Palaeobiodiversity and Palaeoenvironments

, Volume 92, Issue 3, pp 343–352

Origin, palaeoecology and stratigraphic significance of bored and encrusted concretions from the Upper Cretaceous (Santonian) of southern Israel

Authors

    • Department of GeologyThe College of Wooster
  • Michał Zatoń
    • Department of Stratigraphy & Palaeontology, Faculty of Earth SciencesUniversity of Silesia
  • Yoav Avni
    • Geological Survey of Israel
Original Paper

DOI: 10.1007/s12549-012-0082-8

Cite this article as:
Wilson, M.A., Zatoń, M. & Avni, Y. Palaeobio Palaeoenv (2012) 92: 343. doi:10.1007/s12549-012-0082-8

Abstract

Reworked concretions have been significant substrates for boring and encrusting organisms through the Phanerozoic. They provide large, relatively stable calcareous surfaces in systems where sedimentation is minimal. Diverse sclerobiont communities have inhabited reworked concretions since the Ordovician, so they have been important contributors to our understanding of the evolution of these ecological systems. Here, we describe reworked concretions from southern Israel where they are critical for interpreting the stratigraphy and paleoenvironment of an Upper Cretaceous sedimentary sequence. These cobble-sized concretions (averaging roughly 1,000 cm3) are found at the base of the Menuha Formation (Santonian to lower Campanian, Mount Scopus Group) unconformably above the top of the Zihor Formation (Turonian-Coniacian, Judea Group) exposed in the Ramon region of the Negev Highlands. The concretions are almost entirely composed of micritic limestone, and many are exhumed, cemented burrow-fills apparently from 10–20 m of upper Zihor Formation strata removed by erosion. There are also a few cobbles of dolomitic limestone and rare vertebrate bone. The cobbles are moderately to heavily bored by bivalves (producing Gastrochaenolites) and worms (forming Trypanites), and a few have cemented oysters. They are densely arrayed in a single layer, often touching each other or only a few centimeters apart. The sclerobionts associated with the cobbles, along with their hydrodynamic arrangement, strongly suggest that these cobbles accumulated in very shallow water above normal wave base. Most of them (77%) are encrusted on their top surfaces only, indicating that they were bored in place and not later delivered to a deeper environment by submarine currents. The rest of the Menuha Formation above is a chalk with relatively few macrofossils (primarily shark teeth and oysters) and a few trace fossils (Planolites and Thalassinoides are the most common). These reworked cobbles show that the initial deposits of the Menuha Formation accumulated in very shallow water. This has important implications for the development of the Syrian Arc structures in this region, especially the Ramon Monocline.

Keywords

Reworked concretionsDisconformitySclerobiontsUpper CretaceousSantonianIsrael

Introduction

Exhumed concretionary bodies that originated within the sediment during early diagenesis and were subsequently colonised by various benthic sessile organisms are referred to as reworked concretions (or “hiatus concretions” sensu lato; see Voigt 1968). They are thus a form of lag deposit with evidence of an interval not recorded by sedimentation. Reworked concretions have a long history of study most recently reviewed by Zatoń et al. (2011a). They have been described from the Ordovician (e.g. Wilson 1985) through the Recent (e.g. Allison and Pye 1994; Brown and Farrow 1978), sometimes under a variety of other terms such as “cobbles” and “mobile hardgrounds”. Reworked concretions were most commonly formed during calcite sea times (Palmer and Wilson 2004), and so many have been described from the Ordovician–Devonian and Jurassic–Cretaceous intervals (for detailed references, see Zatoń et al. 2011a).

Reworked concretions are important indicators of disconformable surfaces in fine-grained sedimentary sequences such as shales (e.g. Baird 1976, 1981; Brett et al. 2003, 2008) and chalks (e.g. Voigt 1968). They are also useful substrates for studying the evolution and palaeoecology of hard substrate communities (sclerobionts) because of their limited surface areas, mobile nature (often overturning during and after inhabitation), and superb fossil preservation because of burial in fine-grained sediments (see Taylor and Wilson 2003; Wilson 1987).

