SEISMIC STRATIGRAPHY OF THE 1.1 GA MIDCONTINENT RIFT BENEATH WESTERN LAKE SUPERIOR PART I: SUBSIDENCE AS A MAJOR DRIVER ON BASIN DEVELOPMENT (Invited Presentation)

Seismic facies and reflection geometries mapped across 2D reflection profiles characterize basin fill and inform development of the 1.1 Ga Midcontinent Rift beneath western Lake Superior. We reprocess three industry lines and one public GLIMPCE line, and we digitize and geolocate seven published images of industry lines for interpretation in a 3D visualization software.

Subparallel, continuous reflections record subaerial volcanic flows and perhaps sills. Discontinuous high-angle reflections locally abut low-angle, arcuate reflections, suggesting sheet intrusions within preexisting sedimentary basins. Thick intervals of chaotic seismic facies and discontinuous, subparallel reflections indicate pre-rift crust. Grand Marais Ridge (GMR), a gravity low and seismic high south of Grand Marais, MN, is previously recognized as pre-rift crust that separates rift sub-basins. Subparallel, continuous reflections infill separate sub-basins with synform geometry and increasing width between reflections towards synform axes. Locally south of GMR a discrete interval of subparallel continuous reflections thickens and abruptly terminates towards GMR, suggesting fault low in the volcanic section. Sub-basin geometry differs east and west of this potential fault with a deeper, strikingly symmetric basin to the west. South and east, basins flatten and synforms roll into small antiforms against GMR. Antiforms likely overlie reverse fault splays with relatively minor offset that sole into high amplitude, continuous reflections.

Doubly-plunging synform geometry with increased reflection spacing towards synform axes is the dominant feature under western Lake Superior, locally modified by geometries indicative of normal and reverse faulting. Here, syndepositional subsidence may be the primary control on basin development and preserved configuration. Normal and reverse faulting likely have local impact. Thickening against a possible fault may represent offset of material that thickened into the axes of subsiding basins, rather than growth strata against a fault. Sills intruded into sedimentary basins may be significant. Mafic underplating below basins is an intuitive mechanism for subsidence but is not supported by seismic or gravity data. Mechanisms of subsidence remain unresolved.