The GIS

Data synthesis and structure

Download this page (word 97)

 - The different layers of information that can be combined in any way are geographic, geologic and metallogenic:

• geographic base of the Peninsula;
• digital elevation model of the Peninsula;
• geologic and metallogenic map coverage of the Peninsula;
• 1:1,000,000-scale geologic synthesis of the Shield;
• structural sketch map mosaic of the Shield;
• 1:1,000,000-scale structural map of the Shield;
• 1:1,000,000-scale metamorphic map of the Shield;
• aeromagnetic synthesis of the Shield;
• 1:1,000,000-scale surficial deposit map of the Shield;
• geochronological database of the Shield;
• geochemical database of the Shield;
• seismic database of the Shield and surrounding regions;
• metallogenic synthesis of the Shield;
• reevaluation of past geologic, geochemical and geophysical exploration;
• geochemical exploration of the Arabian Shield;
• reserves and grades of the main mineral occurrences of the Shield;
• exploration mining leases.

ArcView Data files and Data structure

GIS Arabia is a homogeneous information system of the Arabian Shield and Peninsula conceived as a tool both for the mining sector, as an aid to minerals exploration and development, and for the academic sector as an aid to developing new metallogenic models. GIS Arabia is based on original syntheses, compilations and reevaluations.

A flow chart of the GIS explaining the underlying philosophy of the project :  

2.2 - Geographical Base of the Peninsula

The Digital Chart of the World is a comprehensive 1:1,000,000-scale vector basemap. It consists of geographic, attribute, and textural data that can be accessed, queried, displayed and modified with ArcView software. The primary source for the database is the United States Defense Mapping Agency Operational Navigation Chart series produced by Australia, Canada, the United Kingdom, and the United States.

The database contains several thematic layers and a description of its different fields, layers, and attributes can be found on http://www.esri.com/data/catalog/esri/esri_dcw.html.

 The thematic layers of the DCW used in the geographic base of GIS Arabia are:

  2.3 - Digital Elevation Model of the Peninsula

The 30'' arc digital elevation model of the Peninsulaused in GIS-ARABIA was extracted from the GTOPO30 (USGS/EDC) database.

Structural analysis of the detailed topography was carried out using Spatial Analyst and SynARC software.

2.4 - Geologic and Metallogenic map coverage of the Peninsula

This metadata layer compiled by BRGM contains information about the geologic and metallogenic map coverage of the Arabian Peninsula. It includes all the availbale 1:50,000-, 1:100,000-, 1:200,000-, 1:250,000-, 1:500,000-, 1:1,000,000- and smaller-scale regular and non-regular geologic, geophysical, hydrologic and metallogenic maps.

The attached database contains the following fields:

Attribute table of the map coverage layer

No "qualitative" selection has been made concerning the cited maps: all known maps are recorded, including some that are obviously obsolete and thus of limited interest. Similarly, the successive editions of small-scale national maps are mentioned so as to provide a base that is as exhaustive as possible.

2.5 - 1:1,000,000-scale geologic synthesis of the Arabian Shield

The aim of the 1:1,000,000-scale geologic synthesis is to present a homogeneous geologic map covering the entire Arabian Shield and favoring a litho-stratigraphic delimitation.

The base information behind the synthesis is easily accessed by 1) zooming-in on the area of interest on the map, which will display the relevant polygon(s) within that area, and 2) clicking on the relevant polygon(s), which will then display the lithostratigraphic characteristics of the contained formations according to data fields of the attribute tables given below:

Cartographic geologic units

Attribute table of polygonal features of the geologic synthesis map

Structural features

Attribute table of linear features of the geologic synthesis map

Because the synthesis was prepared at a scale of 1:1,000,000, the precision of the boundaries can never be more accurate than ± 1 km. Consequently the authors take no responsibility for any consequences resulting from using the map data at a larger scale.

2.6 - Structural sketch map mosaic of the Shield

This image data layer is a mosaic of all structural sketch maps that appear on the 53 regular 1:250,000-scale geologic maps covering the Arabian Shield.

2.7 - 1:000,000-scale structural map of the Shield

The 1:1,000,000-scale structural map of the Shield was produced from the 1:1,000,000-scale geologic map modified and completed with the help of new field data, an aeromagnetic synthesis, the structural sketch map mosaic, and satellite and aerial imagery.

Attribute table of polygonal features of the structural map

Attribute table of linear features of the structural map

2.8. - 1:1,000,000-scale metamorphic map of the Shield

The 1:1,000,000-scale metamorphic map of the Shield was produced from the 1:1,000,000-scale geologic map modified and completed with the information contained in the 53 1:250,000-scale geologic maps and attached explanatory notes. Using petrographic data given in the explanatory notes, ten metamorphic classes have been defined and coded between 1 and 10 as shown below.

