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USGS1995

Silt fraction heavy-mineral distributions in a lateritic environment: The rivers and insular shelf of north-central Puerto Rico

This fraction, which is enriched in heavy minerals relative to the sand fraction, is mainly detrital but contains a strong authigenic component. The authigenic silt heavy-mineral fraction is largely a product of the lateritic weathering and dominated by iron oxides and alterites. Grains of bladed rutile and leached ilmenite are common. Spatial variability in silt-fraction mineralogy is considerable. Within the Rio Cibuco system variability is related to compositional differences in rapidly eroding source rocks. On the shelf, silt heavy-mineral abundances are greatest at the river mouths and decrease seaward. Variability in the shelf samples is controlled primarily by source rivers and shelf sorting processes.

Puerto Rico↗

USGS1980

Opaque minerals as aids in distinguishing between source and sorting effects on beach sand mineralogy in southwestern Oregon

Both source area and wave sorting affect the heavy-mineral distributions of beach sands. Although source and sorting effects are difficult to distinguish, they can be separated on four Oregon beaches by studying the magnetic fraction of the sand. Prominent headlands bound the four beaches, which are located on the southwestern Oregon coast. On three beaches the percentage of magnetite in the sand from the upper swash zone consistently increases toward the north end of each beach, apparently owing to selective sorting during littoral transport. However, the percentages of chromium and titanium in the magnetite are generally independent of the relative location of the samples along the beach and therefore independent of sorting effects. Each beach appears to be characterized by a fairly distinct range of Ti/Cr in the magnetic fraction, and the range differs from beach to beach. The magnetite percentage and the Ti/Cr range suggest that sorting and source effects can be distinguished on beaches rich in "black" sands, and that sand transport around headlands in this area is not extensive enough to mask source differences. The results of this study suggest that sorting and source effects can be distinguished wherever there are chemical or mineralogical differences in a restricted density fraction.

Journal of Sedimentary Petrology↗