Abstract Hartzog Draw field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sand¬stone at depths from 9,000 to 9,600 ft (2,727 to 2,910 m). The producing interval consists of a large mid- to outer shelf sand-ridge complex deposited well below effective normal wave base more than 100 mi (160 km) from shore. The productive interval in the shelf-ridge complex has a maximum thickness of 60 ft (18 m), and averages 20 ft (6 m) in thickness. The field is 22 mi (35 km) long and is as much as 3-1/2 mi (5.8 km) wide. Since its discovery in 1975, over 175 primary production wells were completed on 160-acre spacing. Initial oil-in-place was estimated to be 350 million barrels. Secondary waterflood was initiated in 1981 and 115 additional infield wells were to be drilled by the end of 1985. The reservoir is completely enveloped in shale, has a solution gas drive, no water table and no produced formation water. Net pay is primarily a product of porosity, permeability and thickness of the sandstone, and is directly related to sedimentary facies. Detailed studies of five cores located in the northern, eastern, and central portions of the field allow definition of nine facies. Three of the facies are primarily high-angle crossbedded sandstones; the other facies show a variety of low-energy features including ripples and abundant burrowing. The Central Ridge Facies, a high-angle trough crossbedded slightly glauconitic quartz sandstone, is a consistently high quality reservoir. The High-Energy Ridge-Margin Facies, a crossbedded highly glauconitic sandstone containing siderite and clay rip-up clasts is also a relatively high quality reservoir; the Low-Energy Ridge-Margin Facies, which consists of interbedded ripples and troughs, and the Inter-Ridge Facies (Shaly), a rippled interbedded sandstone and shale, generally are marginal quality to non-reservoirs. The average porosity for the field is 12% and the average permeability is 12 md. Higher mean values are recorded in the producing intervals of the five cores studied. Values for the Central Ridge Facies are 15% and 15 md and for the High-Energy Ridge-Margin Facies are 14% and 19 md. Sandstone isopach maps and cross sections perpendicular to the field elongation show that the field is asymmetrical and considerably steeper on the northeast flank. Paleocurrent flow directions inferred from oriented cores indicate a southerly flow of the currents responsible for deposition of the Hartzog Draw shelf-ridge complex.

Abstract El Dorado field, which is located in southeastern Kansas, was discovered in 1915 by Cities Service Company. The Admire 650’ sandstone reservoir (Permian Wolfcampian) is present at 650 ft (197 m), ranges in thickness from 11 to 23 ft (3 to 7 m) and has produced 36.5 million barrels of oil through primary and secondary recovery methods. A secondary waterflood was carried out in the field during the 1950s. A tertiary oil recovery project covering 51 acres (21 ha) was initiated in the Admire 650’ sandstone in El Dorado field in 1974. Deltaic facies are encountered at El Dorado field; the various deltaic facies are quite variable in reservoir quality. Documentation of the geometries and locations of reservoir and non-reservoir facies allows prediction of the distribution of flow units within the field. Three types of sandstone reservoirs are recognized in the Admire 650’ sandstone. The major producing reservoir facies is the Distributary Channel Sandstone Facies, which includes High- and Low-Energy Subfacies. The Admire 650’ sandstone was initially deposited from splay channels that prograded into a muddy interdistributary bay. Following deposition of some of the splays, distributary channels prograded into the enhanced oil recovery pilot project area (El Dorado Micellar-Polymer Demonstration Project). Some parts of the project area, however, contain substantially thicker distributary channel sandstones than others. In the southeastern corner no channel sandstones were deposited; the area remained an interdistributary bay environment. The final phase of deposition was one in which transgressive marine deposits spread over the whole area depositing limestones and shales. The Distribuuiry Channel Sandstone Facies is most prevalent along the western margin of the northern (Chesney) and southern (Hegberg) leases, as well as across the center of the project area. Flow directions obtained from oriented cores indicate northward and northeastward paleocurrent flow within a distributary channel system which bifurcates within the lease areas. Reservoir heterogeneities depicted on cross sections, fence diagrams, and isopach maps were recognized in cores, logs, and whole-core analysis. Abundant high quality geologic data were available for this project and were used to formulate detailed reservoir descriptions. The reservoir descriptions were utilized to some degree by engineers to better model the field. One engineering parameter which strongly supports the presence of geologic heterogeneity in El Dorado was pressure-transient analysis. Pressure-transient ratios, as much as 14 in areas of recognizable elongate sandstone body deposition, indicate many areas of strongly preferred transmissibility. Distributary Channel Sandstones average 436 md permeability and 28% porosity, whereas splay channel sandstones average 567 md and 27% porosity. Splay sandstones are thinner and discontinuous and, hence, do not contribute as much to overall production. The variation in permeability and porosity of the Admire 650’ sandstone is affected by diagenesis. Permeability and porosity are reduced by clay laminae, deformation of ductile rock fragments, mica and micaceous laminae, quartz, the secondary leaching of feldspar, and calcite cement.

