William J. Sando

LOWER ORDOVICIAN SECTION NEAR CHAMBERSBURG, PENNSYLVANIA

The Lower Ordovician series (Beekmantown group) is at least 3500 feet thick near Chambersburg, Pennsylvania, approximately 1200 feet thicker than was earlier estimated. Reverse faulting in the upper part of the section may have obscured the youngest Early Ordovician and oldest Middle Ordovician beds in the area, so that the total thickness of the Lower Ordovician series may be comparable to the thickness measured in Maryland. The Beekmantown group is represented by the Stonehenge limestone and the Rockdale Run formation. Moving the base of the Stonehenge downward to include about 200 feet of strata previously referred to the underlying Conococheague formation facilitates regional correlation without impairing mappability. Newly discovered fossils from the Rockdale Run formation add notably to the faunas of the upper part of that formation. Comparison of a trilobite-brachiopod faunule near the top of the Diparelasma zone (Cotter-Powell equivalent) with similar fossils in Zone J of the Utah-Nevada sequence suggests that Zone J may be no younger than the Powell dolomite of the Ozark region. Difficulty in recognizing correlatives of the Smithville and Black Rock formations suggests the need for restudy of these units in the Ozark region. Evidence from the Chambersburg section neither proves nor contradicts the hypothesis of continuous deposition from Early Ordovician to Middle Ordovician time in the central Appalachian area.

Endemism and similarity indices: Clues to the zoogeography of North American Mississippian corals

The distribution of coralliferous facies and the degrees of endemism and generic similarities of Mississippian coral faunas permit recognition of five zoogeographic provinces and five zoogeographic sub-provinces in North America: Alaskan province, Pacific Coast province (including Northern and Southern sub-provinces), Western Interior province (including Northern, Central, and Southern sub-provinces), Southeastern province, and Maritime province. Analysis of indices of endemism and similarity suggest the following major conclusions: (1) Zoogeographic regions on the periphery of the North American continent (Alaskan, Pacific Coast, and Maritime provinces) had favorable connections for migration to other coralliferous areas of the world, which permitted maximum gene flow; (2) zoogeographic regions in the interior of the North American Continent (Western Interior and Southeastern provinces) were relatively isolated genetically and were characterized by coral faunas having low to high endemism throughout Mississippian time; (3) gene flow was highest along continuous shallow-water carbonate shelves and was impeded by areas of terrigenous sedimentation and areas of deeper water; and (4) similarities between faunas of different zoogeographic regions generally tend to vary inversely with the migration-route distance between these regions, but other factors that affected gene flow modified the distribution patterns significantly.

Stratigraphic and Economic Significance of Mississippian Sequence at North Georgetown Canyon, Idaho

Recognition of the Mississippian sequence exposed at North Georgetown Canyon, Idaho, as a facies belt largely different from those already known in ranges on the east and west adds significantly to knowledge of the Mississippian stratigraphy and petroleum geology in the Overthrust belt of Idaho, Wyoming, and Utah. In the newly recognized facies belt in the Aspen Range, the Madison Group is represented by the Lodgepole Limestone and Mission Canyon Limestone, but only the lower part of the latter is present. The greatly thinned Mission Canyon is the westernmost known occurrence of the formation. The overlying beds of middle Osagean to middle Chesterian age are a unique combination of deep-basin shallowing upward to peritidal lithofacies that are included in a new formation, the Aspen Range Formation. An organic-rich phosphatic member at the base of the Aspen Range Formation is a possible petroleum source rock. Also, westward thinning of the Mission Canyon Limestone into the Aspen Range and its absence farther west suggest a westward pinch-out that may provide stratigraphic traps for petroleum beneath a seal formed by the phosphatic member of the Aspen Range Formation.

A New Member of the Madison Limestone (Mississippian) in Wyoming

The new name Bull Ridge Member of the Madison Limestone is proposed for a widespread stratigraphic unit of early Meramec age in central Wyoming. This name replaces the name Sacajawea as used by some authors in the Wind River and Owl Creek Mountains.

