Northern exposures of the Horse Range formation, Trotter's Gorge, East Otago

Mitchell, Mark John, 1969-

The northern exposures of the mid-Cretaceous Horse Range Formation are subdivided into four members, based on dominant lithology and sequence. The basal quartz breccia member comprises of a section of 1-2 m upward fining cycles of subangular quartz- and moderate grade (TZ IV) schist-bearing conglomerate/breccia which is in turn overlain conformably by fine grained carbonaceous mudstone and fine quartz sandstone. This member overlies semischist (TZ IIB) basement in an unconformable depositional contact in the south of the area, closest to the Waihemo Fault Zone.
In the north of the field area, basement is unconformably overlain by the greywacke dominated lower conglomerate member. Well rounded greywacke clasts commonly form up to 95% of basal conglomerate, decreasing slightly to 80- 85% in the upper part of the member. Well rounded cobbles of quartz are the other volumetrically significant component of the lower conglomerate, increasing from 2-5% at the base of the member to 15% at the top. Ubiquitous weakly to unmetamorphosed chert and rarely observed semi-schist clasts form minor components of the conglomerate. Thin, impersistent sandstone lenses and rare, finer grained sediments are minor lithologies. Bedding within conglomerate is typically massive or only crudely developed, and imbrication of clasts rarely observed. The lower conglomerate member extends continuously to the south, where it overlies the quartz breccia member with mild unconformity.
A sandstone/mudstone member, consisting predominantly of poorly sorted quartz sandstone, carbonaceous mudstone and minor coal seams, conformably overlies the lower conglomerate member in the central part of the field area. Both planar and trough cross-stratification is well developed within quartz sandstone beds of the sandstone/mudstone member, with bedding thickness commonly oh the scale of 10-100cm.
Conformably overlying the sandstone/mudstone member is another greywacke dominated conglomerate, the upper conglomerate member. This unit is very similar lithologically, to the lower conglomerate and distinction between the two conglomerate members is very difficult, especially in the north of the field area where the intervening sandstone/mudstone member is absent. Total thickness of the Horse Range Formation within the Trotter's Gorge area is 480 m.
Lithofacies analysis and limited paleocurrent data indicate that the conglomerate members and the sandstone/mudstone member were deposited from braided rivers that flowed from the northwest. The quartz breccia member however, is part of an alluvial fan sequence deposited from currents flowing from the southwest.
The Horse Range Formation was deposited within a half-graben formed immediately adjacent to the northwest trending Waihemo Fault Zone during a period of normal faulting associated with mid-Cretaceous extension. The subangular nature of clasts, and the presence of moderate grade (TZ IV) schist within conglomerate/breccia of the quartz breccia member suggests local derivation from the uplifted southern side of the Waihemo Fault Zone. The dominance of greywacke within the overlying conglomerate members however, suggests that these members were derived from an area of uplifted Torlesse, possibly the result of mid-Cretaceous normal fault movement along the northeast trending Dansey Pass Fault. The Horse Range Formation therefore represents the interaction of two fault sets; one set controlled sediment supply while the other set localised deposition via the creation of a half-graben.
Renewed fault activity and warping in the Late Cretaceous was responsible for the unconformity developed between the Horse Range Formation and the overlying, shallow marine, Herbert Formation. The Herbert Formation consists predominantly of parallel laminated, well sorted, fine quartz sandstone and minor, well rounded, quartz granule conglomerate.
Petrographic studies indicate that both the Horse Range and Herbert Formations have been affected by intra-stratal solutions during diagenesis, resulting in the removal of minerals of reduced chemical stability (e.g. epidote), the alteration of feldspar, and pervasive cementation by calcite.
Minor Eocene to Oligocene volcanic activity is recorded by the olivine dolerite caps present at North and South Peaks.
Late Cenozoic regional compression, associated with the development of the Alpine Fault and modern plate boundary, has resulted in the reactivation of the Waihemo Fault Zone with reverse movement.
Quaternary deposits within the field area consist mainly of Pleistocene stream terraces, rock falls and slumps. Chemical deposits, in the form or travertine and sodium carbonate salts, are presently forming.

101 p., [1] folded leaf of plates : ill. (some col.), maps ; 30 cm.

1990Mitchell

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