Variability of Mudstone Microstructure in a Freshwater to Marine Transect: Upper Cretaceous Delta System, Alberta and British Columbia

Abstract

Although the microstructure of marine mudstone has now been documented in a number of studies, little is known of the microstructure of mudstone deposited in fluvial and lacustrine settings. The present study took advantage of a well-established allostratigraphic framework in the Cenomanian Dunvegan Formation in Alberta and British Columbia that enabled varied depositional environments to be related in time and space. The Dunvegan Fm. includes sediments deposited in coastal plain lakes and rivers and floodplains, through large paleovalleys, to delta distributaries, proximal and distal prodelta. Box cores from unlithified mudstones were collected across a dip-oriented transect of allomember E of the Dunvegan alloformation that was deposited in a range of terrestrial, nearshore, and offshore environments. Scanning electron microscopy (SEM) surface and backscattered electron imaging techniques were used to observe the relationship between the mudstone microstructure and depositional environments. Backscattered electron images of mudstones deposited within a coastal plain lake exhibit an array of clay domains (∼0.5 to 1 μm) and face-face aggregates (∼2 to 5 μm) that commonly have a bedding-parallel preferred orientation. Coeval valley fill siltstones contain quartz grains and mudstone lithoclasts (∼10 to 80 μm) within a matrix of randomly oriented clay particles. Contemporaneous prodelta mudstones mark the first appearance of abundant clusters of face-face aggregates forming intraclastic aggregates (∼5 to 20 μm). These intraclastic aggregates can be difficult to differentiate from the surrounding matrix due to similar backscatter coefficients. Images taken from prodelta mudstones sampled further offshore display similar domain and face-face aggregates as the lateral shallow water equivalents. The relatively deeper marine intraclastic aggregates, however, are frequently well defined from the matrix material. In summary, SEM analysis of sedimentary facies shows a transition from an absence of intraclastic aggregates in fluvial and estuarine settings to more abundant intraclastic aggregates in proximal to distal prodelta locations. These observations suggest that clay mineral particles behave differently in fresh and marine environments, therefore, salinity may be a fundamental factor responsible for converting clay sediments into consolidated clay aggregates as they are reworked along the sediment transport system.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016