Modelo pieza de bloqueo en L

The principal rock unit of the FRV at Cadia is a massive and unsorted, polymict pebble to cobble volcanic lithic conglomerate, with volumetrically minor sandstone units (Lithofacies 1). Locally these rocks are normally graded, and pass with decreasing grain size from massive, unsorted pebble and cobble conglomerates at the base to fine grained thinly bedded volcanic sandstones at the top. Reverse grading occurs locally and graded units are typically 10 – 25 m thick. The well- rounded nature of the majority of lithic fragments indicates storage and reworking of clasts in a high-energy shoreline environment prior to transport and deposition as massive and very thickly bedded units. Volcanic conglomerates of Lithofacies 1 form laterally extensive sheets (2 – 3 kilometres E-W) throughout the Cadia district, indicating that deposition of the conglomerates was not focused along submarine channels or canyons.

Rocks of Lithofacies 1 are interpreted to be subaqueous high-density gravity flow deposits (Fig. 3.17A). Most units show many of the characteristics typical of volcaniclastic debris flows, as outlined by McPhie et al (1993). Clast transport in debris flows is gravity-driven, with particle support provided by a sediment-water matrix (Lowe, 1982). Debris flows are very poorly sorted and comprise sandy ma- trix- or clast-supported pebble- to cobble-size particles (McPhie et al., 1993). These

features are typical of the volcanic conglomerate subunits (OVC, OVP) of Lithofacies

1. Additionally, the tabular geometry and thick nature (1 - > 100 m ) of

volcaniclastic debris flows (McPhie et al., 1993) are characteristic of the OVC and

OVP subunits. Volcaniclastic debris flows can travel for considerable distances (up

to 100 km; McPhie et al., 1993) and consequently are of little use in determining proximity to a volcanic edifice.

Squire (2001b) interpreted the coarse grained, massive to very thickly bedded units of the FRV to have been deposited entirely from high-density turbidity

Chapter 3. District Geology 62 s 1 s 1 s3 s3 A. 465 - 455 Ma.Forest Reefs Volcanics deposition

B. Structural setting of RIC emplacement Basaltic debris flows

Ridgeway Intrusive Complex (RIC) and Western Cadia Intrusive Complex

Monzonite to quartz monzonite porphyry Mafic monzonite Late Or dovician

Basaltic clast volcanic conglomerate Forest Reefs Volcanics

Lithofacies 1

Andesitic clast volcanic conglomerate Plagioclase-rich volcanic sandstone

OVC

OVP

OVS

Massive clinopyroxene-phyric lava Lithofacies 4 Clinopyroxene-phyric subvolcanic intrusions Plagioclase-phyric subvolcanic intrusions OVF OIC OIP Weemalla Formation

Laminated feldspathic siltstone Middle

Ordovician Andesitic debris flows

C. 449 - 448 Ma.Late Ordovician (Ea3) uplift

Bedded calcareous volcanic sandstone Lithofacies 3 OB2 Late Ordovician Andesitic debris flows? Figure 3.17. A. B.

Geological model for the development of the Cadia district. Mid Ordovician to early Late

Ordovician (~465-455 Ma). Sketch of the geology of 5280RL level, Ridgeway, showing the interpreted

stress conditions at time of intrusion (see Figure 3.13 for geological details). Middle Late Ordovician

period of limestone deposition in the eastern half of the Cadia district. See text for discussion. C.

AMG North _North

Fault RIC Western CIC Northwest _Southeast Ea3 limestone North Fault Purple Fault? Ridgeway

currents. Tubidity currents are gravity-driven flows of cohesionless sediments, in which particles are suspended by interstitial fluid turbulence (Lowe, 1982; McPhie et al., 1993). Two end member types exist, low-density and high-density turbidity currents (Lowe, 1982). The products of low-density turbidity currents are characterised by upwardly-fining packages of sand- and silt-sized sediments commonly referred to as Bouma sequences (Bouma, 1962). In contrast, high-density turbidity currents transport sand- to granule-size particles (Lowe, 1982), although up to 15% pebble- and cobble-size particles may also be transported (McPhie et al., 1993). High-density turbidites comprise basal units of normal- to locally reverse- graded, coarse-grained (granule to cobble) conglomerates deposited through traction and suspension (Lowe, 1982). These coarse-grained units are ideally overlain by massive or normally graded, fine-grained sandstone and siltstone deposited from direct suspension. High-density turbidites may travel several hundreds of kilometres from their source (McPhie et al., 1993), but Walker (1975) and Lowe (1982) suggest that the coarse-grained facies of these sequences will be deposited proximal to the source area.

The predominance of andesitic clasts in lithofacies 1 at Ridgeway contrasts with an abundance of basaltic clasts in the same rock type east of the Cadiangullong Fault Zone (Figs. 3.3 and 3.17A). This suggests either that volcanic debris was derived from two compositionally district source regions (Fig. 3.17A) or that the andesite-clast and basaltic clast volcanic conglomerates were not coeval and have been structurally juxtaposed along the Cadiangullong Fault Zone. Determination of the relative age of both of these units may help to resolve this issue.

Normally graded packages of volcanic lithic conglomerate to feldspathic siltstone exist in the upper part of the FRV at Ridgeway and locally at Cadia East and may be high-density turbidites. At Ridgeway, repeated units of normally graded basal pebble conglomerate pass upwards into thinly bedded feldspathic volcanic

sandstone (OVS subunit). The presence of these units higher in the FRV sequence

may mark the transition from a distal setting, as suggested by the Weemalla Formation siltstones and overlying volcaniclastic debris flows of the FRV, to one more proximal to a igneous source.

Chapter 3. District Geology

Short-lived periods of below wave-base sedimentation by direct suspension are suggested by the planar laminated volcanic siltstones at Cadia Far East (Lithofacies 2, Figs. 3.3 and 3.4E). Conditions of shallow-water marine sedimentation during the late Eastonian are indicated by the bedded calcareous volcanic sandstone (Lithofacies 3; Fig. 3.17B) at Big Cadia and Little Cadia. Limestones of this age are widespread throughout the Macquarie Arc (Section 2.3.2) and appear to mark a period of regional uplift, possibly in response to growth of the volcanic arc.

The massive clinopyroxene- and plagioclase-phyric rocks that occur throughout the FRV (Lithofacies 4) have both intrusive and extrusive origins (Fig. 3.17A). Quenched groundmass textures and brecciated lava flow tops with finely laminated inter-fragment sediment, in addition to pillow basalts reported by Squire (2001b), are textures that can be interpreted to suggest that at least some of the massive volcanic units were extrusive in nature. Conversely, some of these units have a holocrystalline groundmass, indicative of relatively slow cooling in a hypabyssal environment. Squire (2001b) reported the presence of blocky and fluidal peperites, indicative of the emplacement of a magma into wet, unconsolidated sediments (Busby-Spera and White, 1987). This indicates that some rocks of Lithofacies 4 are sub-volcanic intrusions and that this intrusive activity was broadly coeval with sedimentation.

In summary, interpretation of the Weemalla Formation and FRV lithofacies suggests that the Cadia district underwent a transition from a positional distal to a volcanic centre during the Darriwilian to early Gisbornian (469 – 458 Ma; Fig. 3.17A) to one proximal to an emergent volcano during the late Eastonian (450 – 447 Ma; Fig. 3.17C).