Tuesday, June 12, 1:00pm

Dr. Nicholas Lemon

Principal Petroleum Geologist/Senior Lecturer, NCPGG, The University of Adelaide
 

Reef backstepping driven by sea level and diapir movement
 
 

Abstract

A series of stromatolite reefs fringe the western edge of the Enorama
Diapir in the central Flinders Ranges. The reefs grew during deposition of
the Neoproterozoic Umberatana Group sediments, at a time when the diapir
was active. Continual movement on the diapir maintained an island in the
middle of a marine shelf, regardless of sea level fluctuations in the
basin. Cyanobacterial growth precipitated lime as stromatolites in the
shallow waters around the island. The stromatolites accumulated
substantial reefs, up to 35m high and 200m across the base. The reefs
interfinger with both shallow water limestones of the Etina Formation and
deep water siltstones of the Enorama Shale.

Diapir activity is indicated by tilting of the surrounding sediments and by
inclusion of insoluble debris shed from the diapiric island. Progressive
movement is shown by cannibalisation and reworking of coarse shoreline
conglomerates. Diapir movement continued after deposition of the reefs so
that the entire succession close to the diapir is tilted up to near vertical.

Fourteen reefs have been mapped to date but there are more to be mapped.
The reefs have been dolomitised and now stand out in the landscape as a
line of hills in contrast to the underlying diapir and overlying shale
which both weather to give subdued relief. The oldest reefs are at the
southern end of the line of small hills. The shales which drape over the
older reefs abut the younger reefs to the north and this pattern is
repeated around most of the reefs.

Tape and compass grid mapping has established the relationships between the
reefs and with pulses of movement on the diapir. The base to the middle of
the Enorama Shale is a transgressive systems tract, deposited during a
steady rise in sea level. Onlap onto the diapiric island is marked by
backstepping of each reef higher in the section. Backstepping occurs in a
series of pulses each marked by channels of conglomerate shed from the
diapir and by progressive local angular unconformity surfaces. Small
pulses of movement cause reefs to recolonise on higher ground, sometimes on
previous reef growth, sometimes establishing a new mound. The reefs drown
as the sea floor next to the diapir subsides into the peripheral sink that
flanks the diapir.

On a meso scale, the reefs are composed of a series of stromatolite mounds
up to 6m high and 15m long. The lamination within the mounds is wrinkled
and quite unlike the laminar stromatolites which grew in the shallow water
Etina limestones which are interbedded with the older parts of the reef
complex. The microscale structure of the reefs is also quite unlike the
stromatolites of the surrounding formations. Instead of being finely
laminated micrite, the reefs show a distinct clotted texture with
filamentous linkages, reminiscent of calcareous alga like Epiphyton or
Renalcis.

The entire arrangement of reefs fringing a diapiric island should provide
an ideal hydrocarbon reservoir, trap and seal combination. Even in rocks
as old as these, there are traces of bitumen in the still porous dolomites
of the reefs.

Biography of the Speaker

Nick has spent ten years in industry with BHP working on projects ranging
from Proterozoic iron formations and base metal exploration to coal and oil
shale exploration and the assessment of low grade energy reserves. This was
followed by a return to the University of Adelaide to undertake a PhD
degree. The thesis topic was "Diapir recognition and modelling, with
examples from the late Proterozoic Adelaide Geosyncline, Central Flinders
Ranges, South Australia." This involved detailed mapping of a wide range of
sedimentary environments, and a recognition of the modifications to these
environments and their sediments induced by nearby active diapirs.
Current research interests include the chemical modification of depositional
environments around diapirs, tectonic controls on sedimentation in the
Cooper-Eromanga Basin, and the diagenesis of Permian Cooper Basin
sandstones. An interest in producing analogue models of diapirs has expanded
to include 3D models of extensional fault systems. Nick supervises the
petrography lab at the NCPGG and is interested in improving the
quantification of sediment size and composition through image analysis of
thin sections.