Pebble Count (Modal) Analysis of Four Outcrops of the Upper Siwalik Conglomerates of the Jammu Region, Northwest India and their Textural Implications

The pebble count data from the four outcrops of the Upper Siwalik Conglomerates of the Jammu region, northwest India are compared with the results expected from appropriate theoretical models of constant probability of occurrence from point to point. The resulting degree of fit is applied to characterise the textural arrangement of each constituent.

The experiment shows that within an outcrop for Jarger pebble counts the Positive Binomial and for lower counts Poisson are the appropriate theoretical models. For the combined data over the four outcrops, the proper models are Lexian and Lexian Poisson. This would suggest at anyone outcrop, 5, 10, or 100 pebble counts obtained from anywhere in the outcrop would yield unbiased sample estimate of the outcrop population parameter. Over many outcrops, however, the results are inconsistent. Combined distributions for the four outcrops indicate heterogeneity in the distribution of the variates. Therefore, it is suggested that no one outcrop or one sample from an outcrop represents the entire target population (Upper Siwalik Conglomerates of the Jammu region). The population has to be sampled regionally over several outcrops preferably over an orthogonal grid spread over the target population. The results also suggest that at anyone outcrop the process responsible for the deposition of the conglomerates is uniform but has varied from place to place thus suggesting more than one episode in the deposition of the conglomerates. One has to explain this multi-episodic deposition by a sedimentation model that would explain adequately the quantitative variability present in the data.