Menefee Formation Research Articles

Variations in Cretaceous Coal-Bearing Strata, Gallup Coal Field, New Mexico: ABSTRACT

Cretaceous coals of the western Gallup field, New Mexico, occur with detrital wedges that interfinger to the northwest with brackish-marine sediments of the San Juan basin. This study documents detailed stratigraphic relations and the relation of coal occurrence to depositional environments of the Gallup, Crevasse Canyon, and Menefee formations. One hundred and fifteen sections form the basis for three-dimensional reconstructions of a 30-sq km area northwest of Gallup. The regressive Gallup Sandstone represents reworking of river-mouth sands into coastal barriers where coals accumulated in back-barrier subfacies. This formation grades upward into the Dilco Coal Member of the Crevasse Canyon Formation characterized by varve-like fine-grained sediments, thin sandstones, and coals, which pass upward into thick, northwesterly transported fluvial sandstones. These merge laterally and upward into northerly oriented, thick paleochannel sandstones of the Bartlett Barren Member of the Crevasse Canyon Formation. The coal-bearing Gibson Coal Member of the Crevasse Canyon Formation, which cannot be differentiated from the Cleary Coal Member of the overlying Menefee Formation, contains finer grained sediments and coals deposited in a broad interfluvial depressio bounded on the west by Bartlett alluvial channel facies. Coal accumulation in this depression was terminated by southeasterly oriented, crevasse-like deposits associated with thick fluvial sandstones (Menefee Formation). Stratigraphic variations of coal beds are directly related to their proximity to contemporaneous channel facies. Uniformly thick coals trend subparallel to channel facies; near the channel facies, coals become erratic and pass into rooted, carbonaceous overbank detritus. Coals are offset locally by faults caused by differential compaction beneath overlying channel sandstones. End_of_Article - Last_Page 689------------

Coal Deposits of San Juan Basin: ABSTRACT

Coal is present in some degree through nearly the entire range of Cretaceous strata in the San Juan basin. Economically significant deposits in the southern part of the basin are present in the Gallup Sandstone, the Dilco and Gibson Coal members of the Crevasse Canyon Formation, and in the Cleary Coal Member of the Menefee Formation. All these units belong to the Mesaverde Group. In the central and northern parts of the basin, minable and potentially minable coals occur in the Lower and Upper coal members of the Menefee Formation (Mesaverde Group) and in the younger Fruitland Formation. Coal in the Dakota Sandstone may have an economic potential in the Cortez area of southwestern Colorado. The coal is the result of accumulation of vegetal debris principally within the shore-marginal paludal environmental band. Deposition in this zone was more or less constantly shifting as the sea either invaded the land area southwestward or withdrew toward the northeast. The orientation of the strand line, averaging about N55°W, is thought to have influenced the axial alinement of individual coal lenses. Zones and individual beds deposited under transgressive conditions become younger on the southwest, and, conversely, regressively deposited coal beds and zones are younger on the northeast. Particularly thick accumulations of coal are likely to occur in (1) areas of relative shoreline stability which are likely to be associated with major reversals in the direction of shoreline m vement and (2) in conjunction with minor reversals in shoreline movement during major phases. End_of_Article - Last_Page 955------------

Symmetry, Stratigraphy, and Petrography of Cyclic Cretaceous Deposits of San Juan Basin: ABSTRACT

Late Cretaceous strata of the San Juan basin consist of cyclically interstratified non-marine, nearshore marine, and offshore marine clastic sediments which were deposited both during marine transgressions and regressions. Thickness of the transgressive and regressive parts of these cyclic sequences varies, permitting subdivision into two types of cycles: symmetrical and asymmetrical. In symmetrical cycles the thickness of transgressive and regressive parts are nearly equal; in asymmetrical cycles the transgressive sandstone is absent. The Hosta-Point Lookout wedge is an example of a symmetrical cycle. At its base the transgressive marine Hosta Sandstone overlies continental strata of the Crevasse Canyon Formation. The Hosta Sandstone grades upward into the offshore marine Satan Shale. The Satan Shale marks the midpoint of the cycle and the maximum marine inundation; it grades upward into the regressive marine Point Lookout Sandstone. The Point Lookout is overlain by the continental Menefee Formation. Southwestward, toward the former shoreline, the Satan Shale pinches out and the transgressive and regressive sandstones merge into a single massive sandstone, which still farther southwestward grades into continental strata. The Mulatto-Dalton cycle is asymmetrical for it lacks a basal transgressive sandstone. Instead, the offshore Mulatto Shale directly overlies the continental Dilco Coal with only scattered marine sand lenses at the contact. The Mulatto Shale grades southwestward (toward former shoreline) and upward into the regressive marine Dalton Sandstone which in turn grades southwestward into, and is overlain by, continental deposits of the Crevasse Canyon Formation. Petrography is closely related to the sandstone depositional environments as follows. Regressive sandstone type: Upward increase in maximum and median grain diameter; upward decrease in abundance of primary dolomite grains. These vertical changes reflect progressive shallowing of the basin during regression. Transgressive sandstone type: Upward decrease in maximum and median grain diameter; upward increase in abundance of primary dolomite grains. These vertical changes reflect progressive deepening during transgression. Non-marine sandstone type: Wide range of grain sizes; primary dolomite grains absent; abundant carbonaceous material; higher proportion of non-quartz detritus. Petrography and the cycle concept are useful for recognizing and solving correlation problems.

