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.

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