A Middle/Late Silurian paleomagnetic pole has been obtained from reefal limestones in north central Indiana. Eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization. One additional site gave the same characteristic direction after great circle analysis. These 12 site‐level directions were used to calculate the mean paleomagnetic pole. Reef flank beds dip steeply (about 38°) away from reef centers. Field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds. Present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12°. A 12° tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement, indicating that the magnetization was acquired early. The presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence. The mean of the site paleomagnetic poles (with sites from steeply inclined flank beds corrected for 12° of tilt) is 16.8°N, 124.9°E (K = 74, A95 = 5.1°), in good agreement with the Middle Silurian Rose Hill pole for North America.

The Mud Hills expose pre-, syn-, and postextensional strata related to Miocene development of the Waterman Hills detachment fault. Synextensional breccia (Mud Hills Formation) was deposited during the final stages of crustal extension of the upper plate (20-18 Ma). Previous workers have misinterpreted fault contacts as stratigraphic contacts, and have developed intricate pseudostratigraphy to explain their observations. Detailed mapping, combined with stratigraphic and sedimentologic data, documents that the volcaniclastic Pickhandle Formation (Miocene) is conformably overlain by the plutoniclastic Mud Hills Formation (Miocene) with no interfingering. Repetition of these south-dipping lithologic units is caused by imbricate, north-dipping listric faults. These relations are demonstrated by the systematic northward "v"ing of fault contacts and southward "v"ing of stratigraphic contacts in canyons. Stratigraphic dips decrease upsection, consistent with incremental rotation of basinal strata during deposition. Most of the Mud Hills Formation consists of rockavalanche breccia and megabreccia derived from Mesozoic granodiorite, which is on June 16, 2015 specialpapers.gsapubs.org Downloaded from identical to basement exposed beneath the Pickhandle and Jackhammer (Miocene?) Formations to the north. A distinctive rhyolite-tuff breccia with granodiorite matrix provides a marker horizon within the Mud Hills Formation; the rhyolite tuff is presumed to have been derived from the Waterman Hills area to the south. The Mud Hills Formation was derived from now-buried granodiorite of a stranded upper-plate block to the south, as demonstrated by northward paleocurrents, and the presence of fine-grained units close to the presumed master fault (as is typical of half-graben sedimentation). Unconformably overlying the Mud Hills Formation is the Owl Conglomerate (basal member of the Barstow Formation), which has mixed provenance with southward paleocurrents; the Owl Conglomerate was derived from residual highlands after extension ceased. Our palinspastic model for development of the Mud Hills and Waterman Hills area includes the following: (1) eruption and deposition of andesitic to rhyodacitic volcaniclastic detritus (Jackhammer and Pickhandle Formations) at 24-21 Ma;(2) initiation of extension no later than 21 Ma; (3) development of a supradetachment basin that was quickly dismembered (22-21 Ma) (strata now located in Waterman Hills); (4) initiation of an upper-plate normal fault with down-to-the-north movement, beginning Mud Hills deposition, just prior to eruption of rhyolite at 20.0 Ma; (5) continued movement on the fault and subsidiary faults, which rotated half-graben strata coincident with sedimentation; (6) cessation of extension before 16 Ma, coincident with deposition of the basal Barstow Formation; (7) accumulation of the Barstow Formation, derived from residual highlands both to the north and the south; and (8) strike-slip faulting and related folding after 10 Ma, which formed the Barstow syncline.Integration of structura...

No abstract

No abstract

No abstract