Coding of the metamorphic map

A code between 1 and 10 representing the metamorphic grade has been assigned to each geologic polygon mapped on the 1:1,000,000-scale geologic synthesis map. When no information was available and no interpretation possible, the code 0 has been ascribed. The boundaries of metamorphic facies thus fit with the boundaries of the lithologic formations and cannot be considered as metamophic isograds.

2.9. - Aeromagnetic synthesis of the Shield

Twenty three aeromagnetic surveys have been flown out under the auspices of the Ministry of Petroleum and Mineral Resources of the Kingdom of the Saudi Arabia and the supervision of the BRGM and USGS. These surveys, which have covered the entire Arabian Shield (650,000 km²) were conducted from 1962 to 1983 in several blocks and by different companies. Ground clearance was mainly 150 m, 300 m or 500 m, and line spacing was mainly 800 m. (see following table and figure). The 1962 and 1965-67 surveys, supervised by the BRGM and covering the whole Shield, were flown using fluxgate Gulf Mark III magnetometers with analog recording. In addition to these five general surveys, several less extensive surveys were carried out over targets of economic interest using a CSF cesium-vapor magnetometer with digital recording for the 1976 and 1981 surveys and a Geometrics G 813 proton precession magnetometer with digital recording for the 1983 survey.

Parameters of the different aeromagnetic surveys

Map showing the extent of the different aeromagnetic surveys

Magnetic surveys measure variations in the earth's magnetic field that are related to the magnetic properties of the rocks, also to spatial and temporal variations of the Earth's magnetic field for which the measurements are corrected.

The 1962 and 1965-67 data were originally presented as a series of total-intensity contour maps compiled at 20-gamma intervals to a scale of 1:50,000. This data was later recompiled graphically as a series of colored 1:500,000-scale 100-gamma interval total-intensity maps (Andreasen and Petty, 1973, 1974) and subsequently photographically compiled and reduced to a 1:1,000,000-scale map by Blank and Andreasen (1991). Until now, only this incomplete photographic compilation of graphically reduced contour maps, along with its filtering effects, was available for an overview of the Shield.

To produce the aeromagnetic maps for the GISArabia we used the original analog analytical data that was digitized from the original records, and not the contoured maps. Although more arduous than gridding the contour maps, this method allowed us to capture all the original data without the filtering effect of the contouring (Georgel and others, 1985).

The original data obtained by digitizing the different magnetic profiles does not correspond to the total magnetic field nor to its anomalies. Thus a regional field was calculated and substracted from the original profile data. These polynomial surfaces used the coefficients, given in the following tables..

The data were then compiled to produce a map with a 300 m terrain clearance and a 500m grid spacing in an UTM 38 projection. The absence of discontinuities between the different blocks implies that the regional fields were well calculated.

The obtained aeromagnetic map is of very good quality, except between latitudes 22-25°N and longitudes 43-45° Ewhere a NE-SW linear fabric is observed that probably corresponds to the flight direction; This certainly results from bad leveling, which we could not correct without the original flight measurements.

To produce the reduced-to-the-pole aeromagnetic map, we subdivided the Shield into three parts and applied the following parameters taking the assumptions that the induced-field is largely predominant:

Southern Panel: I = 21.1°, D = 0.76°

Central Panel: I = 29.6°, D = 1.22°

Northern Panel: I = 37.06°, D = 1.6°

The aeromagnetic data and images are provided on a separate CD, subdivided into three directories:

• Data-description

  • Read.txt

  • Figure (location of aeromagnetic surveys)

  • Table (parameters of the different surveys)

• Data

  • Arabxx.Dat.Z (XYZ ascii files, UTM 38 coordinates)

• Images

  • Anorpm.tif (reduced to pole)

  • Anorpm.tifw (attributes of image)

  • Anorpvgm.tif (vertical gradient)

  • Anorpvgm.tifw

  • Anorpp10m.tif (upward continuation to 10 km)

  • Anorpp10m.tifw

  • Attached legends

2.10 - 1:1,000,000-scale surficial deposit map of the Shield

The 1:1,000,000-scale Quaternary surficial deposits map of the Arabian Shield is a synthesis of the 56 1:250,000-scale geologic sheets covering the Shield. More than 50 different types of surficial deposits or groups of deposits ranging in age from Pleistocene to Holocene were taken into account and divided into six units: alluvium, depression deposits, sand deposits, gravel sheets, undifferentiated sand and gravel and undifferentiated deposits.

Surficial deposits are ubiquitous throughout the Arabian Shield, but are particularly abundant along the Red Sea coast to the west and in the depression between the Proterozoic basement and the Phanerozoic cover to the east and north. Where the deposits significantly mask the underlying bedrock, they are mapped as total cover (90-100% surficial deposits). Where they are either lacking or represented in insufficient detail on the source maps to be reported at the synthesis scale, they are mapped as partial cover (0-10%, 10-50% or 50-90% surficial deposits).