Symmetry, Stratigraphy, and Petrography of Cyclic Cretaceous Deposits in San Juan Basin

Late Cretaceous strata of the San Juan basin consist of cyclically interstratified non-marine, nearshore marine, and offshore marine clastic sediments which were deposited both during marine transgressions and regressions. Thickness of the transgressive and regressive parts of these cyclic sequences varies, permitting subdivision into two types of cycles: symmetrical and asymmetrical. In symmetrical cycles the thickness of transgressive and regressive parts are nearly equal; in asymmetrical cycles the transgressive sandstone is absent. The Hosta-Point Lookout wedge is an example of a symmetrical cycle. At its base the transgressive marine Hosta Sandstone overlies continental strata of the Crevasse Canyon Formation. The Hosta Sandstone grades upward into the offshore marine Satan Shale. The Satan Shale marks the midpoint of the cycle and the maximum marine inundation; it grades upward into the regressive marine Point Lookout Sandstone. The Point Lookout is overlain by the continental Menefee Formation. Southwestward, toward the former shoreline, the Satan Shale pinches out and the transgressive and regressive sandstones merge into a single massive sandstone, which still farther southwestward grades into continental strata. The Mulatto-Dalton cycle is asymmetrical for it lacks a basal transgressive sandstone. Instead, the offshore Mulatto Shale directly overlies the continental Dilco Coal with only scattered marine sand lenses at the contact. The Mulatto Shale grades southwestward (toward former shoreline) and upward into the regressive marine Dalton Sandstone which in turn grades southwestward into, and is overlain by, continental deposits of the Crevasse Canyon Formation. Petrography is closely related to the sandstone depositional environments as follows. Regressive sandstone type: Upward increase in maximum and median grain diameter; upward decrease in abundance of primary dolomite grains. These vertical changes reflect progressive shallowing of the basin during regression. Transgressive sandstone type: Upward decrease in maximum and median grain diameter; upward increase in abundance of primary dolomite grains. These vertical changes reflect progressive deepening during transgression. Non-marine sandstone type: Wide range of grain sizes; primary dolomite grains absent; abundant carbonaceous material; higher proportion of non-quartz detritus. Petrography and the cycle concept are useful for recognizing and solving correlation problems.

Symmetry, Stratigraphy, and Petrography of Cyclic Cretaceous Deposits of the San Juan Basin: ABSTRACT

Late Cretaceous strata of the San Juan Basin consist cyclically interstratified non-marine, nearshore marine, and offshore marine clastic sediments which were deposited both during marine transgressions and regressions. Thickness of the transgressive and regressive parts of these cyclic sequences varies, permitting subdivision into two types of cycles: symmetrical and asymmetrical. In symmetrical cycles the thickness of transgressive and regressive parts are nearly equal; in asymmetrical cycles the transgressive sandstone is thin or absent. The Hosta-Point Lookout wedge is an example of a symmetrical cycle. At its base the transgressive marine Hosta Sandstone overlies non-marine strata of the Crevasse Canyon Formation. The Hosta Sandstone grades upward into the offshore marine Satan Shale. The Satan Shale marks the mid-point of the cycle and the maximum marine inundation; it grades upward into the regressive marine Point Lookout Sandstone. The Point Lookout is overlain by the non-marine Menefee Formation. Southwestward, toward the former shoreline, the Satan Shale pinches out and the transgressive and regressive sandstones merge into a single massive sandstone, which is also called the Point Lookout Sandstone. Still farther southwestward this massive sandstone grades into non-marine strata of the Crevasse Canyon and Mene ee formations. The Mulatto-Dalton cycle is asymmetrical for it lacks a basal transgressive sandstone. Instead, the offshore Mulatto Shale directly overlies the non-marine Dilco Coal with only scattered marine sand lenses at the contact. The Mulatto Shale grades southwestward (toward the former shoreline) and upward into the regressive marine Dalton Sandstone which in turn grades southwestward into, and is overlain by, non-marine deposits of the Crevasse Canyon Formation. Petrography is closely related to the sandstone depositional environments as follows. Table These petrographic properties may be used to identify and correlate units in problem areas. End_of_Article - Last_Page 369------------

Revised Nomenclature of Mesaverde Group in San Juan Basin, New Mexico

Mesaverde group is substituted for Mesaverde formation throughout the San Juan basin, and the formations of the type locality, the Point Lookout sandstone, Menefee formation, and Cliff House sandstone, are also extended throughout the basin. Several names for units formerly called members of the Mesaverde formation in the southern part of the basin are retained as names of tongues or members of the formations of the Mesaverde group. The name Gallup sandstone replaces the name Tocito sandstone lentil of the Mancos shale. Crevasse Canyon formation of Allen and Balk (1954) is accepted for that part of the Mesaverde group between the Gallup and Point Lookout sandstones, with the Gibson coal member (restricted) at its top. A new name, the Cleary coal member of the Menefee form tion, is proposed for the beds formerly included in the upper part of the Gibson coal member of the Mesaverde formation.