sediments to improve characterisation of palaeoenvironmental conditions. In addition, ichnofacies were analysed to elucidate how bottom currents control ichnofacies distribution and can modied their attributes. Turbidite deposits are typied by vertical trace fossils (i.e., Ophiomorpha), conforming the Ophiomorpha rudis ichnosubfacies. Contouritic sandstones exhibit high density and low diverse trace fossil assemblage, with predominant Macaronichnus and Scolicia, resembling a proximal expression of Cruziana ichnofacies. Shallow marine environments are dominated by vertical trace fossils (e.g., Conichnus, Ophiomorpha and Skolithos), allowing an assignation to Skolithos ichnofacies. This study reveals energy to be a major controlling factor determining ichnofacies attributes and distribution in ancient deep seaways. Within these seaways, highly energetic conditions typical of shallower settings are present in deeper environments (i.e., slope), contributing to ichnodiversity impoverishment in ichnofacies.


Introduction
Oceanic gateways play a key role in controlling global ocean circulation and climate systems 1 . Ancient seaways are unique environments in which a complex interplay of processes may take place (i.e., oceanic-, tidal-, bottom-, turbiditic-and wind-currents) 2,3 . The constricted morphology of the seaway usually funnels and ampli es the currents that shape the sea oor (i.e., tidal currents) 4 . Previous sedimentological studies of ancient seaways have been largely focussed on shallow counterparts (generally between 100-150 m water depth) [4][5][6] . Few examples of published evidence on deep ancient seaways (> 150 m) and associated deposits can be found. However, oceanographic studies have shown that deep seaways are different from shallow ones, with bottom-currents sometimes playing a dominant role [7][8][9] . The Ri an Corridor is one of those few examples ( Fig. 1) 2,3,10,11 .
During the Late Miocene, the Atlantic Ocean and the Mediterranean Sea were connected by a complex network of marine gateways through south Iberia and north Africa -the Betic and Ri an corridors, respectively 12,13 . The Ri an Corridor was a main deep seaway of this network (ref, Fig. 1). This gateway progressively closed (<6.9 Ma) due to tectonically induced uplift, leading to the onset of the Mediterranean Salinity Crisis in the Late Miocene 13,14 . During the Late Tortonian the seaway evolved into a narrow, deep corridor hosting a complex interplay of processes 2,3 .
Ichnological analysis comprises a wide range of tools (i.e., ichnofabric approach, ichnofacies model) that prove very useful in sedimentary basin research 15 . The ichnofacies model is of special interest for detailed palaeoenvironmental reconstructions and for recognizing, differentiating and interpreting sedimentary environments [16][17][18][19] . Recent steps in ichnological research have established means of recognising and characterising contouritic processes, revealing the importance of ichnology as a proxy for discerning between contourites, turbidites, hemipelagites and pelagites 20-24 , but not without scepticism 25 . At any rate, the relationship between deep-sea settings and trace fossils is very complex and depends highly upon the palaeonvironmental factors that affect trace makers 26 .
Trace fossil research on seaway/strait environments mainly refers to shallow marine settings as brackish ecosystems (i.e., estuarine complexes, resulting in the so-called "brackish-water model" 27,28 ), beachshoreface complexes with evidence of tidal processes 29 , or compound dune elds 30 . Still, detailed trace fossil analysis of ancient deep seaways has never been carried out. The aim of this research is to conduct a detailed ichnological analysis of selected outcrops of the Ri an Corridor, as a unique opportunity to assess trace fossil variations to interpret an ancient deep-water seaway where shallow marine processes (i.e., tidal variations), pelagic/hemipelagic settling, turbiditic supplies and contouritic ows closely (less than 20 km) interact 2,3 . We evaluate the importance of palaeoenvironmental factors such as nutrients, oxygenation, and ow velocity in a setting dominated by bottom currents, and their incidence on the trace maker community. The utility of the ichnofacies model is underlined in the framework of improving high-resolution palaeoenvironmental reconstructions in different depositional environments of ancient deep gateways.
Trace fossil assemblages at the Ri an Corridor In both contouritic and turbiditic deposits, ichnodiversity is low, whereas trace fossil abundance is high in the former and moderate in the latter. In general, shallow marine deposits from the southern Ri an Corridor feature an abundant and moderately diverse trace fossil assemblage. Within the selected outcrops, the clear ichnological variability can be attributed to the different facies.
