麻豆传媒

Pedogenic mud aggregates are sand and silt sized particles composed largely of clay flakes, and are commonly formed in modern vertisol type soils during seasonal wetting and drying. Where the soils are rich in smectite, clays expand and shrink seasonally, causing aggregation of the flakes under pressure, and the aggregates may be transported as fluvial bedload. A major problem associated with studying mud aggregates is their low preservation potential, as they tend to be destroyed by compaction during burial. The presence of calcretes in the Pennsylvanian Boss Point and Little River formations near Joggins, suggesting semi arid and seasonal conditions, led to a successful search for aggregates using thin sections and the scanning electron microscope (SEM).

Samples from fluvial channel sandstones, crevasse splay sandstones and floodplain mudstones all contain preserved aggregates. Although sand sized aggregates are present, most aggregates are typically 10 15 碌 in diameter in all three sediment types, suggesting a common origin. The abundance of aggregates in these samples is highly variable, ranging from 10 50%. Some of the best occurrences are concentrated in thin laminae of floodplain mudstones, where they occur with a few silt sized grains of quartz and feldspar. Small aggregates are also prominent in large mud fragments within channel sandstones, probably eroded from river bank soils. Chemical analysis of clays using the SEM EDS and XRD techniques shows a predominance of illite with lesser chlorite and kaolinite. Smectite was not observed but may have altered to illite during deep burial diagenesis.

The Boss Point is now one of few formations in the rock record that have yielded pedogenic aggregates. However, many aggregates are an order of magnitude smaller than those identified in most previous studies. They are also unusually well preserved within floodplain muds, where small aggregates were probably concentrated by overbank floods and transported across floodplains and into channels. Aggregate preservation probably reflects early carbonate and hematite cementation and shielding by silicate grain frameworks in coarser sediments, as well as fast burial in the rapidly subsiding Cumberland Basin.

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Pages: 125
Supervisor: Martin Gibling