Here, we document Upper Cretaceous reworked concretions, as well as associated carbonate cobbles, from the Negev Highlands of southern Israel. Occurring at the base of the Menuha Formation (Santonian to lower Campanian), they record palaeoenvironmental and tectonic events in the early development of the Syrian Arc in the Levant and northeastern Africa (Bosworth et al. 1999; Krenkel 1924). In the Negev region, these events led to the development of several monoclines of which the Ramon Monocline became the largest and most uplifted. This structure eventually developed into the Makhtesh Ramon, an erosional feature formed in the core of the breached anticline (Avni 1993). In this context, the reworked concretions are important indicators of uplift and shallowing in an otherwise fine-grained carbonate sequence. These cobbles were bored by bivalves and polychaete worms, and their surfaces were encrusted by oysters. This cobble assemblage thus records an ephemeral sclerobiont (hard substrate-dwelling) community as well as the start of regional tectonism. It is also worth noting that, so far, Cretaceous reworked concretions have been thoroughly studied only by Kennedy and Klinger (1972) and Kennedy et al. (1977) from South Africa and North America, respectively. This study thus adds new and significant data on such mobile substrates from the Cretaceous marine world.

Geological setting

The reworked concretions in this study are found at the base of the Menuha Formation (Santonian to lower Campanian, Mount Scopus Group; see Steinitz 1976) disconformably on top of the Zihor Formation (Turonian-Coniacian, Judea Group; see Figs. 1 and 2). The Zihor Formation is a series of chalky limestones and marls representing an outer ramp carbonate platform with transgressive deposits in its upper part (Buchbinder et al. 2000). Its top surface resembles a flat, wave-cut platform with the reworked concretions arrayed one-layer deep across it (a thickness of about 5–15 cm). The basal Menuha Formation is a glauconitic chalk and marl with oysters (Pycnodonte), shark teeth and phosphatic peloids along with the reworked concretions.
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Fig. 1

Stratigraphy near the C/W-450 and C/W-451 localities, southeastern margin of Makhtesh Ramon. The Menuha Formation is a white chalk often covered with chert debris from the overlying Mishash Formation. The Santonian erosion surface with the reworked concretions is between the Zihor and Menuha Formations

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Fig. 2

Stratigraphic column for the top of the Zihor and bottom of the Menuha Formations. Note the reworked concretions at the boundary between the two formations

Avni (1993) described the structural evolution of the Ramon region in three stages: (1) the Platform Stage (Triassic to Turonian) of low-relief shallow carbonate platforms and ramps; (2) the Island Stage (Late Turonian to Eocene) with several tectonic uplifting and folding events that lifted the Ramon structure above sea level and produced numerous angular unconformities and siliciclastic units eroded from the structural high areas; lastly (3) the Continental Stage (Oligocene to Miocene) of deep erosion truncating the structures in some cases down to Triassic rocks. Faulting and large-scale folding during the Late Miocene uplifted the region to its present elevation, tilted it toward the northeast and led to the development of the Makhtesh Ramon as a large eroded valley located in the core of the breached Ramon monocline. The Ramon monocline was uplifted along the Ramon Fault, which is an old tectonic suture situated along the southern flank of the Ramon structure (Garfunkel 1993). The reworked concretions of this study were formed early in the Island Stage of Avni (1993) as the region was slightly uplifted and downcut by marine abrasion. This is evident by the fact that the Menuha Formation along the northwestern flank of the Ramon monocline has a reduced section (21–0 m) of early Campanian age, whereas in the synclines around the Ramon monocline the section is much thicker (97–63 m) and the basal layers of the Menuha Formation are of Late Coniacian–Santonian age (Avni 1991, 1993). This differential sedimentation is typical for the Menuha Formation deposited on the Syrian Arc structures (Krenkel 1924), the Ramon monocline of which is the largest in the Negev region of southern Israel.

