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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Early Diagenesis, Atherton Formation (Quaternary): A Guide for Understanding Early Cement Distribution and Grain Modifications in Nonmarine Deposits
W. C. James
The Atherton Formation represents an extensive sheet of outwash and related materials of Quaternary age. These sand (Q77F8R15) and gravel sediments are locally lithified and present an excellent opportunity for evaluating the effects of early diagenesis on a terrigenous deposit.
The principal cement present is calcium carbonate (> 98%), occurring as euhedral to anhedral grain contact, pore-lining and porefilling varieties. Limonite and gypsum are present as rare cements. Cement distribution was controlled at various levels by (1) stratification character, (2) grain size and sorting, (3) framework grain composition, and (4) proximity to near-surface processes. Initial cement development and subsequent early growth were primarily parallel to laminae within beds. This precipitation-site preference is related to initially higher relative permeabilities (estimated at 2 to 2.5 x ) parallel versus normal to stratification. When associated with trough cross-strata, cements preferentially are located in the lower portion of troughs or directly below trough-set boun aries. There is a slight preference for early formed cements to be present along the axis, instead of the flanks of a trough. About 50% of pebble gravel and sandy pebble gravel units contain locally cemented zones, while 17% of finer-grain-size strata are locally lithified. Over 95% of locally cemented units are within 7 m of the present ground surface, suggestive of a causal relation to near-surface, perhaps soil-related processes.
The sources for the various constituents composing cements are related to the dissolution of previously cemented layers and framework components such as carbonate rock fragments, calcareous pelitic rock fragments, and iron-bearing silicate grains. The examination of oversized voids, elongate pores, and intraparticle porosity suggests that, at least locally, multiple episodes of cementation followed by dissolution have taken place. Reactions between carbonate cement and limestone rock fragments have destroyed a high percentage of detrital carbonate-grain boundaries. Such reactions combined with early dissolution can result in an information loss in excess of 50% of the limestone rock fragment component in a sandstone. A combination of organic acids and CO2 mixed with meteori waters is likely responsible for the dissolution and transportation of cement constituents.
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