Abstract A wide variety of processes have operated on the seabed of the shelves of the world in the past including perhaps storms, permanent currents, wind induced alongshore currents, wave modified currents, subtidal tidal currents and turbidity currents. These processes generated sand bodies with different geometries which commonly contain different sedimentary structures or different sequences of sedimentary structures. On ancient shelves the most common sedimentary structures observed in vertical sections of sandstones are planar-tangential to planar-tabular cross beds, horizontal to subhorizontal laminations, current ripples, wave rippleswave modified current ripples and burrowed and bioturbated (>75% burrowed) sandstones. This sequence is in approximate order of decreasing energy (fluid power). Where consistent vertical sequences of sedimentary structures are observed, one of the most common reflects upward increase in depositional energy. However, a sequence reflecting upward increasing energy and consequent increase in grain size is not unique to shelf sandstones; a similar coarsening upward pattern is reflected in subsurface log patterns in both river- and wave-dominated deltas and in beach/ barrier dominated shorelines. Ancient sandstone examples used to characterize a variety of these processes, geometries, and shelf locations include the “Gallup” (Tocito), Shannon, Fales and Frontier Sandstones from the Cretaceous of the Western Interior. In addition modern Atlantic shelf and North Sea systems are discussed. Shelf sandstones may be classified on the basis of their position on the shelf (shoreface-attached, inner shelf, middle shelf, outer shelf) and on the basis of whether they are deposited during a transgression, regression, or a stillstand. Both vertical and lateral sequences of lithologies vary with position on the shelf, processes of deposition, and position within a transgression-regression spectra. On the middle and outer shelf, shelf sandstones are almost always surrounded by shale. On the inner shelf, and where attached to the shoreface, shelf sandstones overlie a variety of lithologies (sandstone, siltstone and shale) dependant in part on whether they were deposited during a transgression, regression or stillstand. Lithologies deposited lateral to shelf sandstones also vary with the position of the sand body within the spectra of transgression-regression. Vertical and lateral sequences of lithologies areprobably the most variable on the inner shelf. Local topography also may affect the distribution of shelf sandstones. Winnowing of the seabed in areas which are topographically high may concentrate sand into sand ridges. Depressions in the shelf sea floor may result from erosion of lithologies which have different susceptabilities to erosion (i.e., strike valley sands), during sea level drops, by shoreface retreat or as a result of submarine erosion. Depressions on the sea floormay fill with fine- to coarse-grained sand.

Abstract As an introductory step, this paper defines continental shelves and briefly discusses their origin and evolution. Most of the paper is concerned with the large-scale tidal and storm-driven fluid circulation patterns of the continental shelves and the manner in which these flows entrain and move sediment. It is essential to understand these circulation patterns in order to understand the distribution of facies on continental shelves. However, oceanic currents on a rotating planet are complex and their pattern is not intuitively obvious. Therefore, a considerable portion of the chapter is devoted to an analysis of the mechanisms of shelf flow, and the importance of these mechanisms in determining shelf sediment transport. Storm-driven and tidal currents are considered in turn. The shoreface and inner shelf together constitute a gateway through which all shelf sediments must pass, and the complex flows of the shoreface and inner shelf are described in detail. Finally, fluid and sediment dynamics at the shelf edge are reviewed.

Abstract The storm and tidal currents that sweep the surfaces of continental shelves imprint a variety of morphologic and textural patterns on these surfaces. As the surfaces aggrade, the grain size gradients and bedform arrays become the textures, structures and stratification patterns of the resulting sedimentary sequences. This paper describes textural gradients and bedform arrays characteristic of shelf surfaces, and the process of strata formation.

