Cyclostratigraphic Analysis of the Lower Cambrian Shales in Western Hubei and Southern Guizhou
[Objective]The Lower Cambrian organic-rich shale is an important marine shale exploration target in western Hubei and southern Guizhou.Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration.[Methods]In this study,we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian well ND1 in western Hubei and the well HY1 in southern Guizhou and determined the primary astronomical periods.Using pre-viously reported zircon ages as anchoring points,we constructed an astronomical time scale for the Lower Cambrian.Using a sedimentary noise model,relative sea-level changes in the Lower Cambrian were reconstructed.[Results]Our results reveal that wavelengths of 36 m and 9 m in well ND1 and 16.6 m and 4 m in well HY1 correspond to orbital 405 kyr and 100 kyr cycles,respectively.Furthermore,we calculated the durations of the Niutitang Formaion in well Niedi-1 and the Jiumengchong Formation in well HY1 to be 3.30 and 2.64 Myr,respectively,The astronomical age scales of two options are presented.In Option 1,the top of the Niutitang Formation in well ND1 serves as the starting point for the anchor point,with an age interval of 535.20±1.70 Ma to 538.50±1.70 Ma.Option 2 employs the top of the Jiumengchong Formation in well HY1 as the starting point of the anchor point,with the corresponding age interval spanning from 535.20±1.70 Ma to 537.84±1.70 Ma.Through correlation sea-level change curves(DYNOT and ρ1)with eccentricity cycles and sedimentary cycles,our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences(SQ1 and SQ2).Comparing the total organic carbon(TOC)content of the Lower Cambrian organic-rich shale with orbital eccentricity,sedimentation rates,and sea-level changes,our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr,whereas those with low TOC value correspond to the minimum value in well ND1.In addition,the high TOC value corresponds to both the high sea level period and the low sea level period.[Conclusions]Because of the chaotic behavior of the solar system,accurately predicting the mechanism that drives organic-rich shale during the Early Cambrian is impossible.Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time,strong seasonal variations have triggered the enrichment of black shale over a more extended period.Furthermore,the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.
Niutitang FormationJiumenchong Formationorbital forcingeccentricitysedimentary noise model
国家科技期刊平台
NSTL
万方数据
