Lower Eocene Crustacean Burrows (Israel) Reflect a Change from K- to r-Type Mode of Breeding Across the K-T Boundary Clarifying the Process of the End-Cretaceous Biological Crisis - Advances in Data, Methods, Models and Their Applications in Geoscience - page 315

2. Crustacean burrows in pelagic chalk
2.1 Lower eocene burrows
The road cut exposes the upper part of the greenish clay of the Taqiya Formation (Fig. 2)
containing the latest Paleocene (Thanetian) planktonic foraminifer Morozovella velascoensis
(Cushman). It ranges into the overlying 20 cm of argillaceous chalk which forms a lithological
transition to chalk of the Lower Eocene (Ypresian) Mor Formation. The formation begins with
75 cm of lithified chalk with chert nodules containing the Lower Eocene Morozovella formosa
formosa Bolli and Morozovella aragonensis (Nuttall). The layer above is comprised of 12 cm of
argillaceous chalk with secondary gypsum at the base and the crustacean burrow system in
the upper part. The overlying sequence consists of 0.5-1.0 m thick units of hard chalk with
chert nodules alternating with 10-15 cm thick beds of argillaceous chalk. The chalk-filled
crustacean burrows consist of horizontal galleries connecting between chambers. Vertical
shafts are not preserved. The present elliptical shape of the horizontal galleries attests to
sediment compaction to 60-65% of the original vertical dimensions of the galleries as well as of
the chambers. The periphery of these chalky casts is friable and part of the external features of
the chamber cast is erased. None of the gallery fillings shows any scratches and their walls
seem to have been smooth, forming tubes of 1.0-1.5 cm in diameter (Fig. 5G). About 27 heart-
shaped casts were collected, probably representing the only kind of chamber fill. Their
orientation in the layer is with the heart-shape on the horizontal plane. A gallery enters into
the middle of the floor and another one is connected close to the constricted end of the
chamber ceiling, pointing a little upward and continues horizontally (Figs. 3, 4B). The chamber
casts vary a little in dimensions and proportions whereby the longitudinal length (along the
connecting galleries) may be shorter than the transversal width in some specimens. Length
ranges between 55-75 mm and width between 55-73 mm. Despite sediment compaction, the
thickness (chamber height) of all these casts decreases toward the end with the gallery opening
(Figs. 4C, F, H, 5C, E) strengthening the pear-shape of the chamber in side view. Well-
preserved casts show ovoid blister-like elevations 5-6 mm broad and 5-9 mm long covering the
whole cast, being compressed on the flanks by later compaction. These ovoid structures on the
chamber ceiling (upper surface without a gallery entrance) of some specimens tend to orient
into transversal lines forming slightly arched ribs about 7 mm in width (Fig. 5B). The
horizontal galleries and the flattened chamber casts are concentrated in the upper half of the
12-cm-thick layer.
Fig. 3. Lower Eocene burrowing crustacean chamber and connecting gallery within the
chalky sediment (80% of natural size).
The absence of any relict of vertical shaft indicates the truncation of the unconsolidated
sediment reaching close to the horizontal burrows. This sediment removal from the deep sea
bottom by deep marine currents could have occurred before the fill of the burrows by the
following deposition of foraminiferal ooze, or after sediment filled the burrows and formed
a stabilized level at which sediment removal stopped.