Chlorite Morphotypes in Upper Ordovician Proglacial Sequences – Insights From Backscattered SEM and EDS Analysis

Grain-coating chlorite plays a crucial role in preserving sandstone porosity during burial by inhibiting quartz cementation from occluding primary interparticle pore spaces (Ehrenberg, 1993). The physical characteristics of clay coats are critical in this process, with laterally continuous coats generally resulting in greater porosity preservation. Although clay coats do not completely inhibit the precipitation of quartz overgrowths, they strongly quench their growth and development. Experimental data and physical observations reveal that micro-quartz overgrowths can nucleate behind the clay coats, exploiting the microporosity between the clays and the detrital grains. However, the coats act as a physical barrier that limits the ability of these incipient quartz crystals to coalesce into larger pore-filling fabrics (Ajdukiewicz & Larese, 2012). While chlorite generally has a positive impact on porosity preservation, extensive chlorite precipitation may occlude pore throats, reducing rock permeability (Dowey et al., 2012; Worden et al., 2020). Additionally, pore-lining chlorite can significantly impact the petrophysical properties of sandstones, leading to a low resistivity response, which may result in the misidentification of economically viable fairways as low pay-zones (Durand et al., 2000). The formation of grain-coating chlorite is controlled by factors such as closed diagenetic systems, precursor clay availability, and iron supply (Worden et al., 2020). A detailed review of published studies by Dowey et al. (2012) found that chlorite coats are found in a wide range of depositional settings, including deltaic, fluvial, marine, and lacustrine settings, but they are most commonly found in delta-related environments (Dowey et al., 2012). Iron-rich chlorites are most commonly found in coastal environments, whereas mixed iron- and magnesium-rich chlorites are more commonly associated with marine and terrestrial sandstones. Typically, Fe-rich chlorite coats characterize marginal marine environments, such as estuaries and deltas, in a wide range of latitudinal settings. The Fe-rich chlorites usually occur between 50°N and 50°S (Worden et al., 2020; Rateev et al., 2008). In such settings, detrital iron-bearing minerals are delivered to marginal marine systems by rivers. Early diagenetic alteration of these components releases iron, which can be fixed into iron-silicate precursors, such as berthierine, under reducing conditions (Slack et al., 1992; Dowey et al., 2012). These metastable precursors later transform into Fe-rich chlorite during burial diagenesis and metamorphism at depths greater than 2-3 km and temperatures between 60°C and 100°C (Jahren & Aagaard, 1989; Burley & MacQuaker, 1992; Ehrenberg, 1993; Grigsby, 1999; Aagaard et al., 2000). The onset temperature of 60°C is lower than the onset temperature of 80°C for significant quartz overgrowth cementation (Walderhaug, 2009). Sequence stratigraphy also appears to influence the distribution of chlorite, which tends to be more common in highstand and falling stage system tracts (Morad et al., 2012). The distribution of clay minerals and clay coatings can vary significantly depending on the specific sequence stratigraphic setting and local geological conditions. This work investigates somehow atypical Fe-rich chlorite rims that characterize sandstone sequences deposited in proglacial marine settings during the retreat of the Hirnantian ice sheet from the Gondwana passive margin of Arabia.