The Sidi Harazem turbiditic ichnoassemblage consists of 5 ichnogenera -Ophiomorpha (O. rudis), Planolites, Spirophyton, Thalassinoides, and Zoophycos ( Fig. 3 E-H)-and the thick sandstone beds are more bioturbated than the marly ones. Ophiomorpha is the most abundant ichnogenus and appears in the thick turbiditic sandstone beds; Thalassinoides are common, Planolites rare, and Zoophycos and Spirophyton are occasionally found. The trace fossil assemblage of marly pelagic and hemipelagic deposits from the Sidi Harazem consists of abundant undifferentiated structures and scarce Planoliteslike and Thalassinoides-like traces.
The sandy contourites in Kirmta and Sidi Chahed comprise a highly abundant and scarcely diverse trace fossil assemblage (4 ichnogenera), dominated by Macaronichnus and Scolicia, frequent Planolites, and common Thalassinoides (Fig. 2). Traces fossils were predominantly found in the planar strati ed and cross-bedded sandstones. Turbidites show an absence of discrete trace fossils.
The Ain Kansera section is characterised by a shallow marine ichnoassemblage with high ichodiversity and an abundance of vertical structures, including 9 ichnogenera localized in the sandstone beds: Conichnus, Diplocraterion, Macaronichnus, Ophiomorpha, Parahaentzschelinia, Planolites, Scolicia, Skolithos, and Thalassinoides ( Fig. 3 A-D). The sandstone beds with swaley cross-strati cation show a change in the trace fossil assemblage towards the top of the outcrop. The lower sandstone beds present dominant Conichnus and Macaronichnus, common Parahaentzschelinia and Thalassinoides, and rare Diplocraterion, Planolites, and Scolicia. The upper sandstone beds register the disappearance of Conichnus and Parahaentzschelinia, while Ophiomorpha and Skolithos become dominant.

Ichnofacies characterisation
The trace-fossil assemblage of Sidi Harazem is typi ed by Ophiomorpharudis and Thalassinoides. Ophiomorpha is generally but not exclusively characteristic of high-energy environments (i.e., shoreface) in well-sorted, shifting sandy substrates, constituting a common element of the Skolithos and Cruziana ichnofacies 17,18 . However, the appearance of Ophiomorpha in deep-sea environments is also registered, and usually explained as an effect of casual transport of the trace makers by currents from shallow marine environments into the deep-sea 32,33 . Uchman 34 proposed the Ophiomorpharudis ichnosubfacies within the Nereites ichnofacies for the record of ichnoassemblages dominated by Ophiomorpha rudis registered in thick sandstone beds related with channels and proximal lobes in turbiditic systems 35 .
Accordingly, the Sidi Harazem trace fossil assemblage could be associated with Ophiomorpharudis ichnosubfacies. Ichnosubfacies/ichnofacies assignation is tentative due to the absence of other components of this ichnosubfacies (e.g., Scolicia, Nereites, graphoglyptids); this uncertainty is tied to outcrop limitations, e.g. the low exposure of turbiditic soles and di culties in observing discrete trace fossils in the non-compact hemipelagic and pelagic deposits.
The trace fossil assemblages of Kirmta and Sidi Chahed feature high abundance and low ichnodiversity, being dominated by Macaronichnus and Scolicia. Macaronichnus is usually interpreted as a shallow marine (up to foreshore) trace fossil 36 that occasionally appears in deeper environments 37,38 and is commonly associated with the Skolithos ichnofacies 17,18,19,39 . Scolicia presents a wide environmental range, but is a typical element of the shelfal Cruziana ichnofacies 39 . The proximal expression of the Cruziana ichnofacies is de ned as a transition between the distal expression of the Skolithos ichnofacies and the archetypal Cruziana ichnofacies, dominated by deposit-feeding structures having numerable dwellings and suspension feeding forms 40 . The low ichnodiversity observed within the contourite facies from Kirmta and Sidi Chahed outcrops, together with the ubiquity of the dominant trace fossils, hamper a conclusive ichnofacies assignation. Still, though Macaronichnus is typical of the Skolithos ichnofacies, it may punctually appear in proximal Cruziana ichnofacies 40 . Considering the dominance of horizontal traces produced by deposit and detritus feeders over dwellings and suspension feeding structures, contourite ichnoassemblages at the Ri an Corridor, registered at Kirmta and Sidi Chahed outcrops, can therefore be tentatively assigned to an impoverished proximal Cruziana ichnofacies 18 .
