290 Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

The session focuses on deltas and adjacent depositional environments from fluvial systems to the continental shelf. Research on modern, ancient or extraterrestrial deltas that emphasize their importance to our society is welcome.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 303AB

Sedimentological expressions of the mid- and end-Guadalupian events in eastern Australia

Paper number: 290-13
Program: Technical Sessions
Time: 4:06 PM – 4:21 PM

Authors

Abstract

The Sydney and Gunnedah basins of eastern Australia preserve a sedimentary record of the mid-Guadalupian event (c. 262 Ma; AKA: the Kamura Event) and the end-Guadalupian event (c. 259 Ma). This study aims to provide sedimentological context to putative paleoenvironmental disturbances and biotic crises at these levels that have been recognized elsewhere in the world. In the northern Sydney Basin, the basal Tomago Coal Measures are correlative with the mid-Guadalupian interval. An interval of coastal plain deposits with evidence of frequent transgressive events and progradational cycles disconformably overlies homogeneous, glaciomarine deposits (gravel-bearing, fossiliferous, coarse-grained siltstones and very fine grained sandstones with glendonites) of the underlying Mulbring Siltstone. A low-diversity trace fossil suite in the basal Tomago Coal Measures suggests brackish water or otherwise stressed conditions, and rhythmic lamination in heteroliths indicate tidal exchange. This interval contrasts with the overlying, coastal to alluvial plain and coal-bearing lower Tomago Coal Measures. This stratigraphy is evident in multiple field sites and drillcores.

The end-Guadalupian in the Sydney Basin corresponds to a widespread glaciomarine interval, the Kulnura Marine Tongue and equivalents. Tens of meters of gravel-bearing, fossiliferous, bioturbated sandstones abruptly overlie the coastal plain deposits of the lower Tomago Coal Measures. These strata also preserve a restricted trace fossil assemblage comprising only 2–4, “facies-crossing” ichnotaxa. The glaciomarine interval can be correlated across the entirety of the Sydney and Gunnedah basins (several hundred km), suggesting an external control on sediment accumulation. Furthermore, the top of the interval is everywhere a disconformable to angular unconformable contact, overlain in many areas by an alluvial conglomerate (Marangaroo Conglomerate and equivalents).

Although the origin of the mid-Guadalupian facies changes is not yet clear, the end-Guadalupian succession can reliably be attributed to a period of restricted marine transgression and cold climate with ice formation (first pulse of the P4 glaciation), followed by a major relative drawdown of sea-level, all at a regional scale. These relationships will help inform a fuller understanding of Guadalupian environmental disturbances and an assessment of any biotic crises at this time.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412479

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