Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA

Abstract

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline.

Gamma-ray Spectrometry in Geothermal Exploration: State of the Art Techniques

“…The vast majority of times this was not necessary, but the repeated measurements did provide extra confidence in the results particularly when readings were unusually low or high. Each measurement lasted 3 min, a period previously established as adequate for a reliable sample of rocks with similar counts per second as granite [46,63], albeit measurement times can be shorter in high activity areas or longer in lower activity areas such as metamorphic basement [26]. To ensure 3 min measurements were long enough to be reliable we checked against half hour measurements (Figure 8), obtaining reassuringly similar results.…”

Assinatura gamaespectrométrica de depósitos fluviais: estudo de caso na Formação Rio do Rasto, Permiano Superior da Bacia do Paraná

“…A correlação estratigráfica de depósitos fluviais é difícil utilizando apenas dados convencionais de poços (Hampson et al, 2005), pois o ambiente fluvial apresenta um elevado grau de heterogeneidade interna e a continuidade lateral das camadas é, muitas vezes, menor que o espaçamento entre os poços (Evans et al, 2007). Por esses motivos, a análise de afloramentos auxilia a interpretação da arquitetura fluvial.…”

Caracterização Das Principais Feições Superficiais De Fundo E Em Subsuperfície No Limite Sudoeste Da Baía De Macapá, Canal Do Norte, Rio Amazonas

“…A análise espacial dos refletores sísmicos revelou pelo menos cinco sismofácies, com o indicativo de associação as alterações morfológicas de leito. As informações obtidas no processamento também corroboram a correlação entre as unidades estruturais e o conjunto de sismofácies, contribuindo para evidenciar a forte relação entre as estruturas mostradas em superfície e em subsuperfície (Hampson et al, 2005;Shi and Wang, 2010). A configuração morfológica pretérita auxilia na compreensão dos processos costeiros, favorecendo o conhecimento atual dos ambientes dinâmicos em desenvolvimento (Miall, 2006;Weschenfelder et al, 2010).…”

“…Entretanto, dependendo do método utilizado e logística de levantamento, podem-se obter resultados satisfatórios em relação a investigação em subsuperfície destas formas de fundo (Hampson et al, 2005).…”

“…Embora seja apresentado um elevado grau de horizontalidade nos refletores, percebeu-se a presença de desconformidade em algumas áreas isoladas em aproximadamente dois metros de profundidade abaixo do substrato. Esta característica pode estar relacionada com a variação abrupta no regime de fluxo, onde há anormalidade no processo sedimentológico natural devido a alteração do nível de base, originado a formação de superfícies erosivas (Hampson et al, 2005;Stow et al, 2013). A disposição dos refletores neste arranjo também pode ser verificada nas calhas de canais, onde essa formação é derivada dos extremos valores de velocidade de fluxo (Daniell and Hughes, 2007;Nagata et al, 2000).…”