New Biostratigraphic and Paleotectonic Interpretation of Devonian and Mississippian Rocks in Southwestern Montana Thrust Belt: ABSTRACT

The structurally complex area of previously undifferentiated Mississippian rocks below the Meramecian Kibbey Sandstone in the southwestern Montana thrust belt of the northern Tendoy Range actually comprises an unusual combination of Devonian and Mississippian basinal, slope, platform, and nearshore deposits. The lower part of this sequence comprised two widespread western Montana formations: the Devonian part of the Three Forks Formation over 65 m (213 ft) thick, containing all three members, and the basinal Kinderhookian and Osagean Paine Limestone, 228 m (748 ft) thick. However, the Paine is succeeded by an eastern tongue of the older, Osagean part of the lower-slope Middle Canyon Formation, 266 m (872 ft) thick. This tongue consists of clinoform cherty micrite and bedd d chert, with some encrinite debris flows that increase upward in number and thickness. The overlying Osagean and Meramecian Mission Canyon Limestone comprises 102 m (335 ft) of an upper-slope encrinite lower member and 74 m (243 ft) of a mainly shelf-margin wackestone and encrinite upper member. Thus, the Mission Canyon here represents the distal part of a broad carbonate platform. Succeeding the Mission Canyon is 140 m (459 ft) of the newly named Meramecian McKenzie Canyon Limestone, which comprises a sabkha, back-mound, and lagoonal sequence of evaporite-solution breccia, micrite, dismicrite, and pelmicrite, with interbeds of crinoidal wackestone, encrinite, and bio-oopelsparite. This formation represents beds absent at an unconformity elsewhere in western Montana and western Wyoming. Thus, the sequence from the Paine through the McKenzie Canyon, constituting the newly named Tendoy Group is 810 m (2,657 ft) thick and represents one of the most complete records of Kinderhookian to Meramecian carbonate deposition in the northwestern United States, displayed in the predominantly upward-shallowing part of a eustatic megacycle. The reconstructed detailed biostratigraphy aids new structural interpretations and provides several new plays for petroleum exploration in the Montana thrust belt. End_of_Article - Last_Page 865------------

Coral Zonules: New Tools for Petroleum Exploration in Mission Canyon Limestone and Charles Formation, Williston Basin, North Dakota: ABSTRACT

Study of the distribution of corals and rock types in the Mission Canyon Limestone and the lower part of the Charles Formation (Tilston, Frobisher-Alida, and Ratcliffe intervals) in 29 cores from wells in the Williston basin of western North Dakota resulted in recognition of four coral zonules and three regressive carbonate cycles. Although coral diversity and abundance decrease eastward toward the basin margin and upward in the sequence because of the influence of increasingly restricted environments, two of the zonules in the lower part of the Mission Canyon extend into areas of western North Dakota where marker-defined intervals are difficult or impossible to recognize. The Nesson anticline, or a paleotopographic ridge following the same trend, may have been a barrier hat hindered coral development in the east during later Madison deposition. Parallelism between the zonules and marker beds used to define standard intervals employed in subsurface stratigraphic correlation indicated that the marker beds are essentially time lines within the area studied. The first records of Stelechophyllum micrum and S. banffense in Madison rocks in the United States indicate a connection with the Alberta shelf and indicate that North Dakota was probably a part of the Central Western Interior subprovince during Osagean time. End_of_Article - Last_Page 870------------

Structural Geometry of Newly Defined Blacktail Salient of Montana Thrust Belt: ABSTRACT

Complexly imbricated Upper Devonian and Mississippian rocks in the northeastern Tendoy Mountains, Montana, form the previously unrecognized McKenzie thrust system, which is south of and structurally above the south-plunging Armstead anticline and north of the Tendoy thrust sheet. The northern margin of the McKenzie system, east of Garfield Canyon, displays a minimum of 4 mi (6 km) of eastward displacement. The southeastern margin is south of Kelmbeck Creek, near McKnight Canyon. The eastern edge of the system is buried under Quaternary to Late Cretaceous cover at or east of Red Rock Valley. East of the McKenzie system, the front of the Montana thrust belt extends north-northeast from Dell, Montana, to the eastern Blacktail Range, on the basis of unpublished mapping by J. . Haley and W. C. Pecora, Jr. The convex eastward curvature of the thrust belt in this area, including the McKenzie thrust system, is herein designated the Blacktail salient. Imbricates of the McKenzie thrust system comprise two duplex fault zones between Bell and McKenzie Canyons. The lower duplex involves a unique suite of platform to basinal Kinderhookian to lower Meramecian (Mississippian) carbonate rocks as well as Upper Devonian rocks. The End_Page 858------------------------------ floor thrust of this imbricate stack appears to lie within the Upper Devonian Three Forks Formation; the roof thrust lies within the middle Meramecian Kibbey Sandstone. The upper duplex involves Upper Mississippian rocks above the Kibbey Sandstone. Its roof thrust closely follows bedding near the top of the Mississippian sequence. The geometry of imbricate stacks within the McKenzie plate demands shortening of greater than 100%, resulting in at least 2 mi (3 km) additional eastward displacement of its trailing edge. Recognition of the Blacktail salient with its complex structural patterns and unusual platform to basinal carbonate sequence provides new exploration targets in the southwestern part of the Montana thrust belt. End_of_Article - Last_Page 859------------

