Fake Rocks: How Triassic Carbonate Sediments Converted to Siltstones; An Example From the Montney Formation of Western Canada

Abstract

The Lower Triassic Montney Formation in Alberta is regarded as a classic turbidite sequence sourced from the N-American craton that resulted in finely laminated siltstones. However, new extensive research using high-resolution geochemistry, petrography, cathodoluminescence (CL) SEM, and geostatistical analysis of XRF data implies a carbonate origin. Principal component analysis reveals three groups of correlating elements: 1) Ti, K, Al, etc; 2) Si and Na; and 3) Ca and Mg. In addition, petrographic and CL-SEM investigations illustrate that very few grains are detrital and most classical detrital components, such as quartz and feldspars, are diagenetic in origin. These findings are stunning as this implies that carbonate rocks can be diagenetically altered into what appear to be classic siltstones.

Here we present a full range of new findings including a continuous high-resolution XRF profile of a 220 m (720 ft) core. The Montney Formation was deposited in a back-arc setting along the western margin of the N-American craton and has experienced normal burial pressure and heat gradients. Petrographic and geochemical evidence from selected samples suggests that the original sediments were thick porous bioclastic deposits composed of aragonite, high Mg-calcite, biogenic silica and rare detrital and organic matter. Upon burial, high porosity and excessive diagenetic alterations and dissolution transformed the carbonate/siliceous units into condensed and altered (i.e. laminated) dolomitic siltstones or shales. Pseudo-sedimentary bed structures are preserved due to the nature of the dissolution process.

A new depositional model is proposed for the Lower Triassic Montney Formation, which illustrates a semi-restricted shallow carbonate shelf with prolific shell productivity due to limited species diversity following the Permian-Triassic extinction event. For the new depositional model, oxic bottom water conditions are required. Although previous studies have hypothesized bottom water anoxia, the trace elements and total organic carbon (TOC) contents in this study infer oxic to sub-oxic sea water conditions. A novel and unique diagenetic pathway is put forward demonstrating how quartz and various feldspars (K-feldspar, albite) are generated during diagenesis. This study may have marked implications for understanding other shales and fine-grained rocks as the proposed mineralogical changes appear as a common process.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018