Locations studied

Reworked concretions from the base of the Menuha Formation were collected and measured from five locations in the Ramon area of southern Israel (Fig. 3):
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Fig. 3

Location of the reworked concretion localities on a Google Earth image. Inset shows the image area in Israel with a black rectangle

C/W-450: Southeastern side of Makhtesh Ramon approximately 1.3 km east of Route 40, at the base of a complete exposure of the Menuha Formation chalks (30.57777°N, 34.89689°E).

C/W-451: Southeastern side of Makhtesh Ramon, approximately 2.0 km east of Route 40, on the cliff’s edge (30.57869°N, 34.90100°E).

C/W-452: Wadi Aqrav, approximately 1.5 km northwest of Route 171 (30.56370°N, 34.64201°E).

C/W-453: Wadi Aqrav, approximately 1.4 km northwest of Route 171 (30.56280°N, 34.64285°E).

C/W-454: Wadi Aqrav, approximately 1.3 km northwest of Route 171 (30.56207°N, 34.64289°E).

Materials and methods

Reworked concretions from the base of the Menuha Formation were examined at five locations in the Ramon area (see, for example, Fig. 4) with specimens collected from each. Two sets of concretions were measured in the field: 43 from locality C/W-451 and 56 from C/W-454.
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Fig. 4

Overhead view of the Menuha cobble horizon in Wadi Agrav (location C/W-452). Hammer is 28 cm long

Stratigraphic columns were constructed of the Zihor and Menuha Formations at the five localities using standard field methods including measuring staffs, inclinometers and compasses. Lithologic units were distinguished in each formation and sampled for later analysis.

The sizes of reworked concretions were estimated in the field by measuring three mutually perpendicular axes: length, width and thickness. Cuboid volumes were calculated by multiplying these dimensions together (see Wilson and Taylor 2001), providing an estimate of relative mass and thus mobility of the concretions. The reworked concretion horizons we studied were in situ, meaning that the cobbles were in their original burial positions. We could thus also record whether borings and encrusters were found on the tops, bases and/or sides of each measured cobble.

Collected reworked concretions were examined by thin-section and by acetate peels using standard techniques.

All specimens are housed in the geological collections at the College of Wooster in Wooster, Ohio, USA.

Results

Size, morphology and composition of the reworked concretions

The 99 measured reworked concretions at two localities (C/W-451 and C/W-454) ranged in lengths from 5.5 to 28.6 cm, with cuboid volumes from 53.5 to 3,774.5 cm3 (Table 1).
Table 1

Dimensions of the reworked concretions measured in this study

Locality

n

Minimum length (cm)

Maximum length (cm)

Average length (cm)

Average volume (cm3)

Sclerobionts on all surfaces

C/W-451

56

5.5

28.6

14.1

1,062

16%

C/W-454

43

6.1

14.5

8.7

215

77%

All but one of the cobbles studied with thin-sections and acetate peels (20 from all five localities) are composed of light grey, micritic, and sometimes slightly dolomitic limestone. The micrite contains recrystallised ghosts of foraminiferans and rare echinoderm spines, but is otherwise unfossiliferous. The one exception is a cobble formed of highly abraded reptilian bone.

Distribution of sclerobionts on cobble surfaces

All cobbles were bored on at least one surface, and a few had encrusting oysters. The borings are almost always proximally truncated, meaning the cobbles were considerably abraded before final burial. Deep borings are still preserved, but we may be missing shallow borings (like those of sponges and barnacles) and most of the original encrusters. The surviving cemented oysters have thick shells and usually show only the portion that was directly affixed to the cobble.

As shown in Table 1, the percentage of cobbles bored and/or encrusted by sclerobionts on all surfaces is 16% in the southeastern Makhtesh Ramon collection (C/W-451; the larger cobbles) and 77% in the Wadi Aqrav set (C/W-454; the smaller cobbles).

