Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa

A characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found. The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup. In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation. Palaeocurrents of the Middelvlei placer are towards the south-southeast in the western parts of the study area and towards the south-east in the east. Maximum pebble size, however, decreases towards the west-southwest, that is in a direction roughly perpendicular to the palaeocurrents. A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike. The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward-thinning wedge of underlying strata. The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.     

Multiple dolomitization events along the Pozalagua Fault (Pozalagua Quarry,Basque–Cantabrian Basin,Northern Spain)

The Pozalagua Quarry in the Basque–Cantabrian Basin of northern Spain exposes a unique set of fault‐associated dolomites that can be studied on a decametre scale. The dolomites developed along the Pozalagua Fault system in slope‐deposited limestones of Albian age. Following marine phreatic diagenesis, the limestones were subject to meteoric karst formation. The resulting cavities were filled either by angular limestone fragments in a black clay‐rich matrix, or by cave floor/pond (now dolomitized) sediments. The subsequent diagenetic history reflects repeated periods of fracturing, fluid expulsion, dissolution and cementation. Contrasting fluid pulses resulted in the formation of a network of hydrothermal karst and the subsequent development of coarse‐crystalline calcite cement, zebra dolomite, recrystallized coarse‐crystalline dolomite, elongated blue–grey coarse‐crystalline dolomite cement in the open fault and, finally, coarse‐crystalline saddle dolomite. Decimetre‐size reworked host‐rock fragments present in the latter two dolomite phases probably reflect roof collapse fragments of a cave system that developed along the Pozalagua Fault system. However, there are also metre‐scale host‐rock fragments that apparently ‘float’ in the coarse‐crystalline saddle dolomites, implying that either fragment assimilation was a widespread process or violent expulsion of fluids occurred along the Pozalagua Fault system. The presence of pre‐dolomite and post‐dolomite stylolites, parallel to bedding, supports a linkage between the diagenetic events and the Late Albian tectonism that affected the region.