The form of several sheet intrusions in a sandstone-shale host rock is regular, but much more complex than the term “sheet intrusion” implies. The periphery of a sill emanating from the Shonkin Sag laccolith in Montana is composed of fingers of igneous rock with lengths of about 100 m, widths of 3 to 5 m, and thicknesses which vary from about 1.2 m near their points of coalescence to about 0.6 m near their terminations. At finger terminations, the host rock is wedged aside and compacted, and along the sides of fingers, buckling and shearing of strata are common. Energy expended by magma to dilate and flow through a sheet is small compared to that for fingers. Initiation of fingers is attributed to instability of the advancing interface between a viscous magma and a more viscous host rock. Fingers coalesce into a sheet such that cusp-shaped grooves filled with deformed host rock remain in the contact. Systematic offsets of sheet-intrusion contacts form where offset fingers coalesce. Several large dikes (5 to 20 m thick) near La Veta, Colorado, exhibit contact offsets of as much as 3 m in magnitude, which give the dike the appearance of a clapboard wall. Small dikelets, horns, often project from the outer edge of offsets. If fingers grow in a surface perpendicular to the least principal compressive stress, offsets may develop as the intrusion propagates through a region where this stress changes orientation. Grooves and offsets form parallel to finger lengths and thus indicate the propagation direction of the intrusion. Flow of magma subsequent to finger coalescence may not be parallel to the propagation direction.

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