The magmatic stratigraphy of the Sudbury Igneous Complex (Canada) is thought to have resulted from closed-system differentiation of an initially homogeneous impact melt sheet. The topography of its upward-growing chamber floor is therefore thought to have been planar and subhorizontal. However, we report on the discovery of a large pothole-like depression (~300 m in depth and ~550 m in width) in the chamber floor of this complex. The depression has been revealed through two-dimensional mapping of igneous layering that is defined by systematic vertical changes in cumulus assemblages and bulk rock chemistry. Although the formation of the depression by syn- to post-magmatic folding and/or slumping of chamber floor cumulates cannot be completely excluded, we favor an alternative explanation that follows from the recent recognition that the Sudbury Igneous Complex melt sheet crystallized concurrently from the floor and roof inward. The roof sequence was subsequently disrupted and collapsed as large discrete blocks onto the floor sequence. This may have resulted in local irregularities in topography of the upward-growing chamber floor so that crystal deposition onto and between the neighboring blocks produced pothole-like depressions. The phenomenon of physical disruption of roof sequences appears to provide a reasonable explanation for the common lack of the rocks that grew from the roof downward in layered intrusions.

中文翻译：

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加拿大萨德伯里火成岩体室底的坑洞状凹陷

萨德伯里火成岩杂岩体（加拿大）的岩浆地层被认为是由最初均质的冲击熔体片的封闭系统分异产生的。因此，其向上生长的房间地板的地形被认为是平面和亚水平的。然而，我们报告在该综合体的腔室地板上发现了一个大型坑洞状凹陷（深度约为 300 m，宽度约为 550 m）。该凹陷已通过火成岩分层的二维映射揭示，该分层由积云组合和大块岩石化学的系统垂直变化定义。虽然不能完全排除由同岩浆后褶皱和/或室底堆积物的坍塌形成的凹陷，我们赞成另一种解释，即根据最近的认识，即萨德伯里火成岩复合体熔体板同时从地板和屋顶向内结晶。屋顶序列随后被破坏并作为大的离散块倒塌到地板序列上。这可能导致向上生长的腔室底板的局部地形不规则，使得晶体沉积在相邻块上和相邻块之间产生坑洞状凹陷。顶板序列的物理破坏现象似乎为普遍缺乏从顶板向下生长成层状侵入体的岩石提供了合理的解释。屋顶序列随后被破坏并作为大的离散块倒塌到地板序列上。这可能导致向上生长的腔室底板的局部地形不规则，使得晶体沉积在相邻块上和相邻块之间产生坑洞状凹陷。顶板序列的物理破坏现象似乎为普遍缺乏从顶板向下生长成层状侵入体的岩石提供了合理的解释。屋顶序列随后被破坏并作为大的离散块倒塌到地板序列上。这可能导致向上生长的腔室底板的局部地形不规则，使得晶体沉积在相邻块上和相邻块之间产生坑洞状凹陷。顶板序列的物理破坏现象似乎为普遍缺乏从顶板向下生长成层状侵入体的岩石提供了合理的解释。