EVALUATION OF DUPLICATE SAMPLES REGIONAL SEDIMENT SURVEYS , BRITISH COLUMBIA
INTRODUCTION The ability to discriminate real trends related to geological and geochemical causes from those that result from spurious factors such as sampling and analytical errors is of paramount importance in the suI:cess of geochemical data interpretation. Since the estimate of reproducibility (precision) allows us t o quantify the amount of variation due t o sampling and laboratory analysis it is an integral part of the evaluetion of geochemical data and should be conducted prior to carrying out any detailed interpreation. As part of our continuing study of rapid but thorough evaluation procedures for multi-element stream-sediment data (for example, Matysek. et a/., 1981) we designed a systematic computer-oriented method of evaluating the quality of geochemical survey data based on field-site duplicates. It irlcorporates a bias test, an analysis of variance technique, and the Thompson and Howarth (1976) approach to qua'ytifying precision. This detailed procedure utilizes the type and quality of data incorporated in various regional geochemical programs undertaken by the British Columbia Ministry of Energy, Mines and Petloleum Resources (the Ministry), but can be adapted easily for other programs, GENERAL METHODOLOGY steps: In brief, our general approach t o evaluation of the quality of geochemical data sets involves the following (1) Extraction of a t least 50 independent, field-site duplicate pairs from a geochemical data set f cmr use in subsequent data analysis. (2) Determination of the degree of systematic bias between duplicate pairs based upon the number of cases in which the first observation is greater than the duplicate. (3) Evaluation of the duplicated geochemical data set in terms of metal variability a t the regional, between-sample site and a t the local, within-sample site, sampling, and analvtical levels by a two-factor analysis of variance technique. (4) Quantification of the within-sample site variability by estimating precision utilizing the Thsmpson and Howarth (19761 method. D A T A Data obtained from the three most recent regional geochemical surveys undertaken Iby the Ministry (1981 I were evaluated using this method. The base data consist o f analyses of stream sediments and water samples collected a t an average density of one sample per 17.3 square kilometres over NTS map-sheet areas 92H, 921, and 92J. Silt samples were field dried and the minus 80-mesh (177 microns) fraction retained for subsequent analyses. The samples were analyzed for zinc, copper, lead, nickel, cobalt, silver, manganese, iron, molybdenum, arsenic, and antimony by the atomic absorption …
The determination of sampling and analytical errors in exploration geochemistry
Analytical precision may be less important than sampling error as a factor governing whether or not errors are significantly smaller than overall data variability. Identical treatment of original and replicate samples, and collection of replicate samples at some 10 percent, but at least 30, of the sample sites of a survey should provide enough information to determine whether or not variability represents relevant information or has a significant chance of being due to a combination of sampling and analytical errors.
Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo-Paris Basin
Using key lithological, biostratigraphic and geophysical borehole-log markers a correlation of the Upper Cretaceous Chalk stratigraphies across the whole Anglo-Paris Basin has been achieved. A common lithological marker bed stratigraphy applies and key fossil markers (inoceramids, Micraster and Echinocorys ) are consistently related to the same lithological markers throughout the basin. The major lithological changes consistently occur at the same levels but are not necessarily equally strongly developed. The magnificent dip section along the French Channel coast between the Somme and the Seine, and the inland sections at Precy-sur-Oise and Verlin (Loiret) illustrate the influence of long-lived tectonic lines on Chalk sedimentation history recognised by: (i) rapid thickness variations; (ii) channel flooring hardgrounds and associated anomalous bedding dips: (iii) stratigraphically anomalous infills of channels which may be metres to kilometres across; (iv) redeposited chalks containing abundant Bathichnus paramoudrae ; (v) tectonically induced slumps and slides with flint and chalk shatter beds (palaeoseismic horizons). These features are found along may axes including the Anticlinal du Bray at Precy and Dieppe, the Anticlinal de Margny-les-Compiegne at Nourard le-Franc and Le Treport (the Bresle Valley Axis) and the Fecamp and Verlin fault systems. The ‘events’ recognised along these axes link with similar events recognised in Sussex (e.g. Kingston Anticline), on Portsdown, at Culver Cliff, Isle of Wight (Sandown Pericline) and on the Weymouth Anticline (White Nothe). Such events are common in the high Turonian, low Coniacian, basal Santonian, and the low and mid Campanian. The recurrence of key marker beds dominated by a particular trace fossil association or sequence, Bathichnus-Thalassinoides suevicus-Zoophycos associated with major changes in lithology, micro and macro-fauna indicates more widespread ‘oceanographic’ cycles of transgression and regression related to Atlantic opening.