Grun et al.—Drilling Predation on Miocene Echinocyamus from Malta
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or omission surface. Various terms have been used to designate these beds (see Bennett, 1980; Pedley et al., 1978, 1985; Föllmi et al., 2008) as well as the preceding hardground surface (Gruszczyński et al., 2008). The top of the MGLM shows distinct relief, with a
Figure 3. Field photograph of the Qolla I-Bajda hill near Xwejni, seen from the west. Boundaries of lithostratigraphic units are indicated by white bars, as is the sampling area (hatched in inset). Inset shows the exact position of the site in relation to the surrounding area.
Limestone members (MGLM and UGLM, respectively) are exposed in the Qolla I-Bajda section (36°4'47''N, 14°14'59''E) at Xwejni Bay. This section is directly exposed to the open sea and thus subject to intense weathering. The uppermost few mil- limeters of the otherwise very durableGlobigerina Limestone are relatively soft, presumably due to the constant cycle of seawater soaking and drying. The more resistant parts of the succession of Qolla I-Bajda, namely the terminal hardground of the Middle Globigerina Limestone and associated phosphate conglomerate (UpperMain PhosphateBed –UMPB), formledges onwhich the weathered material accumulates.
Lithostratigraphy and sedimentary environment.—Deposition in the western margin of the Malta-Ragusa Rise represents a major province of Miocene phosphogenesis with a complex origin, transport, and sedimentation of the phosphatized clasts (Pedley and Bennett, 1985). The occurrence of large amounts of phosphatized components is generally attributed to marine settings with high organic productivity, low oxygen levels, low sedimentation rates, and low terrigenous input (e.g., Föllmi et al., 2008; Tapanila et al., 2008). Following Pedley and Bennett (1985), the phosphatized clasts were deposited during periods of turbulence, interrupted by nondeposition resulting in the formation of hardgrounds as well as the deposition of normal pelagic marine sediments. Rehfeld and Janssen (1995) proposed a multiphased development of beds rich in phosphate controlled by sea-level oscillations. The studied beds follow a prominent horizon in the upper
part of the Globigerina Limestone Formation (GLF), which ranges from Aquitanian to Langhian in age. The GLF is predominantly composed of pelagic carbonate limestones deposited offshore. It is subdivided into three members—Lower (LGLM), Middle (MGLM), and Upper Globigerina Limestone (UGLM) members—on the basis of two prominent horizons that can be followed throughout the Maltese Archipelago. The second horizon separates the pale-gray marly limestones of the MGLMfrom yellowish marly limestones of theUGLM(Pedley, 1992; Föllmi et al., 2008). The basal sediments of the UGLM consist of an ~1m thick
bed containing phosphatic nodules and clasts, phosphatized fossils, and nonphosphatized components within a marly limestone matrix (Pedley and Bennett, 1985; Föllmi et al., 2008). The sediments overlie an intensely borrowed hardground
topographic seafloor high in NW and W Gozo (Pedley et al., 2002), where it ends with a distinct terminal hardground with a thalassinoidean burrow system extending up to 1.5m into the MGLM. In that area, bed thickness of theUMPB is greatest and clast size is largest (Bianucci et al., 2011). The phosphorite intraclasts and a matrix of planktonic foraminiferal packstones of the overlying UGLM infill this hardground. According to Bianucci et al. (2011), the areas where the UMPB is associated with an underlying hardground represent autochtho- nous phosphatization associated with topographic seafloor highs.
Toward the south and east, clast size and bed thickness decrease. The UMPB Bed is composed of two horizons of
phosphorite intraclasts floating in the matrix at Malta and up to five similar horizons at Gozo (Pedley and Bennett, 1985; Carbone et al., 1987; Rose et al., 1992). For these occurrences, an allochthonous origin and clast transportation over short distances, presumably from a topographic high inNWGozo, are assumed.
Different interpretations for the depositional depth esti-
mates assigned to the UMPB can be found in the literature; Carbone et al. (1987) estimated depositional depths at 25–65 m, while Bennett (1980) considered the environment to range from deep shelf margin to open shelf sea (see Boggild and Rose, 1984). According to Challis (1980), theUMPB was deposited at shallower conditions than the MGLM, which is currently thought to have been deposited at depths in excess of 400m on the basis of the absence of planktonic foraminifera and presence of chert nodules (see Bianucci et al., 2011). More agreement exists for the paleoenvironment of the UGLM, which is consistently attributed to an upper bathyal setting at 500–600m (Bellanca et al., 2002; Abels et al., 2005) or 500–800m depth (Bianucci et al., 2011). The fauna of the UMPB has been intensively studied and
includes both phosphatized and nonphosphatized components. The former comprises vertebrate bones, mollusks including pteropods and cephalopods, corals, serpulids, barnacles, terebratulid brachiopods, bored pebbles, and echinoderms. Nonphosphatized biogenic components include bivalves, barnacles, and echinoids (Pedley and Bennett, 1985). Phospha- tized skeletons can be well preserved including thin-shelled pteropods, which are preserved as phosphatic internal molds (Rehfeld and Janssen, 1995; Janssen, 2012).
Biostratigraphy.—The terminal hardground of the MGLM is associated with a hiatus of unknown duration, and hence dates given for the UMPB in the literature vary from late Burdigalian to Langhian. The history of chronostratigraphic position of the UGLM is discussed in detail in Bianucci et al. (2011) and Foresi et al. (2011). The base of the UGLMhas been traditionally assigned to the Langhian on the basis of calcareous nannoplankton and planktonic foraminiferans (nannozone CN4, NN5;Giannelli and Salvatorini, 1972; Mazzei, 1985). Föllmi et al. (2008, fig. 11) place the UMPB at the Burdigalian/Langhian
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