Print this pageAbstract: Speleothems and Evaporite Solution Collapse in Athapuscow Aulacogen (Middle Precambrian), Great Slave Lake, Northwest Territories
Basinal limestone rhythmites of the Pethei Group are sharply overlain by the Stark Formation, a polyphase megabreccia about 1 km thick, in which blocks of indigenous shallow-water carbonate rocks are dispersed chaotically in a matrix of brecciated red beds. Toward the top of the megabreccia, progressively more coherent red beds, containing beds of reworked detritus from the carbonate blocks below, pass gradationally upward into nonbrecciated alluvial sandstone. Halite casts are ubiquitous in the red beds and, more important, also occur in the uppermost 10 cm of the basinal limestone. Chalcedonic concretions, perhaps originally anhydrite or gypsum, are present in the basinal limestone below the halite casts. It is hypothesized that the basinal Pethei limestone once was ove lain by a thick basin-filling sequence of salt, above which the shallow-water Stark carbonate sediments and red beds were deposited. The megabreccia is believed to be the result primarily of solution collapse, further disrupted by gravity sliding during red bed sedimentation.
An unusual lithostrome, here termed LAB rock (laminated accretionary breccia), occurs in three modes: (1) as a single bed about 1 m thick between the basinal limestone and the megabreccia; (2) as an incrustation around certain carbonate blocks within the megabreccia; and (3) as dislodged blocks within the megabreccia. The LAB rocks consist of plastered fragments and fragmental layers blanketed with laminated travertine, in places characterized by centimeter-scale tubercles that give the deposit a strongly accretionary aspect and which, although probably not biogenic, resemble certain stromatolites. The LAB rocks typically contain variable amounts of silica, in the form of tiny chalcedonic blebs or prismatic crystals of low-temperature quartz. The LAB rocks must have been formed during solution collapse, perhaps as cave deposits.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma