PRELIMINARY REPORT ON SWEAT BEE NESTS IN TIDALLY INFLUENCED DELTA PLAIN DEPOSITS OF THE EOCENE QUEEN CITY FORMATION, SAN ANTONIO, TEXAS

Trace fossils in the sandstone-dominated strata near the top of the Eocene Queen City Formation south of San Antonio, Texas, have been interpreted as subaqueous burrows of marine organisms in tidal flat to sand flat and bay environment. Herein we provide a preliminary assessment of the trace fossils and sedimentary units as sweat bee nests in tidally influenced, medial to distal distributary channel and overbank deposits in the delta plain. Sedimentary facies include thin- to thick bedded, fine- to medium-grained sandstones with thin mudstone interbeds. A series of beds thicken upward to a maximum and then thin upwards from there; this pattern repeats. These bed sets are interbedded with mudstone units with few very thin sandstone beds in thickness from 5 to 15 cm. The top bed is ~75 cm thick and sandstone dominated with very thin mudstone interbeds similar to the pattern described earlier. The mudstone-dominated heterolithic bed sets in the lower part of the outcrop contain thin and sparse rhizoliths and flask-shaped traces that are circular in cross-section. The sandstone-dominated heterolithic bed sets in the upper part of the outcrop contain abundant flask-shaped traces, rare thin rhizoliths that are discontinuous, and subhorizontal burrows assignable to Steinichnus. The circular to flask-shaped trace fossils actually represent a compound structure composed of flask-shaped cells arranged in bilateral to cruciform patterns around a central shaft that can be traced from 10 to 20 cm vertically. Cells parallel to the face of the outcrop exhibit a flask shape. Cells perpendicular to the outcrop exhibit a circular shape. Cells in a position between these two orientations show a variety of elliptical to ovoid shape. Shafts (vertical) and tunnels (horizontal) are ~4 mm in diameter. Complete cells are ~1.4 cm long. These compound trace fossils are most similar to present-day subterranean nests produced by sweat bees, and comparable to those nests produced by the Halictidae. Similar nest morphologies have been found in present-day soils developed on levee and proximal overbank sediments along the Kansas River. The Eocene bee nest is also similar to the compound flask-shaped cell and shaft and tunnel components in ripple-laminated fluvial deposits of the Upper Cretaceous Dakota Sandstone interpreted as halictid bee nests.