The description of the six units is summarized from the explanatory notes of the 56 maps that were used.

Alluvium (Qa) comprises 1) wadi bed deposits filling the main channels of the active drainage system, 2) present and ancient flood-plain and basin deposits, 3) terrace deposits with or without pediment, and 4) alluvial-fan deposits.

The channels of the active drainage system are filled with unconsolidated, crudely bedded to well-stratified gravel, sand, silt and scattered subangular to well rounded boulders, and locally including reworked loess or scoria derived from volcanic extrusions. The deposits are generally well defined, with crossbedding and channel structures, but certain are extremely diffuse and overlapped by eolian sand. Toward the coast, the sedimentary load of most wadis becomes distributed as deltas over sabkhahs and tidal mudflats, with only a few maintaining a channel as far as the open sea. Most active wadis of the Proterozoic domain, and those of the fossil system in the eastern depression, are too narrow to be represented at the synthesis scale where they are not associated with extensive flood-plain or terrace deposits; they are thus assigned to a different unit.

The flood-plain and basin deposits of the main present-day channels and certain fossil channels are covered by silt, clay, sand and subordinate gravel, locally interlayered with boulder beds, sand bars, overbank deposits or reworked loess. In the Jizan quadrangle, they are correlated to the north with the alluvial terraces of the Sabya quadrangle, thought to be Pleistocene to Holocene in age.

The terrace deposits consist of silt, sand, cobbles and gravel, and may or may not show a desert pavement and/or a desert varnish, and locally a well-defined red soil horizon. In places, several levels of terraces are present above the present-day channels. In the Wadi Baysh quadrangle, they are thought to be Pleistocene in age.

Alluvial fans occur at the foot of the Red Sea escarpment and along certain larger wadis. The deposits consist of an unconsolidated, poorly sorted talus of pebbles and boulders, with intercalated lenses of pebbles and sand, formed by recent active erosion of the escarpments.

Depression deposits and duricrusts (Qb) include sabkhah, khabra and lacustrine deposits, duricrusts and reef complexes.

This unit is present along most of the Red Sea coast where it predominantly comprises sabkhahs and tidal mudflats that consist of silt, mud and clay interstratified and encrusted in the sabkhahs by halite and gypsum. On their landward side, the deposits are locally covered by alluvium or eolian sand, whereas on their seaward side, they are protected from coastal erosion by an outer line of beach dunes or reef complexes. The latter consist of submarine, actively growing, reef limestone; uplifted, inactive reef-limestone terraces, rising a few metres above sea level; calcareous beach sand deposits, arenaceous limestone and calcareous breccia ("beach rock"). In the Jiddah quadrangle, a reef limestone gave a 14C isotopic age of 40,000 years BP.

Many small patches of sabkhah, khabra and lacustrine deposits occur in the Proterozoic domain, but few are mapped at the present scale. These deposits are developed predominantly in the eastern depression where they are generally associated with a network of fossil alluvial channels that drained the basement and the Paleozoic escarpment. Sabkhah and lacustrine deposits also commonly occur in interdunal depressions. Most of these deposits are thus inactive. The khabra deposits consist of clayey-silty playas or sandy material, the lacustrine deposits of clayey material, carbonates and sandy silt locally containing abundant gastropod or bivalve shells, and the sabkhah deposits of silt and clay with interbedded evaporitic deposits.

Duricrusts, or thin hardened layers resembling duricrust, are commonly developed over several rock types of the eastern Phanerozoic cover. Two episodes of encrustation are recognized and are the result of a long and complex evolution during the Early Quaternary and possibly the Late Pliocene.

Sand deposits (Qs) predominantly include eolian sand fields, and locally, beach sand or extensive loess silt, as along Wadi Bishah (Wadi Bishah quadrangle).

Although some extensive sand sheets occur with well-developed dune fields in the Proterozoic domain, the sand deposits are essentially represented by extensive large sand sheets and dune systems in the eastern and northern parts of the Shield. The sand sheets vary in thickness, ranging from a thin film overlying gravel-plain deposits or older layers, to deposits several metres thick. The dune systems comprise extensive mobile or fixed dune fields. Longitudinal dune chains may be more than 1 km wide and as much as 35 km long, separated by interdunal belts. Most are oriented northwest, others are northeast- or east-trending, with their direction not everywhere corresponding to the present prevailing winds, and thus possibly reflecting at least two periods of aridity.

