Eocene animal trace fossils in 1.7-billion-year-old metaquartzites

Abstract

The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren Group in southwestern Australia contain burrows indistinguishable from ichnogenera Thalassinoides, Ophiomorpha, Teichichnus, and Taenidium, known from firmgrounds and softgrounds. The metamorphic fabric in the host rock is largely retained, and because the most resilient rocks in the sequence, the metaquartzites, are too hard for animal burrowing, the trace fossils have been interpreted as predating the last metamorphic event in the region. Since this event is dated at 1.2 Ga, this would bestow advanced animals an anomalously early age. We have studied the field relationships, petrographic fabric, and geochronology of the rocks and demonstrate that the burrowing took place during an Eocene transgression over a weathered regolith. At this time, the metaquartzites of the inundated surface had been weathered to friable sandstones or loose sands (arenized), allowing for animal burrowing. Subsequent to this event, there was a resilicification of the quartzites, filling the pore space with syntaxial quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic fabric was seemingly restored, and the rocks again assumed the appearance of hard metaquartzites impenetrable to animal burrowing.

Keywords: Eocene; Paleoproterozoic; geochronology; regolith; trace fossils.

Fig. 1.

Map of the Mount Barren Group showing location of three fossil localities and U–Pb geochronology of major units. Paleoproterozoic and Tertiary stratigraphy. Coordinates: Locality 1 [locality R2 of Thom (1)], S 33°56.4, E 119°56.4; Locality 2, S 33°56.1, E 119°57.7; and Locality 3, S 34°03.5, E 119°37.1.

Fig. 2.

Field images (except for J) of trace fossils in Kundip Quartzite. Yellow lines indicate approximate direction of remnant bedding or foliation. (A–C) Thalassinoides isp. (ichnospecies), top surface of rock with subhorizontal branching galleries and vertical shafts; overview (A) and details (B and C). White arrow in B points to region with millimeter-wide branches; white arrow in C points to Y-branch. S 33°56.107, E 119°57.782. (D) Thalassinoides isp., subhorizontal burrow with repeated Y-branching. S 33°56.444, E 119°56.365. (E) Thalassinoides isp., subhorizontal branch aligned with remnant bedding or foliation in quartzite. S 33°56.444, E 119°56.365. (F) Thalassinoides isp., complex subhorizontal branching system. S 33°56.066, E 119°57.702. (G and H) Thalassinoides isp., T-branching. Enlargement in H shows homogenous ferruginous sandstone infill and absence of burrow wall. S 33°56.008, E 119°57.743. (I) Thalassinoides isp., top surface of rock with subhorizontal branching galleries and tendency toward alignment with remnant bedding or foliation in quartzite. S 33°56.082, E 119°57.714. (J) Burrows with light halo. UWA180711. (K) Meniscate burrow with irregular light halo. S 33°56.037, E 119°57.699; UWA180705.

Fig. 3.

(A–E) Ophiomorpha in Kundip Quartzite, with pelletal linings in the burrow walls. Pellets mostly weathered out, leaving voids. Note smooth inner and knobbly outer wall surface in A. (F) Unidentified burrow with granular infill. Specimens collected from loose material at Locality 1 (Fig. 1) but subsequently lost.

Fig. 4.

Kybulup Schist phyllite with trace fossils. Material consists of loose slabs not in situ, collected at Locality 3 (Fig. 1). Traces mainly aligned along parting planes of phyllite, at narrowly oblique angles to metamorphic cleavage. (A) Cylindrical traces with globular protrusion and possible vertical shafts; UWA180696. (B) Complex bioturbation fabric; UWA131809. (C) Taenidium (broad burrow with meniscate fill) cut by narrower burrows of different color. Narrow cylindrical burrow (Top) ending in an expanded termination following a constriction; UWA131812. (D) Heavily bioturbated fabric mostly consisting of Taenidium; UWA131818. (E) False branching in Taenidium-like juxtaposed burrows; UWA131803. (F) Cylindrical burrow with abutting narrower burrow; possible vertical shafts; UWA131801. (G) Burrow traversing the phyllite parting planes and cleavage; UWA131843. (H) Section through burrow (outlined) showing obliterated cleavage fabric in burrow fill; UWA180739. (All scale bars, 5 mm, except H, 1 mm.)

Fig. 5.

Thin section through cylindrical burrow in Kundip Quartzite retaining metamorphic fabric; UWA180740. (A) Petrographic microscopy, crossed nicols, lambda plate. Target rotated 90° between the views separated by oblique line. White arrows in A are perpendicular to cleavage. (B and C) Details of A. (D–G) SEM-BSE (D and E) and CL (F and G) images of regions in B and C. Porosity seen as black in D and E and white in F and G.

Fig. 6.

Probability density plots and histograms (50 million years bin width) of U–Pb ages of detrital zircon (red curves in A–D) and authigenic xenotime (green curve in A). Baddeleyite age of the mafic sill intruding the quartzite is also shown (blue band in A and B). Weighted mean ages for the oldest authigenic xenotime in A and baddeleyite in B are at 95% confidence limits.