Abstract The first commercial quantities of natural gas were gathered and marketed in 1927 from the Muskegon field of southwestern Michigan. From this date to the present, gas production in Michigan has been from formations that range in geologic age from Mississippian to Ordovician. The total gas produced in the state and metered to market through December 1965 was 485.8 billion cu ft. The remaining recoverable reserve from known productive structures is approximately 236.6 billion cu ft. The highly populated centers in Michigan demand far greater quantities of gas than can be produced in the state. Accordingly, Michigan must rely upon interstate pipeline facilities for adequate natural gas supplies. Because of weather variations, approximately one-third of such supplies must be stored in the summer months and withdrawn during the winter months from local underground gas-storage reservoirs. The quantity of gas imported into the state during 1965 was 546.7 billion cu ft, exceeding by about 61 billion cu ft the total quantity of gas produced throughout the history of the state. Certain gas fields in the Michigan basin have provided excellent underground storage reservoirs for the natural gas industry and are very important to the general economic welfare of the state. Future gas reserves from within the state must come from areas and depths where there has been little or no exploratory drilling. Recent discoveries in the Trenton-Black River of south- central Michigan and the Salina-Niagara of southeastern Michigan have given impetus to the search for oil and gas in the state.

ABSTRACT The Carboniferous Michigan Basin is the subject of conflicting interpretations resulting from the lack of detailed stratigraphic analysis of relevant rock units. In this study, an ~610 m (2000 ft) section of recently acquired core material was evaluated on the basis of lithofacies and stacking patterns, stratigraphic contacts, and well-established regional geologic relations of Mississippian and Pennsylvanian strata. The Bayport formation is composed of seven distinct primary depositional lithofacies reflecting open-marine and shoal-water to restricted peritidal environments, typically capped by an exposure surface. Carbonate-dominated strata of the Bayport formation are interstratified but ultimately transition up section into siliciclastic-dominated strata (previously called the Parma Sandstone) deposited in tidally influenced, estuarine facies. Late Mississippian Bayport strata are sharply overlain by Pennsylvanian-aged siliciclastic lithofacies of the Saginaw Formation. These facies were deposited in a range of terrestrial and marginal-marine environments, from coarse-grained fluvial sandstones at the base (previously known as the Grand River Formation), to the finer-grained channel sandstones and floodplain mudstones of mixed fluvial and estuarine systems in the middle Saginaw Formation. Carbonaceous shales, mudstones, and thin coal intervals characterize the middle to upper Saginaw Formation. In the southern Michigan Basin, an important unconformity at the Mississippian-Pennsylvanian contact is represented by either an incised valley-fill succession or a prominent paleosol above the Bayport formation at the base of the Absaroka section in the Saginaw Formation. In upthrown areas adjacent to a major wrench fault, the Lucas fault in south-central Michigan, the Bayport formation is transitional upward from an intensely karsted limestone to a red-bed paleosol and then to primarily carbonaceous mudrock of the Saginaw Formation. In downthrown areas adjacent to the fault, the formation contact, and systemic unconformity, is a sandstone-on-sandstone contact. Climate-sensitive strata indicate a significant transition from predominantly arid conditions in the Mississippian Bayport formation to humid climate conditions in the Pennsylvanian Saginaw Formation across the Mississippian-Pennsylvanian systemic boundary. Previously, the Bayport formation was considered Meramecian in age; however, palynologic analyses of samples collected from core within the interval indicate a Chesterian (late Mississippian) age, representing a significant revision of existing Michigan Basin stratigraphy.

This paper describes three mid-Tertiary intrusions from the Henry Mountains (Utah, USA) that were assembled from amalgamation of multiple horizontal sheet-like magma pulses in the absence of regional deformation. The three-dimensional intrusion geometries are exceptionally well preserved and include: (1) a highly lobate sill; (2) a laccolith; and (3) a bysmalith (a cylindrical, fault-bounded, piston-like laccolith). Individual intrusive sheets are recognised on the margins of the bodies by stacked lobate contacts, and within the intrusions by both intercalated sedimentary wallrock and formation of solid-state fabrics. Finally, conduits feeding these intrusions were mostly sub-horizontal and pipe-like, as determined by both direct observation and modelling of geophysical data. The intrusion geometries, in aggregate, are interpreted to reflect the time evolution of an idealised upper crustal pluton. These intrusions initiate as sills, evolve into laccoliths, and eventually become piston-like bysmaliths. The emplacement of multiple magma sheets was rapid and pulsed; the largest intrusion was assembled in less than 100 years. The magmatic fabrics are interpreted as recording the internal flow of the sheets preserved by fast cooling rates in the upper crust. Because there are multiple magma sheets, fabrics may vary vertically as different sheets are traversed. These bodies provide unambiguous evidence that some intrusions are emplaced in multiple pulses, and that igneous assembly can be highly heterogeneous in both space and time. The features diagnostic of pulsed assembly observed in these small intrusions can be easily destroyed in larger plutons, particularly in tectonically active regions.

ABSTRACT The Middle Devonian Sylvania Sandstone in the Michigan Basin, United States, is noteworthy for its mixed, cherty dolomitic carbonate, limestone, and sandstone composition and excellent reservoir quality and fluid-flow properties in many areas of the basin. Substantial commercial brine production was initiated in the middle twentieth century, and liquid waste disposal continues today in the Sylvania Sandstone, although little or no hydrocarbon production is known from this unit. The Sylvania Sandstone pinches out to the south and west in the basin and overlies either the Bass Islands Group at the regional base Kaskaskia unconformity to the south or the Bois Blanc Formation, with which it is proposed to be in facies relationship to the northeast. The Sylvania Sandstone is overlain by and apparently interfingers with the Meldrum Member of the Amherstburg Formation throughout the basin. The Sylvania unit predominantly consists of siliciclastic rocks in the southeastern Michigan Basin and in outcrop in Ohio, but it is transitional to predominantly cherty carbonate in the northwest along a depositional hinge striking from southeast to northwest through the central basin. This hinge zone is dominated by mixed carbonate and siliciclastic strata deposited in normal-salinity, tidally influenced paralic and shallow-marine environments. Excellent reservoir quality is present in quartz sandstones and, especially, mixed sandy dolomite lithofacies in the central basin. Cherty facies may also possess significant porosity, but typically with low permeability. Limestone lithofacies are commonly non-reservoir-quality facies. Multiple high-frequency, low-magnitude relative sea-level cycles within the Sylvania Sandstone are suggested from regional stratigraphic analysis. The Sylvania Sandstone in the Michigan Basin is interpreted as a mixed carbonate and siliciclastic, basin-margin facies assemblage deposited in an overall transgressive systems tract above the base Kaskaskia unconformity and in conformable relationship with more basinal facies of the Bois Blanc Formation.