Abstract Two linear sand ridges from the nearshore and middle portion of the New Jersey Continental Shelf were sampled using vibracores and box cores. Lithologic descriptions were made of the cores based on epoxy peels, X-ray radiographs, and impregnated core slabs and grain size analysis. Vibracores obtained for the study have an average penetration of 6 m (20 ft.) and 95% recovery. Box cores sampled lithologies and relative abundance of physical and biogenic structures found in the upper 25 to 46 cm (9.8 to 18.1 in.) of the sediment. Bottom topographies were established on the basis of 3.5 kHz seismic data. The nearshore sand ridge sampled (72°22'W, 39°19'N) exceed 5 km (3 mi) in length and ranges up to 2 km (1.2 mi) in width and has a relief of 6 to 10 m (20 to 33 ft.). The mid-shelf ridge (74°08'W, 39°09'N) is nearly 4 km (2.5 mi) long, up to 1 km (0.6 mi) wide, and has a relief of 10 to 11 m (33 to 36 ft.). Three to four general lithologic units were recognized; these may be common to both ridges. At the base of many of the cores, nonskeletal mud and poorly sorted sands are present; some of the interlayered sands and muds contain laminations and abundant pebbles. Overlying this unit in the nearshore ridge is a shell-rich mud and sand interval that is relatively massive (bioturbated). This lithology was also recovered in one core from the middle shelf ridge. C-14 dates taken from the shell-rich

Abstract This paper is designed to review the geological evidence for storm deposits, and will only peripherally discuss modern processes. The papers in this volume by Swift give excellent coverage of the day-by-day and year-by-year processes which operate on modern shelves. However, the rarer events have a low probability of being observed or measured, yet the deposits of such events are probably abundant in the geological record. Thus, the record adds to, as well as compliments the body of knowledge acquired by oceanographers and marine geologists. It will be suggested, for example, that turbidites occur in ancient shallow marine situations, commonly with a periodicity of about 1000–10,000 years. There are no well established examples of modern turbidity currents that have deposited preservable beds in a shelf or shallow marine situation. It must be emphasized, therefore, that the geologist will inevitably have a different perspective on shelf storm deposits from that of a geological oceanographer. This part of the notes is chronologically subdivided as follows: 1899, Silurian storm deposits in New York State; discussion of G. K. Gilbert's deduction of 20 m storm waves in the Medina ocean. 1967, Hurricanes Carla and Cindy. The work of Hayes can be taken as the beginning of “modern” geological studies of storm deposits. 1971-1975, Miscellaneous storm deposits. In this section, several storm interpretations are introduced, emphasizing both the nature of the deposit and the emplacing mechanism. 1975, Hummocky cross stratification. This sedimentary structure, present in both siliciclastic and carbonate rocks, is

Abstract There are many examples of ancient sand bodies which have been interpreted as having a tidal origin. Most of them, however, are intertidal to very shallow subtidal, and represent lagoonal sand flats, tidal channel or estuarine channel and sand flat environments. There are remarkably few well described examples of ancient shelf/shallow marine subtidal sand bodies -- the few examples that exist will be the topic of these notes.

Abstract Two vertically stacked shelf-ridge (bar) complexes in the Shannon Sandstone member of the Cody Shale (designated upper and lower sandstones) crop out in the Salt Creek anticline of the Powder River Basin, Wyoming. The shelf-ridge complexes are composed primarily of moderately to highly glauconitic, fine-to medium-grained lithic sandstone and attain thicknesses of over 70 feet. The shelf-ridge complexes were deposited at least 70 miles from shore at middle to inner shelf depths by south to southwest-flowing shore-parallel currents intensified periodically and frequently by storms. Ridges in each sequence trend north-south, slightly oblique to current flow. A possible source of sediments for the shelf ridges was the Eagle Sandstone shoreline and deltaic deposits of southern Montana 200 miles to the northwest. Eleven facies were defined in outcrop on the basis of physical and biologic sedimentary structures and lithology. Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections, and, in general, facies are stacked in coarsening-upward sequences with Central Bar Facies commonly immediately overlying Interbar Sandstone Facies. Porous and permeable potential reservoir facies include: Central Bar Facies, a clean, cross-bedded sandstone; Bar Margin Facies (Type 1), a highly glauconitic, cross-bedded sandstone containing abundant shale and limonite (after siderite) rip-up clasts and lenses; and Bar Margin Facies (Type 2), a cross-bedded to rippled sandstone. These facies were formed by sediment transported and deposited in the form of medium- to large-scale troughs and sand waves on and across the tops of ridges by moderate to high energy