The trace-fossil assemblage of Ain Kansera is characterised by moderate ichnodiversity with a dominance of vertical (Skolithos and Ophiomorpha), cylindrical or conic-shaped (Conichnus) dwelling burrows of suspension feeders and passive predators. Horizontal traces produced by mobile fauna are scarce, mainly associated with Macaronichnus trace makers. According to these ichnological features, shallow marine facies at the Ri an Corridor -represented by Ain Kansera sediments-can be clearly assigned to the Skolithos ichnofacies, with predominant burrow systems having vertical, cylindrical, or Ushaped components of suspension feeders and passive predators, and a scarcity of horizontal traces 17,18,19,39,41 .
Ichnofacies in seaways: hydrodynamic energy and the incidence of bottom currents inthe Ri an Corridor Over the past years, detailed ichnological research has revealed the major incidence of particular environmental factors (e.g., organic-matter content, oxygenation, and sedimentation rate) on ichnological attributes from deep-sea environments, including ichnofacies characterisation and distribution 26 . The deep sea is a complex environment where several depositional processes co-exist, including pelagic/hemipelagic settling, bottom currents and gravity ows 9 . Trace fossil analysis has proven useful for discerning and characterising such sedimentary environments and associated deposits 21 .
Hydrodynamic conditions are a very signi cant limiting factor for trace makers, inducing variations in distribution and behaviour, hence in the preservation of trace fossils 19,29,42,43 . Typically, ichnoassemblages related to high energy conditions are characterised by vertical dwelling structures of infaunal suspension feeders and/or passive predators, forming low-diversity suites; ichnoassemblages related to low energy conditions are dominated by horizontal traces of deposit and detritus feeders, as well as higher diversity 19 . Ichnofacies identi cation is mainly based on the recognition of key features that connect biological structures with physical parameters (i.e., environmental conditions) 17,18,19 . Accordingly, ichnofacies re ect speci c combinations of organisms´ responses to a wide range of environmental conditions.
In the case of seaways, prevailing hydrodynamic conditions are a main environmental factor, along with controlling depositional processes and sedimentation regimes 6 . Even though the number of trace fossil studies is considerably lower than in other clastic shallow or deep marine environments, ichnological analysis has proven to be useful to characterise waves, tides or storms in shallow seaways 29 , overlooking deep seaways and their implications. Deep seaways with narrow palaeogeographical con guration, as is the case of the Ri an Corridor 10 , would promote higher energetic conditions than those typical of deepsea environments. In the study area, clearly distinct sedimentary environments -in terms of hydrodynamic conditions, bathymetry, rate of sedimentation, etc.-are closely spaced 2 , passing from shallow marine to turbiditic slope systems in less than 20 km (Fig. 4). Such variations in palaeoenvironmental conditions are supported by ichnofacies characterisation and distribution.
Turbidite deposits from Sidi Harazem, emplaced on the slope of the Ri an Corridor, are typi ed by vertical trace fossils, mainly by the record of Ophiomorpha rudis. These ichnological attributes are similar to those associated with particular sub-environments (e.g., channels and proximal trubiditic lobes) of the turbiditic systems conforming the Ophiomorpha rudis ichnosubfacies inside the Nereites ichnofacies 35 .
Sandy contourite 2D-and 3D-dune facies (upper slope environment) (Fig. 4) from Sidi Chahed and Kirmta are related to high-energy deep-water environments. However, they are dominated by horizontal trace fossils (Macaronichnus and Scolicia) produced by mobile deposit-and detritus-feeders, discarding a direct assignation to the Skolithos ichnofacies. In this case, palaeoenvironmental conditions other than hydrodynamic energy must be considered to explain the dominance of horizontal forms and the absence of vertical biogenic structures. The record of densely Macaronichnus ichnoasemblages in these contourite sediments was recently linked to high nutrient supply provided by ancient bottom currents 38,44 . This agrees with the record of Scolicia: its abundance and size usually increase in conjunction with greater amounts and nutritious values of benthic food 20,45 . Thus, the strong palaeo-MOW bottom currents that dominated the slope may have created well-oxygenated and nutrient-rich benthic environments, favouring colonisation by trace makers that could exploit such accumulations of organic matter inside the sediment. Macaronichnus and Scolicia producers could develop an opportunistic behaviour, determining a rapid and complete bioturbation, avoiding colonisation by other trace makersincluding suspension feeders-and an ichnodiverse trace fossil assemblage. These ichnological features resemble Cruziana ichnofacies attributes. Notwithstanding, the high ichnodiversity so common in Cruziana ichnofacies is absent here. The great abundance and low ichnodiversity observed for the contourite facies appears to indicate the absence of an archetypical Cruziana ichnofacies, but the development of a proximal Cruziana ichnofacies. Bottom currents and their associated deposits (i.e., contourites) have been previously linked to both Cruziana and Zoophycos ichnofacies in Cyprus Miocene carbonate contourite deposits 22,23 , meaning that contourite deposits are not exclusively related to a single ichnofacies. The replacement from Zoophycos to Cruziana ichnofacies was interpreted to be mainly controlled by sea level dynamics 23 .
