DARK STRIPES IN SEDIMENTARY RECORDS: MINERALOGICAL DENSITY LAGS IN SANDS

The evolution of clastic sedimentary systems has been punctuated by high-energy events which are often manifested by unconformities and coarse lag deposits. In modern and ancient sand-dominated depositional environments, morphological indicators of erosion or substantial increase in flow regime are often complemented by lithological anomalies, such as heavy-mineral concentrations (HMCs; >10% mineral fraction with density >2.9). In coastal sequences, HMCs represent density lag formation during episodes of increased wave or wind activity. This paper presents examples from Holocene coastal barrier and dune sequences in different parts of the world to illustrate the modes of HMC occurrence and their use as paleoenvironmental indicators. Common for many post-storm beaches, most enriched horizons are also easily identifiable in sediment cores and trenches. Where HMC thickness is comparable to the resolution of geophysical methods (e.g., ground-penetrating radar) they may produce sharp subsurface reflections. Continuity of geophysical records is particularly valuable where enriched layers are not exposed in outcrops or captured by sediment cores. Whereas long-term variations in the flux of accessory minerals (e.g., due to changes in tectonic or climatic regime) will control their background content in sands and sandstones, individual bed-scale enrichment is a reliable indicator of high-energy events. In aeolian systems, HMCs occur as continuous layers on primary lateral accretion surfaces or as isolated lenses related to small-scale bedform migration. They range from a few millimeters to >5 cm in thickness and likely reflect greater near-surface flow stresses during periods of increased wind action. Where HMCs are enriched in magnetite, magnetic susceptibility may be used to assess its enrichment relative to enclosing quartz-rich sands. For example, depending on magnetite content, in-situ bulk susceptibility values (x10^-5 SI units) range from 50 to >300 in enriched horizons, in contrast to <5 for background dune sands. Quantitative analysis of aeolian HMC texture and composition can be used to constrain sediment transport conditions and may serve as a potential proxy for wind intensity on decadal-to-millennial scales, which is an important component of paleoclimate research.