Sclerobionts

Gastrochaenolites

This boring is the most common sclerobiont on the Menuha cobbles (Fig. 5). It is a clavate boring with a circular cross-section and a narrow proximal neck that expands downward into a flask-shaped chamber (Kelly and Bromley 1984; see Fig. 6). It is one of the most common borings in hard substrates from the Jurassic to the Recent (Taylor and Wilson 2003) and is well known from the Cretaceous of Israel (Lewy 1985; Lewy and Avni 1988). Ichnospecies of Gastrochaenolites are ordinarily easy to distinguish, but these cobbles are too abraded to show characteristics of the chamber or neck dimensions and morphology. This ichnogenus represents the domichnium of a filter-feeding mytilid bivalve, most likely either a gastrochaenid or a lithophagid (Kelly and Bromley 1984).
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Fig. 5

A Menuha Formation reworked concretion viewed from the top. The large hole in the centre is a truncated Gastrochaenolites boring. The smaller holes across the surfaces are Trypanites

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Fig. 6

Polished cross-section of a Menuha reworked concretion showing small, cylindrical borings of Trypanites (T) and larger flask-shaped Gastrochaenolites (G) borings

Trypanites

This boring is cylindrical, unbranched and generally perpendicular to the hard substrate surface into which it is excavated (Bromley 1972; see Figs. 6 and 7). Trypanites is ubiquitous on marine calcareous hard substrates, being found in almost all sclerobiont communities since the Early Cambrian (Taylor and Wilson 2003). A variety of animals groups have constructed Trypanites over geologic time; the most common were probably filter-feeding polychaete worms (Bromley 1972).
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Fig. 7

An eroded oyster attachment on a Menuha reworked concretion. The many small holes in the surface are Trypanites borings

Oysters

The oysters attached to the Menuha cobbles are highly abraded, leaving at most a circular attachment on the cobble surfaces (Fig. 7). These fragments are consistent with the species Pycnodonte vesiculare, which is abundant in the sediments surrounding the cobbles.

Associated fossils

Fossils are also found in the chalk sediments surrounding the cobbles at the base of the Menuha Formation in all five localities.

Oysters

Left and right valves of the oyster Pycnodonte vesiculare are rare to common between the cobbles. The curved attachment surfaces of these specimens are consistent with the curved surfaces of the cobbles, so many were likely dislodged from them during abrasion and rolling. These oysters show no encrustation or boring. P. vesiculare is one of the most common Cretaceous oysters with a nearly cosmopolitan distribution in shallow marine sediments (Bartov and Steinitz 1982; Dhondt et al. 1999).

Shark teeth

Several shark teeth were found associated with the cobbles. The identifiable material represents the species Cretalamna appendiculata and Scapanorhyncus rapax, both of which are extinct and were likely generalist predators (Hamm and Shimada 2002; Shimada 2007). Some teeth are nearly complete, although abraded; others are found only as small fragments.

Irregular echinoids

Tests of the irregular echinoid Mecaster batnensis (formerly Hemiaster batnensis) were found among the cobbles in two of the five localities (C/W-450 and C/W-451). These tests are mostly complete with some chips and abrasion on their edges.

Matrix sediment

The matrix surrounding the cobbles is primarily white chalk consistent with the lithology of most of the Menuha Formation (Fig. 8). The chalk in this horizon has a significant number of irregular glauconite peloids (fine sand size) and smooth ovoid phosphatic peloids (medium sand size). There are also a few silicified and unidentifiable tests of benthic foraminiferans.
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Fig. 8

Bored reworked concretions in a chalk matrix at the base of the Menuha Formation (Wadi Agrav; location C/W-452). Coin diameter is 22 mm

Discussion

Origin of the Menuha reworked concretions

A common trace fossil in the Zihor and Menuha formations is the crustacean burrow system Thalassinoides. These burrows are always filled with cemented sediment in these units, and they are more resistant than the chalks and marls that surround them. The dimensions of the Menuha reworked concretions are congruent with the sizes of these burrows, and the larger cobbles have lengths, widths and depths nearly identical with sections of Thalassinoides (especially at the Wadi Aqrav locality C/W-452; see Fig. 9). The micritic composition of the cobbles is also consistent with this hypothesis for their origin from Thalassinoides.
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Fig. 9