Abstract Hartzog Draw Field, located in the Powder River Basin, Wyoming, was discovered in August, 1975. It is one of the largest oil fields discovered in the Rocky Mountain province in recent years, with initial estimates of ultimate recovery exceeding 100,000,000 barrels of oil. Field development through the fall of 1977 extended more than 20 miles lengthwise in a northwest-southeast direction and up to three miles in width, encompassing in excess of 22,000 productive areas. Development drilling on 160 acre spacing has had a better than 95% success ratio and initial production rates commonly exceed 1,000 barrels of oil per day, with several wells having potentialed in excess of 3,000 barrels per day. Production at Hartzog Draw is from the Upper Cretaceous Shannon Sandstone Member of the Cody Shale, at a depth of 9,000 to 9,600 feet. Oil accumulation is stratigraphically controlled, structure having almost no influence on entrapment. The reservoir sandstones (mostly MARINE CENTRAL BAR and BAR MARGIN FACIES) are quartzose and glauconitic, fine to medium grained, moderately well sorted, highly trough cross-bedded, and occur in stacked sequences up to 60 feet in thickness. Sideritic clasts and shale rip-up clasts occur locally in the high angle trough cross-bedded units. In the reservoir facies, effective porosities average around 13% and permeabilities 12 md. There is no apparent water table, and net pay thickness closely parallels net sand thickness. The reservoir sandstones are associated with a 30 to 80 foot thick package of rippled interbedded very fine-grained sandstone and shale (INTERBAR and

Abstract Hartzog Draw Field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sandstone at depths from 9000 to 9600 ft. The producing interval consists of a large midshelf sand bar complex deposited below effective normal wave base more than lOO miles from shore. The productive interval in the bar complex has a maximum thickness of 65 ft, is over 21 miles long and is up to 3 1/2 miles wide. Over 170 wells have been completed on 160 acre spacing since its discovery in 1975, and ultimate oil recovery may exceed 100,000,000 barrels. The reservoir is completely enveloped in shale, has a solution gas drive, no water table and no produced formation water. Even zones that calculate water saturations of over 65% from logs do not produce water. Net pay is primarily a product of porosity, permeability and thickness of the sandstone, and is directly related to sedimentary facies. Of six facies observed in cores, only one, the central bar facies, a high angle trough cross-bedded glauconitic quartz sandstone, is a consistently high quality reservoir. Two others, the bar margin facies, a ripple to trough cross-bedded sandstone with abundant shale and siderite clasts, and the interbar facies, a rippled interbedded sandstone and shale, generally are marginal quality reservoirs. Data from three cores indicates the central bar facies to have a significantly better average porosity and permeability (12.7%, 6.4 md) than either the bar margin facies (8.1%, 3.7 md) or interbar facies (6.2%, 2.1 md). In addition

Abstract The Upper Cretaceous (Turonian) Cardium Formation (Figs. 1, 2) is a dominantly sandstone unit surrounded by marine shales of the Alberta (= Colorado) Group. It was deposited in the Western Interior Seaway in Alberta. In the subsurface, the Cardium is a major oil and gas reservoir (Table 1, on next page). It crops out abundantly in several thrust slices of the Rocky Mountain Foothills, between the U.S. Border in southern Alberta and Dawson Creek, B.C., a distance of about 800 km. The Alberta Group spans the Cenomanian to early Campanian, a period of about 15 million years (Fig. 2), and the Cardium belongs to the upper part of the Turonian, implying that deposition of the Cardium may have occurred in as little as one million years (Palmer, 1983). In the Canadian Cordillera, there are two main clastic wedges, the late Jurassic-early Cretaceous Kootenay - Blairmore Assemblage, and the late Cretaceous - Paleocene Belly River - Paskapoo Assemblage. The Alberta Group presumably represents a period of relative tectonic quiescence between these two assemblages, with dominantly mudstones accumulating in the Alberta Basin.