Mississippian Shelf Margin and Carbonate Platform from Montana to Nevada: ABSTRACT

The Kinderhookian to middle Meramecian history of a carbonate platform and shelf margin, extending from Nevada to Montana, is documented through four time-rock correlation charts and five successive maps that are synchronized by foraminiferan, conodont, and coral zonations. The platform was bordered on the west by a starved basin, a flysch trough, and orogenic highlands. The history of platform development is an integral part of the sedimentary cycle of the deep-water Deseret starved basin. Antler orogenic activity produced epeirogenic movements on the craton, which affected sea level and caused episodic progradation and retreat of the carbonate shelf margin. The sequential history is: (1) in earliest Mississippian time a narrow, northeast-trending seaway bordered by low oastal plains received mostly fine clastic sediments; (2) during late Kinderhookian time, a carbonate platform and shelf margin formed as a result of eastward expansion of the seaway; (3) during early Osagean time, the shelf margin retreated and a broad, gentle (less than 0°5^prime) clinoform ramp developed; (4) during middle Osagean time, lowering of the basin and craton and rise of sea level changed the pattern and sedimentary regime of the carbonate platform. Progradation of the shelf margin over the former ramp resulted in maximum expansion of the platform concurrent with maximum deepening of the starved basin. The foreslope attained a maximum steepness of 5°; (5) in middle Meramecian time, uplift of the craton and lowering of sea level caused shoaling of the carbonate plat orm and development of a sabkha landward. With increased uplift a karst plain developed over most of the former carbonate platform, and some cratonic sands were transported westward by streams into the basin. Meanwhile, filling of the flysch trough allowed eastward spillover of distal-flysch sediments to almost completely fill the basin. End_of_Article - Last_Page 716------------

Mississippian Carbonate Shelf Margin Along Overthrust Belt from Montana to Nevada: ABSTRACT

A constructional carbonate platform and a generally north-trending shelf margin in Utah and southwestern Idaho were bordered on the west by a starved basin, flysch trough, and orogenic highland during Kinderhookian to middle Meramecian time. The Antler orogeny produced epeirogenic movements, which resulted in sea-level changes that caused the carbonate platform episodically to prograde and retreat. At different times the shelf was bordered either by a narrow foreslope or by a broad ramp. The sequential history is as follows: (1) Late Devonian thrusting raised the continental margin to produce the Antler orogenic highlands, which in earliest Mississippian time had a low eastern coastal plain that bordered a narrow, shallow marine basin lacking a distinct eastern shelf. (2) Widespread marine inundation of the craton on the east was followed by a stillstand, during which a low shelf margin that turned abruptly eastward in Montana was developed and deposition of clinoform micritic limestone beds occurred in moderately deep water across a very broad ramp. (3) Increased downwarping produced an incipient starved basin, separated by a shallow carbonate bank from the flysch trough on the west and by a broad ramp from the northeast-trending shelf margin on the east; coarse encrinites were deposited alternately with micrites on the ramp. (4) Maximum deepening and expansion of the starved basin were accompanied on the west by deepening of the carbonate bank and on the east by westward progradation of a carbonate platform with a narrow, steep foreslope. (5) Lowering o sea level produced a karst plateau on the former carbonate platform and caused cratonic sands to be carried westward into the basin. Meanwhile, filling of the flysch trough allowed an eastward spillover of distal flysch sediments into the basin. The starved-basin sediments, which have organic-carbon values as high as 7% in outcrop, are considered to be source rocks. Coarse sediments of the carbonate platform, particularly where dolomitized, may serve as petroleum reservoirs. End_of_Article - Last_Page 828------------