Sedimentation in the Ritland Impact Structure, western Norway

Abstract

The Ritland structure is the remnants of a 2.7 km diameter, 350 m deep, Early to Middle Cambrian marine impact crater, located in Hjelmeland, western Norway. The impact origin of this structure was proposed in 2001 based on geological mapping. In 2007, further investigation confirmed the presence of rocks of melt origin and the subsequent shock-metamorphic features. Detailed field mapping and sedimentological and mineralogical analyses of the crater infill, explaining the possible depositional mechanisms, sedimentary environments and provenance, were the main objectives of this study. The Ritland crater infill studies were further extended into a comparative analysis with some selected marine crater infills, aiming to achieve a better understanding of the post-impact depositional mechanisms and sedimentary environments.<br><br> The Ritland crater was excavated in the gneissic sub-Cambrian peneplain by a bolide impact of ~115 m diameter. Recent studies on the lower Palaeozoic sediments outside the crater and intercalated ejecta beds indicate that the Ritland target site probably had experienced the Early Cambrian marine transgression when the bolide impacted, possibly into shoreface-close environments in the inner shelf. The Ritland crater infills were broadly classified into three sedimentation stages: A) late syn-impact, B) early post-impact, and C) late post-impact. Late syn-impact crater sedimentation represents an early stage of crater modification, where sediments were derived from collapse of the transient cavity and eventually avalanched down towards the crater centre. In the Ritland crater, extremely clast-rich breccias accompanied with the occasional appearance of reworked particles of melt origin, suggested to be deposited from rock avalanches, probably were deposited within seconds after impact. The rock avalanche deposits were overlain by debris flow deposits representing a water-wet condition related to the resurge of the seawater, thus deposited in the early post-impact stage. A pronounced shift from clast-supported to matrix-supported texture occurred in the upper part of these debris flow deposits. This transition probably reflects flow transformations, where debris flows were transformed into hyperconcentrated density flows during downslope transport, caused by dilution with the gradual ingress of sea water (seeping/breaching through the crater rim). The crater cavity soon (probably hours/days) filled with water and turbidity flows started to dominate. The sandstone succession exposed in the Ritland crater was deposited from turbidity currents, possibly representing minor submarine fans. The coarser clastics were deposited along the unstable, steep crater walls (derived from continued local erosion and slope-failure), while fine-grained sediments were deposited to the crater centre. The clast and matrix mineralogy of the late syn-impact and early post-impact crater-infilling sediments are comparable and dominated by local basement lithologies (target site) suggesting that these sediments were mostly derived from impact-generated debris.<br><br> The late-post impact succession in the Ritland crater is mostly fine-grained, e.g. sandstones, silty shale and shales deposited during later stages in comparatively stable crater conditions. The base of the late post-impact succession is marked by ~6 m of fine- to medium-grained sandstones and alternating fine-grained sandstones and silty shales. A marked increase in the textural maturity (increase in quartz/feldspar ratio), and an evidence of Cambrian marine life within these sandstones (unlike to early post-impact sandstones) suggest a transition from early post impact to late post-impact stage. These successions gradually shift upwards into dark grey to black shales with an about 180 m thick unit in the crater centre. The late post-impact sedimentation in the Ritland crater is suspension dominated with only minor episodes of turbidity current deposition observed in the transitional sandstones. Localised occurrences of coarser clastics within the marine shales along the steep crater wall suggest that gravity slides and screes were sporadically active along the crater wall for a long time after the impact, until the crater was eventually topped by marine clays.<br><br> Comparative analysis of the Ritland crater infill to the Gardnos, Kärdla, Lockne and Chesapeake Bay crater-filling deposits revealed almost a common order of occurrence of different sedimentary processes dominating at the different stages of crater sedimentation. The thickness and compositional variations are explained by differences in crater size, target lithology and water depth.<br><br> Rock avalanches dominated during the late syn-impact stage in the Ritland and Gardnos craters, mainly due to their crystalline target sites covered with only a thin veneer of marine sediments. Bolide impact into target sites with thick sedimentary cover will most likely generate large-scale collapses of the transient cavity and water-saturated rim layers, resulting in repetitive events of rock avalanches, slumping and gravity-slides as observed in the Kärdla and Chesapeake Bay craters. Impact in deeper water target sites, e.g. Lockne, does not show any clear-cut large-scale gravity-collapse but rather represents an early resurge. <br><br> Impact into crystalline target sites with a thin cover of shallow marine sediments in the Ritland and Gardnos craters resulted in well developed and elevated rim formation. The crater rim walls probably acted as a barrier and hindered the large-scale resurge-generated debris flow events within the craters in their early post-impact stage. Impact into thicker sedimentary cover at the target sites resulted in relatively weaker rim formation in the Kärdla and brim development in the Lockne and Chesapeake Bay craters, allowing multiple events of large-scale debris flows. <br><br> Turbidites were commonly observed at the waning stage of the resurge during early post-impact to the late post-impact stage. The Kärdla and Lockne craters represent thick resurge/back-surge turbidites. Ritland and Gardnos turbidites may have partially derived from back-surge but dominantly, most likely, form later slope-failure, erosion and reworking of the impact-derived debris. The turbidites in the Chesapeake Bay crater may either have been derived from the back-surge or from later slope-failure. Suspension deposition dominates in all the compared craters at the late stages when the crater cavities reached a more stable condition.