Sedimentary, microbial and deformation features in the lower Belt Supergroup (ca. 1.45 Ga), western North America: pseudofossils, facies, tides and syndepositional tectonic activity in a Mesoproterozoic intracratonic basin

Abstract

The Belt Supergroup, called the Purcell Supergroup in Canada, accumulated in a broad, rapidly subsiding basin that formed during the oblique collision of an Australian craton with western Laurentia around 1.45 Gyr ago. These rocks are exposed in southwestern Alberta, southeastern British Columbia, and adjacent northwestern Montana, Idaho and northeastern Washington. Emplaced over Upper Cretaceous rocks by the Lewis Thrust fault, the northeastern exposures of the Lower Belt succession are in Waterton and Glacier national parks of Alberta and Montana, as well Castle Wildland Provincial Park of Alberta. The Lower Belt in this region was deposited initially as a west-facing carbonate platform while the basin had considerable bathymetric relief, but it shallowed due to voluminous siliciclastic mud input from the eroding orogen to the west. The Lower Belt has not been studied since the 1980s when the carbonate rocks were interpreted as tidal flat deposits, and a few years later the ‘string-of-beads’ features in mudstones of the lower Appekunny Formation were formally named Horodyskia and regarded as the oldest eukaryotic macrofossils. Thus, the aim of thesis is to approach these rocks in a modern perspective, using a combination of measured sections, photography, serial sectioning, petrography, scanning electron microscopy, X-ray diffraction, CT scanning and synchrotron XRF analysis. Horodyskia has been interpreted as a eukaryotic fossil, and algal, animal and fungal-like affinities have been suggested. Instead, it is shown here to be not a fossil but the preferential binding of mud flakes and flocs onto a wrinkled and tufted microbial benthic mat. The mat, or biofilm, was characterized by elevated pinnacles and ridges that bound these particles creating the appearance of an organised curvilinear structure as the mat continued to build up over time. Although Horodyskia is not a eukaryotic fossil, it is not simply a pseudofossil but a kind of microbially-induced sedimentary structure. The underlying Haig Brook, Tombstone Mountain and Waterton formations indicate that a broad, eastward-shallowing carbonate ramp developed. The carbonate factory was dominated by lime mud production. The overlying Altyn Formation records a prograding carbonate platform succession that culminated in a shallow shelf setting. Five main facies are identified in the Waterton and Altyn formations: laminated mudstone, ribbon limestone, grainstone, oolite and stromatolite patch reefs. The last three were deposited under relatively high energy conditions. In northern sections the quartz and carbonate sand formed shoals reworked by tidal action, whereas in the southern area this facies occurs as allochthonous beds in a deeper, outer shelf setting. Oolite and grainstone locally exhibit large clinoforms which are interpreted as westward-migrating sand bars formed by tidal currents. Extensive tidal flats and sabkhas are inferred to have been present along the coastline on the basis of silicified oolite and anhydrite grains admixed in the grainstone. Not only did tsunamis impact the coast, tsunami off-surge is interpreted as the main agent of transporting coarse inner shelf grains to the middle and outer shelf, and occasionally down the ramp. Tidal effects diminished, then ceased after burial of the carbonate platform by muds from a western source which led to a shallowing of the Belt Basin. Although it was probably major faults in the basin centre that caused the tsunamis, tectonic activity also caused the platform to be wracked by strong earthquakes which generated a variety of syndepositional deformation structures depending on the rheology of the sediments. These seismites include folds, ball-and-pillow structure, cracks, microfaults, breccias, cataclastites, veins and rare molar-tooth structure. Molar-tooth structure also formed in inner-shelf lime muds as evidenced by transported microspar grains in grainstone. The facies-dependent nature of these features corroborates previous work on the Belt Supergroup and other units of pre-Cambrian and Phanerozoic ages. The salient achievements of this study are: (1) elucidation of Horodyskia and the intricacies of mud deposition and microbial mats in the Appekunny Formation; (2) a new interpretation of the sedimentology of the Lower Belt carbonate platform based on facies and inferred environments; (3) the recognition of the effects of strong tides in the Belt Basin early in its development; (4) the recognition of the role of tsunamis in sediment transport on and off the platform; and (5) a better appreciation of the range and attributes of synsedimentary deformation features.