Logarithmic Cons-Pair Lists

What is the name of immutable data structure, which has O(1) access to the head element and logarithmic increasing access cost for the following elements?

It can be implemented by converting tail into an array, when prefix size becomes larger than the tail size. I.e. (cons prefix array)

>>13

(cons x xs) and (cdr xs) would still be O(log2(n))