Seismic profiles in Lake Como show many small (>100 m, 328 ft), low angle reflections overlain and onlapped by horizontal reflections (A-A' and B-B' below). These features represent small-scale subsidence with subsequent infilling. The areas of localized subsidence have been mapped out in the figure showing areas of subsidence. The subsidence features appear to be controlled at depth by collapse in the underlying structure. This is shown in profile B-B' with downwarped reflections and subsidence-related faulting (type 3). The near surface fill is nearly acoustically transparent and is possibly homogeneous sands from the surrounding sand ridges infilling the depressions. In places the overburden appears to be displaced and rotated as it slumps into the depression (north shore red number 2, Track Map).
Location of Subsurface Subsidence Areas
Gamma counts from wells in the vicinity place the top of the Hawthorn Group at about -9 m (-30 ft) NGVD and the top of the Ocala Limestone at about -20 m (-65 ft) NGVD (wells P-0114, P-0246, see Index Map D and gamma log cross-section B-B'). The reflection shown as a red dashed line in the seismic profiles may represent a horizon near the top of the Hawthorn Group. Subsidence in the Hawthorn Group sediments, as a result of structural collapse in the underlying Ocala Limestone, would provide recharge pathways to the aquifer. The near surface undifferentiated fill appears to be relatively intact, although some subsidence or breaches may be present as shown in profile B-B'.