Farah,
I've been looking at this some more, and the marker for Java just toggles back and forth between Java 6 and Java 7. If the Java 7 marker is not present, then a cartridge just defaults back to Java 6.
But you still have to have the version of Java installed on the gear in order to toggle back and forth.
Here are the contents of the /usr/lib/jvm directory on the gear where OpenShift online installed my Wildfly 8.1
cartridge ...
lrwxrwxrwx. 1 root root 26 Apr 22 13:19 java -> /etc/alternatives/java_sdk
drwxr-xr-x. 3 root root 4.0K Feb 18 2013 java-1.5.0-gcj-1.5.0.0
lrwxrwxrwx. 1 root root 32 Apr 22 13:19 java-1.6.0 -> /etc/alternatives/java_sdk_1.6.0
lrwxrwxrwx. 1 root root 26 Apr 22 13:19 java-1.6.0-openjdk -> java-1.6.0-openjdk-1.6.0.0
drwxr-xr-x. 7 root root 4.0K Apr 11 06:57 java-1.6.0-openjdk-1.6.0.0
lrwxrwxrwx. 1 root root 32 Apr 22 13:18 java-1.7.0 -> /etc/alternatives/java_sdk_1.7.0
lrwxrwxrwx. 1 root root 27 Apr 22 13:18 java-1.7.0-openjdk -> java-1.7.0-openjdk-1.7.0.55
drwxr-xr-x. 3 root root 4.0K Feb 3 17:06 java-1.7.0-openjdk-1.7.0.45
drwxr-xr-x. 3 root root 4.0K Apr 22 13:19 java-1.7.0-openjdk-1.7.0.51
drwxr-xr-x. 7 root root 4.0K Apr 22 13:18 java-1.7.0-openjdk-1.7.0.55
drwxr-xr-x. 3 root root 4.0K May 1 2013 java-1.7.0-openjdk-1.7.0.9
lrwxrwxrwx. 1 root root 34 Apr 22 13:19 java-openjdk -> /etc/alternatives/java_sdk_openjdk
lrwxrwxrwx. 1 root root 21 Apr 22 13:19 jre -> /etc/alternatives/jre
lrwxrwxrwx. 1 root root 27 Feb 18 2013 jre-1.5.0 -> /etc/alternatives/jre_1.5.0
lrwxrwxrwx. 1 root root 26 Feb 18 2013 jre-1.5.0-gcj -> java-1.5.0-gcj-1.5.0.0/jre
lrwxrwxrwx. 1 root root 27 Apr 22 13:19 jre-1.6.0 -> /etc/alternatives/jre_1.6.0
lrwxrwxrwx. 1 root root 30 Apr 22 13:19 jre-1.6.0-openjdk -> java-1.6.0-openjdk-1.6.0.0/jre
lrwxrwxrwx. 1 root root 27 Apr 22 13:18 jre-1.7.0 -> /etc/alternatives/jre_1.7.0
lrwxrwxrwx. 1 root root 31 Apr 22 13:18 jre-1.7.0-openjdk -> java-1.7.0-openjdk-1.7.0.55/jre
lrwxrwxrwx. 1 root root 25 Feb 18 2013 jre-gcj -> /etc/alternatives/jre_gcj
lrwxrwxrwx. 1 root root 29 Apr 22 13:19 jre-openjdk -> /etc/alternatives/jre_openjdk
I truly think the answer is that the gear needs to add Java 8 SE in order for the Wildfly cartridge to be able to use it. And the Wildfly cartridge will then need to be updated to respond to a Java 8 marker..