INTENSE STRAIN DURING POROSITY COLLAPSE OF ENTRADA SANDSTONE DURING EMPLACEMENT OF MAGMA FINGERS: VOLUME ACCOMMODATION FOR SMALL INTRUSIONS IN THE HENRY MOUNTAINS, UTAH

Strain analyses on the Entrada Sandstone reveals that the volume for small intrusions in the Henry Mountains may be produced by porosity collapse of crushed clastic grains. The Jurassic Entrada Sandstone is a high porosity eolian red sandstone with m-scale cross-beds. It is very fine to fine-grained quartz arenite and is moderately to well sorted. The Maiden Creek Sill is a 25 Ma shallow intrusion composed of a dominantly covered central portion and four protruding magma fingers that are well exposed and intrude the Entrada Sandstone. Analyses of the magma fingers and surrounding sandstone reveals that there is no uplift of surrounding rock adjacent to or above the fingers. Fingers vary in width between 30 and 100m and can be up to 50m in height. Fracturing is prevalent in the sandstone adjacent to these fingers, at various orientations, but no or very little displacements are observed. There is a 50 cm thick brittle shear zone that separates the sandstone layers containing the sill and fingers to the sandstone layers above. Fry analyses have been conducted on photographs of thin sections from 16 sandstone samples collected at various horizontal distances from one magma finger. Samples were collected from distances varying from 50 cm to 30 m. All sandstone samples exhibit extremely deformed grains. Approximately 50% of grains exhibit multiple fractures that are displaced into pore spaces. Many grains exhibit such intense fracturing that initial grain boundaries are indistinguishable and shapes are defined by surrounding grains. Fry strain analyses were conducted using the program of Vollmer, F.W. (2018). One thin section was analyzed per sample, oriented perpendicular to the margin of the finger and four locations were analyzed from each thin section. Strain ellipses vary in orientation within thin sections and between samples although 48% of strain ellipses have a long axis within 30° of vertical. The long axis of ellipses is oriented top towards and away from the finger margin. Strain ratios vary between 1 and 2.26 with an average of ~1.45. Our model postulates that magma volume is created by horizontal displacement of adjacent sandstone and accommodated by crushing of grains and porosity collapse, consistent with shear zone development at the boundary between the magma finger horizon and overlying undeformed sandstone.