--> Flume Studies With Graded Quartz Powders and a Mixture of Quartz and Kaolinite — Implications for Silt Laminated Shales in the Rock Record

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Flume Studies With Graded Quartz Powders and a Mixture of Quartz and Kaolinite — Implications for Silt Laminated Shales in the Rock Record

Abstract

Natural muds contain a substantial quantity of detrital quartz in the 62 – 1 micron grain size range. For a better understanding of potential transport and depositional processes, a series of flume experiments were conducted with commercial quartz powders that had maximum grain sizes of 50 microns, 40 microns, 30 microns and 5 microns. The finer fraction of each sediment batch was removed in an initial experiment step, and the coarse fraction was tested for critical velocity of sedimentation. These quartz powders show critical velocities of sedimentation of 45cm/sec (50 and 40 microns); 40 cm/sec (30 microns), and 20 cm/sec (5 microns). Ripple migration stopped at 20 cm/sec (50 and 40 microns), 25 cm/sec (30 microns), and 10 cm/sec (5 microns). Floccule formation was observed in the finer grain sizes (30–5 microns) and at smaller flow velocities (probably aided by the formation of biofilm in the flume). The coarser size grades (50, 40, 30 microns) form migrating barchan ripples and the finest grade (5 microns) forms migrating transverse ripples once the flow velocity drops below critical. In a separate step the finest grade (5 microns) and the coarsest grade (50 microns) silica was mixed with kaolinite to observe the interplay between silica and kaolinite under uniform low velocity (20cm/sec) flow regime. Initial observation revealed that the finer component of silica (5 microns) would co-mix with kaolinite forming kaolinite rich laminae with fine silt distributed through the clay matrix. The coarsest component of silica however produces silica rich laminae with little contribution from the kaolinite. Because kaolinite forms bedload floccules and floccule ripples at the used velocity setting, the observations indicate that fine silt grains are integrated into clay floccules and form mixed silt-clay laminae after compaction. The coarse silt forms separate ripples that travel through the flume channel at the same time as floccule ripples, and over time a deposit of interlaminated coarse silt and clay (with fine silt) accumulates. This relationship is directly matched by silt laminated shales form the rock record and suggests a comparable origin. Although in a number of ancient shales, the dissemination of fine silt within clay beds has been interpreted as an indication of eolian input, the process observed in our experiments is most likely a better explanation for the majority of cases.