Abstract Five depositional models have been utilized during the last 30 years to explain the deposition of the Upper Cretaceous Gallup and Tocito Sandstone and Tocito Sandstone Lentil in the San Juan Basin in New Mexico. It has generally been recognized that most of the true Gallup sandstones were deposited as strand plain and beach deposits. There is a continuing controversy as to the relationship of the sand ridge (offshore bar) deposits to the Gallup shoreline sandstones. The offshore deposits have been designated as Gallup by some writers and as the Tocito Sandstone Lentil of the Mancos Shale by others. Scenarios such as described in this paper for the Gallup Sandstones may be more common for shelf sandstones than is presently recognized. The Gallup Sandstone may be divided into two major depositional units. Most of the Gallup Sandstone is a strand plain deposit with a typical transition zone, shoreface, foreshore vertical sequence. Hummocky cross stratification marks the base of the Gallup sequence in some areas. Most of the fluvial portions of the Gallup are designated as the Torrivio Sandstone Member. Some of the earliest correlations (Model I) suggested that the Gallup Sandstone was younger than the Tocito. Other correlations (Model II) indicated that the Gallup consisted of a series of synchronous shoreline and offshore deposits. The third, fourth and fifth models include a major unconformity which separates the shoreline Gallup sandstones from offshore-bar sandstones designated as the Tocito Sandstone Lentil of the Mancos Shale. The third model stresses the importance of

Abstract Comparisons commonly help sharpen our observations and interpretations in depositional environments. It is appropriate here to compare shallow marine/shelf environments with the next major sandstone depositional environments to be found in a seaward direction -- classical deep water turbidite systems. The need for such a comparison is apparent from the problems raised in my paper on “Geological Evidence for Storm Transportation and Deposition on Ancient Shelves,” this volume. We may decide to compare individual beds, groups of beds traced laterally, or groups of beds in vertical sequences. However, it has been shown earlier in this volume that the turbidity current process can operate on the shelf, and that preservable turbidites can be deposited in shallow seas. Deciding what to compare is not so simple as it might first appear.

Abstract This paper reviews studies of Woodbine--Eagle Ford reservoir sandstones from the subsurface of East Texas and evaluates shelf sand depositional models in the light of recent studies of fluid and sediment dynamics on modern shelves. The application of fluid and sediment dynamical principles has reaffirmed some shelf depositional models, traditionally applied to the East Texas basin, but modifies or discredits others; in these cases, new models are proposed. Three distinct types of reservoir-quality shelf sandstones can be recognized in these studies; (1) sand ridge deposits, (2) tabular or sheet sandstones, and, (3) lenticular (topographically controlled) sandstones. This preliminary classification is based on external sand body geometries, facies associations and facies distributions. Sand ridge deposits occur at Kurten Field as stacked, en echelon, linear sandstone bodies deposited on the muddy shelf of the east side of the Cretaceous Interior Seaway. Sandstone bodies are asymmetric in cross-section with steeper eastern flanks and are elongated in a north-south direction. Sand ridge deposits at Kurten Field occur stratigraphically adjacent to deposits of the Harris Delta. Sand ridge deposition probably occurred in an inner to middle shelf environment during small scale transgressive episodes,} possibly associated with the abandonment of delta lobes (autocyclic transgression). Intermittent, alongshelf, geostrophic flows appear to be the most likely mechanism of sand transport and deposition. Tabular shelf sandstones occur in the lower Woodbine at Damascus Field as a complex of single to multistory thin beds within a dominantly shale section. Cores display stacked, massive to laminated, fining-upward sandstone sequences, with abundant soft-sediment deformation and primary structures indicating rapid sedimentation. Sandstones form a series of thin sheet-like deposits elongated across the strike of the paleoshoreline. Sanddeposition took place in an inner to middle shelf environment, during a general period of shoreline regression. Deposition is suggested to have occurred in localized zones of alongshelf flow deceleration and expansion during storms. Bouma-like vertical sequences of primary structures in Damascus sandstones indicate that these beds are tempestites (i.e. suspension deposits produced by storm flows). Lenticular shelf sandstones are present in the uppermost Eagle Ford (Sub-Clarksville) section in Grimes County, Texas. Fining-upward sandstone sequences consist of amalgamated, massive to cross-stratified beds with erosional bases, overlain by bioturbated shaley sandstones. Individual sandstone bodies have restricted areal extents and deposition appears to have been controlled by local, salt-related topographic lows. These Sub-Clarksville sands were apparently deposited during a regional transgression which succeeded a phase of sea level stillstand. Remobilization of the substrate by wind-forced storm currents during transgression appears to have formed broad erosional surfaces, accompanied by deposition of sands swept into zones of local flow deceleration. Transgressive sand ridges may have formed contemporaneously on other parts of the late Eagle Ford shelf.