Bored reworked concretion at Wadi Agrav (location C/W-452) in the form of a Thalassinoides burrow-fill

The two measured sets of cobbles show somewhat different size ranges and shapes, with those at C/W-451 on average larger in length and volume than those at C/W-454. This would be expected in a widespread bed with different hydrodynamic conditions. It is also significant that only 16% of the larger cobbles (C/W-451) are bored or encrusted on all sides, whereas 77% of the smaller cobbles (C/W-454) are (Table 1). The smaller cobbles were overturned more often than their larger equivalents, and thus had more widespread occupation by sclerobionts.

Cobble formation within the sediment

Early cementation of the sediment filling burrows is a common process on the seafloor, especially during Calcite Sea intervals like the Cretaceous (Lee and Morse 2010; Palmer and Wilson 2004). Sulfate-reducing bacteria most likely mediated this cementation by metabolising organic material in the burrows and raising the pH of the pore waters (Braissant et al. 2007; Wetzel and Allia 2000) in combination with increased partial pressures of carbon dioxide (Lee and Morse 2010). Organic matter in the crustacean burrow systems would have been abundant, consisting of mucus lining, faecal pellets, organic detritus and even dead carcasses (e.g. Brown and Farrow 1978; Kaźmierczak 1974; Ziebis et al. 1996).

The open and relatively large burrow systems of crustaceans, such as Thalassinoides, often have sediment fillings cemented through early diagenesis because they allowed easy circulation of water through their networks of tunnels, and the sediments are usually more permeable than the surrounding matrix (see Brown and Farrow 1978; Ziebis et al. 1996). Similar early initiation of cementation associated with burrowing has been recognised both in the fossil record (Kaźmierczak 1974; Zatoń et al. 2011a, and references cited therein) and in modern environments (Brown and Farrow 1978).

Exhumation during erosion

Our hypothesis is that, during the initial Santonian uplift of the Ramon region at the start of the Syrian Arc deformation, the chalky seafloor sediments were raised above wave base and waves and currents washed the fine sediments away. The cemented burrow-fills were retained as a lag deposit with cobble-sized grains too large to sweep away but small enough to be overturned often and thoroughly abraded. It is possible that the substrate beneath the cobble layer (what later became the top of the Zihor Formation) was lithified by this time and became a wave-cut platform on which the cobbles rolled (Fig. 10). This would be similar to the cobble horizons recently described from the Miocene of Spain by Santos et al. (2011). Later, transgressive chalks covered the cobble bed, preserving it between the Zihor and Menuha formations.
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Fig. 10

Eroded surface below the reworked concretion horizon (southeastern Makhtesh Ramon; location C/W-451. This is the top of the Zihor Formation. Hammer is 28 cm long

The cobbles are thoroughly bored by Trypanites and Gastrochaenolites. The Trypanites borings appear to have come first since they are filled with an iron oxide-cemented matrix and are cut by the Gastrochaenolites borings, which are filled with relatively fresh chalk that matches the matrix enclosing the cobbles. The only known encrusters are the oysters Pycnodonte vesiculare. This is again similar to the Miocene mobile substrate community in Spain interpreted by Santos et al. (2011, p. 543) as having no encrusters because of “immature colonisation and a high-energy marine environment”. The limestone cobbles in the Upper Cretaceous Qahlah Formation in the United Arab Emirates and Oman also have relatively few encrusters because of their highly energetic environment of deposition (Wilson and Taylor 2001). Interestingly, oysters and oyster-like bivalves are considered to be among the first colonisers of the Middle Jurassic (Callovian) large bivalve shells (Zatoń et al. 2011b). Thus, it is not unlikely that the oysters on the Menuha reworked concretions also represent the very early stages of encruster colonisation in this high-energy setting, especially since more than one oyster generation can be detected (Fig. 11). In more calm subtidal settings, on the other hand, where disturbance by physical (storm) processes is infrequent, the concretions tend to be encrusted by a variety of different organisms that had time to colonise the substrate and grow before the next overturning (e.g. Zatoń et al. 2011a).
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Fig. 11

