The “cannonballs” concretions are very common geomorphic structures
within the Eocene rocks in both the Faiyum and Bahariya depressions
in Egypt (Plyusnina et al. 2016; Sallam et al. 2018). These concretions
are spheroidal, hard and compact ranging ~ 0.5–4.0 m in diameter. In
the Faiyum Depression, the “cannonballs” concretions occur either
individually eroding out of the Middle Eocene Birket Qarun Formation (Fig. 1a) or protrude from the weathered cliff-sides of this formation
(Fig. 1b). The Birket Qarun Formation is made up of calcareous sandstone,
marlstone and sandy limestone deposited in shallow marine environments.
The “cannonballs” concretions at the Faiyum geosite consist
mainly of calcareous sandstone cemented by calcite.
At the northern scarp of the Bahariya Depression, the “cannonballs”
concretions occur at the boundary surface between the Lower Eocene
Naqb Formation and its overlying Qazzun Formation (Fig. 2a). The
lithology of these two formations points to a marked difference in depositional
environments between shallow marine disturbed by current
action at base (the Naqb Fm.), while quiet slightly deeper at top (the
Qazzun Fm.) (Afify et al. 2016). The “cannonballs” concretions at this
geosite are composed mainly of hard, siliceous, nummulitic limestone,
dolomitic in parts (Fig. 2b).
The “cannonballs” concretions in both the Faiyum and Bahariya
oases are interpreted to be formed by precipitation of cement material
within the voids or cavities of the sediments hosing them (e.g.,
Al-Agha et al. 1995; McBride et al. 1994, 2003; Mozley and Davis
2005; Wanas 2008). They grew as concentric lamellae around nuclei,
which are often represented by tooth, shell fragment or fossil bodies
(e.g., Mozley 1996; Raiswell and Fisher 2000). These concretions
are lithologically harder than the host rocks, and therefore are more
resistant to weathering processes. As a result, the host rocks are totally
removed during differential erosion by sand storms, leaving behind
such “cannonballs” concretions on the upper surface of the older strata
(cf. McBride et al. 1994, 2003). |