The shallow marine facies from Ain Kansera (shoreface environment) are dominated by vertical, cylindrical, or U-shaped dwelling burrows (Conichnus, Ophiomorpha and Skolithos) of suspension feeders (Fig. 4). These attributes are usually related to high energetic conditions developed in shallow marine environments conforming the Skolithos ichnofacies 18 .
In short, at the Ri an Corridor, ichnofacies distributions from proximal to distal settings are controlled by bottom currents (palaeo-MOW), with hydrodynamic conditions being the major palaeonvironmental limiting factor. Especially interesting is the development of proximal Cruziana ichnofacies in deeper settings from the slope environments; bottom currents generated high energetic conditions similar to those of shallow/proximal areas.

Conclusions
During the Late Miocene, the Ri an Corridor (Morocco) connected the Atlantic Ocean and the Mediterranean Sea. The particular palaeogeographical con guration led this ancient deep seaway to be affected by variable palaeoceanographic processes and associated deposits (e.g., shallow marine sandstones, channelized sandstone contourite facies, and turbidites intercalated between deep-sea hemipelagites), inducing different ichnological features in terms of ichnofacies composition and distribution. Turbidite deposits are typi ed by vertical trace fossils (i.e., Ophiomorpha), with assignation to the Ophiomorpha rudis ichnosubfacies. Contourite deposits record ichnological assemblages dominated by Scolicia and Macaronichnus, with a plausible assignation to an impoverished proximal Cruziana ichnofacies. Shallow marine environments are dominated by vertical trace fossils (e.g., Conichnus, Ophiomorpha and Skolithos) conforming Skolithos ichnofacies. Ichnofacies distribution in the studied ancient seaways is mainly controlled by the prevailing hydrodynamic regimes, but further ichnological analysis is needed to determine the incidence of other palaeoenvironmental factors.

Geological setting
The Ri an Corridor (Morocco) connected the Atlantic Ocean and the Mediterranean Sea during the Late Miocene (Fig. 1). The outcrops studied herein are located on the northern ank of the Saiss Basin in the South Ri an Corridor, west of Taza-sill 10 (Fig. 1). This basin comprises middle to upper Miocene foreland deposits 31 and records a unique contourite channel system related to the palaeo-Mediterranean Out ow Water (Palaeo-MOW 2 ), which resulted from net evaporation in the Mediterranean leading to dense water formation. Regional tectonic activity favoured the development of turbiditic and gravity ows, inducing a complex deep system in which variable sedimentary processes interacted 2 .
Four outcrops (Fig. 1)  The Ain Kansera outcrop consists of upper Tortonian (between 7.51 and 7.31 Ma) sandstone and marlstone intercalations 10 . Sandstone bed thicknesses range from 1 to 10 m, the beds containing medium to coarse and very coarse sands. They consist of a compositional mix of siliciclastic and bioclastic sand, and regularly contain hummocky and swaley cross-strati cation. Benthic foraminiferal assemblages and the presence of hummocky cross-strati cation indicate an inner shelf environment (water depths of 50-100 m) 10 . Toward the top, the thick sandstone intervals probably represent a shallower (15-50 m water depth) wave-dominated infralittoral setting.

Methodology
Trace fossil specimens from the aforementioned sections were studied rst at the outcrop and then in laboratory. Outcrop analysis focus on macroscopic morphological burrow features (i.e., orientation, shape, length / diameter, cross-cutting relationships, and taphonomy). Moreover, abundance and facies relationship were study in detail. Collected specimens were examined in the laboratory, with special attention to ichnotaxonomical features. The images of some photographed specimens (outcrop and laboratory) were treated with image software 46 . Collected samples are housed in the Department of Stratigraphy and Palaeontology, University of Granada.