Three attachments of the oyster Pycnodonte on a reworked concretion. The one on the left is overlapping the centre oyster, demonstrating that it is from a later encruster generation

Palaeoenvironmental significance

This layer of reworked concretions at the base of the Menuha Formation is direct evidence of a very shallow interval in the midst of fine-grained carbonate units. The bivalve and worm borings, along with the encrusting oysters (Pycnodonte vesiculare), are typical sclerobionts found in intertidal to subtidal waters (Taylor and Wilson 2003). The abrasion of the cobbles and their accumulation as a lag deposit is consistent with this palaeoenvironmental interpretation. The glauconitic and phosphatic peloids are also typical of shallow marine environments. They likely originated as faecal pellets and are often found in condensed sequences with a reduced sedimentation rate, especially in the Late Mesozoic (Jimenez Millan et al. 1998; Van Houten and Purucker 1984).

The fossils found with the cobbles, other than the oysters, do not appear to be contemporaneous with the cobble fauna. The identifiable shark teeth (Cretalamna appendiculata and Scapanorhyncus rapax) represent animals that most likely lived in the middle to outer continental shelf (Hamm and Shimada 2002; Shimada 2007). The irregular echinoids (Mecaster batnensis) are also known most commonly from these deeper continental environments in the Cretaceous (Manso and Andrade 2008). These fossils may also be lag deposits with the cobbles, and thus were derived from the winnowed sediments above the cobble horizon. It is also possible that the echinoids burrowed down through the chalky sediment down to the cobble level long after they were buried during the transgression.

There have been few previous studies of the palaeoenvironment of the Menuha Formation. Flexer and Starinsky (1970), Steinitz (1976) and Flexer and Honigstein (1984) focused primarily on the lithology and microfossils of the chalk and how the unit compared to contemporaneous chalks in the region. Lewy (1985) mentioned the clastic nature of the basal Menuha Formation in southeastern Israel, including phosphatic fragments and fish teeth, but did not describe a palaeoenvironment.

The evidence from this study shows that the base of the Menuha Formation disconformably rests on an abraded surface at the top of the Zihor Formation. The bored and encrusted cobble horizon lies as a lag on top of the abraded surface, suggesting that this was a wave-cut platform with reworked concretions derived from the erosion of a pre-existing chalk with cemented Thalassinoides burrow-fills. The Menuha Formation chalks were the result of a transgression that buried the cobbles.

Conclusions

  1. 1.

    The layer of reworked concretions at the base of the Menuha Formation (Santonian) in the Ramon region of southern Israel represents a high-energy shallowing event in which the top of the underlying Zihor Formation became a wave-cut platform with a lag of cobbles.

     
  2. 2.

    The reworked concretions were in large part derived from Thalassinoides burrow-fills in the chalky sediment eroded during this regression.

     
  3. 3.

    Sessile benthic filter-feeding bivalves, both as boring mytilids and encrusting oysters, were the most common sclerobionts on these cobbles. The cobbles also hosted filter-feeding polychaetes that bored into their surfaces. Other sclerobionts were probably lost because of the high amount of abrasion in this environment.

     
  4. 4.

    The cobbles represent the beginning of Syrian Arc deformation in this part of the Middle East. Their later burial by the chalks of the Santonian to lower Campanian Menuha Formation demonstrates the initial folding and deepening of the relatively flat surface along the axis of the Ramon monocline.

     

Acknowledgements

We thank The Geological Survey of Israel and Will Cary, Andrew Retzler and Micah Risacher (The College of Wooster) for valuable field assistance. Olev Vinn (University of Tartu) and Carl Brett (University of Cincinnati) provided thoughtful reviews of this manuscript. The College of Wooster supported this work through the Sherman Wengerd Endowment and the Henry Luce Fund for Distinguished Scholarship.

Copyright information

© Senckenberg Gesellschaft für Naturforschung and Springer 2012