This unit also covers most of the coastal plain where it comprises sand sheets and tracts of dunes, and extensive sheets of wind-blown material deposited on loess, old flood-plain deposits and pediment deposits. The unit can form east-trending linear dune ridges of 1 to 5 m high, 15 to 100 m wide and as much as 1 to 5 km long, separated by inter-dune areas. These locally grade into tracts of barchan dunes, not exceeding 10 m wide and 2-3 m high, which are generally open to the east or southeast. In places, dunes encroach upon wadi channels or sabkhahs where sand is reworked.

All these sand deposits consist of fine to medium quartz locally mixed with abundant feldspar, heavy minerals or other material; most are Holocene in age, but some may be Pleistocene.

The wind-blown material consists of silt to medium-grained subangular to subrounded sand, and is generally thin (less than 1 m) with surfaces marked by ripples or small dunes.

Gravel sheets (Qg) include vast expanses of sand and gravel, colluvium, and pediment material derived from Proterozoic outcrops or developed on an unidentified unit.

The gravel sheets are generally vast, containing blocks all of which are derived from older deposits, and form flat glacis. To the north and east, where they are well-developed, the gravel sheets can be divided into three episodes: 1) an old sheet, without recent volcanic clasts, thought to be Early Pleistocene, 2) an inactive sheet, containing recent volcanic clasts, ranging in age from early Late Pleistocene to Holocene, and 3) an active sheet mainly corresponding to present-day piedmont deposits.

The colluvium everywhere comprises detritus formed by weathering of the underlying bedrock and transport by gravity and runoff over short distances. The deposits consist of unsorted angular rock fragments of all sizes forming fan-shape accumulations. Abundant colluvial deposits occur at the foot of and on the dip slopes of the Khuff Formation cuestas in the eastern part of the Shield.

The pediment material consists of a thin veneer of poorly sorted, fine to coarse- gravel, sand and silt, partially covered by or merging with wadi sediments and eolian sand.

Most of the surficial deposits of the Shield belong to this unit, being predominantly composed of piedmont and pediment deposits, but also including alluvium at the bottom of the thalwegs and old entirely eroded alluvial terraces. The latter case is well illustrated by the extensive sandy plain of Sahl Rukbah (Turabah quadrangle).

Undifferentiated sand and gravel deposits (Qsg) comprise eolian sand fields, with or without dunes, gravel sheets, and pediment material. In the Al Qunfudhah quadrangle, this unit also includes sabkhahs along the coast.

Undifferentiated deposits (Q) comprise unidentified Quaternary surficial deposits and a mixed unit formed by deposits of various origin that cannot be differentiated at the present scale of work.

In the Najran quadrangle, this unit is represented by a mixed unit essentially composed of alluvial gravel forming terraces above the present flood plains and underlying widespread gravel plains, including eolian sand and pediment deposits overlying older alluvial deposits and bedrock.

Attribute table of the Surficial Deposits layer

Values of Fs_num

2.11 - Geochronological database of the Shield

Une réévaluation des données géochronologiques concernant l’Arabie Saoudite et contenues dans la banque de données de "JOHNSON P.R., CARTEN R.B., and JASTANIAH A., 1997, Tabulation of previously published U-Pb, Rb-Sr and Sm-Nd numerical age data for the Precambrian of northeast Africa and Arabia (second edition), (editor, P.R. JOHNSON): Saudi Arabian Deputy Ministry for Mineral Resources Open-File Report USGS-OF-97-1, p. 15, electronic disk." a été réalisée à partir des données contenues dans tous les documents cités accessibles.

La liste des âges et les tableaux contenant les évaluations sont ajoutés en annexes.

Ce travail s'est attaché à évaluer la fiabilité des données analytiques de base concernant les âges présentés dans la banque de données. Toutes les données disponibles ont été recalculées. Le programme de calcul qui a été utilisé pour cette réévaluation est le programme ISOPLOT, version 2.50 de K.R. LUDWIG. Les âges présents dans cette banque sont répertoriés en 5 catégories en fonction du degré de confiance que l'on peut leur attribuer:

Attribute table of the Geochronology layer

2.12 - Geochemical database of the Shield

The geochemical database constituting an information layer in the GIS Arabia includes whole-rock analyses (major and trace elements) derived from the literature and BRGM projects.

Sources des données

L’information géochimique a été puisée à trois sources : 1/ la bibliographie scientifique internationale, investiguée via la base de données bibliographique GEOREF ; 2/ les rapports du DMMR ; 3/ les rapports du BRGM.

Par ailleurs, différentes missions, faites dans le cadre des activités de la Direction de la Recherche du BRGM entre 1993 et 1998 (dont le projet " Bouclier Arabo-nubien "), ont permis de réaliser près de 150 analyses de haute précision (dosages des traces par ICP-MS) sur des roches précisément localisées. Ce " corpus " analytique, dont une partie a fait l’objet d’un rapport précédent (Thiéblemont, 1997), a été intégré à la base de données.

En général, seules les données à " vocation " p