Origin, palaeoecology and stratigraphic significance of bored and encrusted concretions from the Upper Cretaceous (Santonian) of southern Israel
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.
KeywordsReworked concretionsDisconformitySclerobiontsUpper CretaceousSantonianIsrael
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.
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.
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
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.
Size, morphology and composition of the reworked concretions
Dimensions of the reworked concretions measured in this study
Minimum length (cm)
Maximum length (cm)
Average length (cm)
Average volume (cm3)
Sclerobionts on all surfaces
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).
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.
Fossils are also found in the chalk sediments surrounding the cobbles at the base of the Menuha Formation in all five localities.
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).
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.
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.
Origin of the Menuha reworked concretions
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
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.
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.
The reworked concretions were in large part derived from Thalassinoides burrow-fills in the chalky sediment eroded during this regression.
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.
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.
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.