--> ABSTRACT: Three-Dimensional Braid-Plain Architecture, Bulgo Formation, Sydney Basin, Australia, by Shawn A. Reynolds and Jennifer L. Glasford; #91022 (1989)

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Three-Dimensional Braid-Plain Architecture, Bulgo Formation, Sydney Basin, Australia

Shawn A. Reynolds, Jennifer L. Glasford

The Triassic Bulgo Formation of the Sydney basin was deposited by a basin-wide braid-plain system. Detailed outcrop analysis of facies relationships and correlation with subsurface data allow for recognition of three-dimensional facies architecture and paleogeography reconstruction.

The Lachlan foldbelt was the primary source area for the Bulgo Formation. Three outcrop sites in varying proximity to the source area were examined: (1) Blue Mountains, (2) Burragorrang Walls, and (3) Royal National Park.

Proximal deposits of the Blue Mountains consist of two members. Both the lower and upper Bulgo are coarse-grained, tabular sandstone sheets. They are separated by a paleosoil horizon which is the only criterion for separation.

Medial deposits at Burrangorang Walls can be distinguished on the basis of shale content. The lower Bulgo remains a coarse-grained, tabular sandstone sheet. The upper Bulgo is also a coarse-grained sandstone sheet but contains a significant proportion of interchannel shale deposits and mud plugs.

Distal deposits of the Royal National Park are characterized by a similar coarse-grained lower member which was deposited by a large braid-plain system where smaller braided systems coalesced into a basin-wide tabular sandstone sheet. However, the upper Bulgo member shows a further downdip evolution to a fine-grained sandstone sheet deposited by small, interconnected braided streams which were separated by numerous flood-plain and paludal deposits. This upsection and lateral evolution reflects waning sediment supply and denudation of the Lachlan foldbelt source area.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.