Abstract The Solar Petroleum Navajo F-151 (NW Sec. 10 T31N R17W), which is the subject of this discussion, is located in Horseshoe Field in the San Juan Basin in San Juan County, New Mexico (Fig. 1). It produces from the Tocito Sandstone Lentil of the Mancos Shale which has been incorrectly included by many workers in the Gallup Sandstone of Turonian and Coniacian (Upper Cretaceous) age (Fig. 2 and 3). The Tocito Sandstone Lentil is typically inter-bedded with or lies at the base of the upper Mancos (Niobrara) Shale in the western and central parts of the San Juan Basin (Molenaar, 1973, 1983a). Substantial amounts of the production attributed to the Gallup Sandstone should probably now be attributed to the Tocito Sandstone Lentil (125 million barrels, to date, Fassett, 1981). The most conclusive evidence for differentiating the Tocito Sandstone from the Gallup Sandstone is obtained from outcrop studies and from cores such as the Solar Petroleum Navajo F-151.

Abstract Hartzog Draw field is a stratigraphically controlled oil reservoir which produces from the Upper Cretaceous Shannon Sandstone at depths from 9000 to 9600 feet. The producing interval consists of a large mid-shelf sand-ridge (bar) complex deposited below effective normal wave base more than 100 miles from shore. The productive (net pay) interval in the bar complex has a maximum thickness of 60 feet, is 22 miles long, and is one to four miles wide. The field was discovered in 1975 and 177 producing wells were completed on 160 acre spacing during the primary production phase of development. Initial oil in place was calculated to be 350,000,000 barrels. The shelf sand-ridge complex is competely enveloped in shale, has a solution gas drive, no water table and no produced formation water. Net pay is primarily a product of porosity, permeability and thickness of the sandstone, and is related primarily to sedimentary facies and the degree of diagenesis. Of the six to eight facies observed in cores, only one, the Central Ridge Facies , a high-angle trough cross-bedded glauconitic quartz sandstone, is consistently high-quality reservoir. Two others, High-Energy Ridge-Margin Facies , a predominantly trough cross-bedded highly glauconitic sandstone with abundant shale and siderite clasts, and Low-Energy Ridge-Margin Facies , inter-bedded trough and rippled sandstone, also may be good quality reservoirs. Inter-Ridge Facies which consist of rippled interbedded sandstone and shale, generally are poor quality to non-reservoirs. Values from the Central Ridge Facies from three of the cores taken early in the development of the northern part

Abstract Three shelf sandstone cores from western Alberta are discussed in terms of lithology, bedding types, log character, and processes of deposition. Two of the cores are Cardium cores, one each from Ricinus and Caroline Fields. A single Viking Sandstone core from Stettler Field is discussed. The general geology of the Cardium field at Ricinus is given in Walker (this volume: “Upper Cretaceous (Turonian) Cardium Formation, Southern Foothills and Plains, Alberta”). The Cardium Formation is a 50-100 m thick sandstone within the dominantly shaley Alberta (= Colorado) Group, and is Turonian (U. Cretaceous) in age. The geologic setting at Ricinus is summarized in Walker (this volume, Figs. 1, 12, 14, 15, 18); the field is basically a channel fill, the channel being at least 45 km long, 4-5 km wide (before palinspastic reconstruction), and 20-40 m deep. Ricinus sits on top of the junction between the Plains (no deformation) and Foothills (multiple imbricate thrusts); in some wells the sandstone is repeated up to about 6 times. The Gulf Ricinus 13-26 well is in the northern part of Ricinus Field, which is structurally simpler than the southern part. The gamma-sonic log (Fig. 1) and SP-resistivity log show the sand to be abrupt and sharp-based; the top is also abrupt. Serrations in the gamma ray indicate either thin mud-stone partings, or zones of ripped-up mud clasts (e.g., 9011.5, 9017, 9019, 9020 feet). As can be seen in the photographs of the slabbed cores (Fig. 2, A-D) the base of the sandstone (9023.6 feet)